David E. Laaksonen

Department of Physiology, University of Kuopio, Kuopio, 70211 Kuopio, Finland

Published (Online): 01 June 2003

© Journal of Sports Science and Medicine (2003) 2, Suppl.1,1-65
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2. Laaksonen, D.E., Atalay, M., Niskanen, L., Uusitupa, M., Hänninen, O. and Sen, C.K. (1996) Increased resting and exercise-induced oxidative stress in young IDDM men. Diabetes Care 19, 569-574.

3. Atalay, M., Laaksonen, D.E., Niskanen, L., Uusitupa, M., Hänninen, O. and Sen, C.K. (1997) Altered antioxidant enzyme defences in insulin-dependent diabetic men with increased resting and exercise-induced oxidative stress. Acta Physiologica Scandinavia 61, 195-201.

4. Laaksonen, D.E., Lakka, H.M., Niskanen, L.K, Kaplan, G.A., Salonen, J.T. and Lakka, T.A. (2002) Metabolic syndrome and development of diabetes mellitus: application and validation of recently suggested definitions of the metabolic syndrome in a prospective cohort study. American Journal of Epidemiology 156, 1070-1077.

5. Laaksonen, D.E., Lakka, H.M., Salonen, J.T., Niskanen, L.K., Rauramaa, R. and Lakka, T.A. (2002) Low Levels of Leisure-Time Physical Activity and Cardiorespiratory Fitness Predict Development of the Metabolic Syndrome. Diabetes Care 25, 1612-1618.

Type 1 and type 2 diabetes mellitus are major worldwide health problems predisposing to markedly increased cardiovascular mortality and serious morbidity and mortality related to development of nephropathy, neuropathy and retinopathy (Zimmet et al., 1997). The metabolic syndrome, a concurrence of disturbed glucose and insulin metabolism, overweight and abdominal fat distribution, mild dyslipidemia and hypertension, is from a clinical and public health standpoint most important because of its association with subsequent development of type 2 diabetes mellitus and cardiovascular disease (CVD) (Reaven, 1988; Kaplan, 1989; DeFronzo and Ferrannini, 1991; Kaplan, 1996; Liese et al., 1998; Lempiäinen et al., 1999; Pyörälä et al., 2000). Roughly one third of middle-aged Americans may have the metabolic syndrome as defined by the National Cholesterol Education Program (NCEP) (Ford et al., 2002). Using a different definition, 17% of men and 7% of women were estimated to have the metabolic syndrome based on a community study in Pieksämäki, Finland (Vanhala et al., 1997).

Physical exercise is a cornerstone of therapy for type 1 and type 2 diabetes mellitus (DM). Observational studies suggest that physical activity and physical fitness may decrease the risk for CVD in both non-diabetic persons (Paffenbarger et al., 1986; Ekelund et al., 1988; Blair et al., 1989; Sandvik et al., 1993; Lakka et al., 1994a; Laukkanen et al., 2001) and those with type 1 (Moy et al., 1993) and type 2 diabetes (Wei et al., 2000). This protective effect may be mediated in part through components of the metabolic syndrome. In non-diabetic persons, intervention studies, physical exercise has in variable degrees and at least in the short term decreased weight and visceral fat accumulation (Ivy, 1997; Rice et al., 1999; Ross et al., 2000), increased high-density lipoprotein (HDL) cholesterol and decreased triglyceride levels (Tran et al., 1983; Haskell, 1984), decreased blood pressure (Arroll and Beaglehole, 1992) and improved insulin sensitivity (Ivy, 1997; Rice et al., 1999; Ross et al., 2000). Physical exercise may also decrease serum low-density lipoprotein (LDL) cholesterol levels (Stefanick et al., 1998). Results from mainly small and uncontrolled studies testing the effects of regular aerobic exercise on the lipid profile in type 1 DM individuals have, however, been variable (Wallberg-Henriksson et al., 1982; Yki-Jarvinen et al., 1984; Wallberg-Henriksson et al., 1986; Lehmann et al., 1997).

Oxidative stress has been increasingly implicated in the accelerated atherosclerosis and microvascular complications of diabetes mellitus (Cameron and Cotter, 1993; Lyons, 1993; Tesfamariam, 1994; Cameron et al., 1996). Oxidative stress can result in widespread lipid, protein and DNA damage (Halliwell, 1994), including oxidative modification of LDL cholesterol, believed to be central in the pathogenesis of atherosclerois, and endothelial dysfunction (Haberland et al., 1988; Lyons, 1993; Tesfamariam, 1994; Witztum, 1994).

Many recent studies suggest that even moderate exercise increases free radical production beyond the capacity of antioxidant defenses, resulting in oxidative stress (Wallberg-Henriksson et al., 1982; Yki-Jarvinen et al., 1984; Wallberg-Henriksson et al., 1986; Stefanick et al., 1998). On the other hand, regular exercise may strengthen antioxidant defenses and decrease resting and acute exercise-induced oxidative stress (Vasankari et al., 1998; Bailey et al., 2001; Miyazaki et al., 2001). Little is known about exercise-induced oxidative stress in diabetes mellitus.

The mechanisms underlying the apparent increased oxidative stress in diabetes are not entirely clear. Accumulating evidence points to many, often interrelated mechanisms (Cameron and Cotter, 1993; Lyons, 1993; Tesfamariam, 1994; Cameron et al., 1996), increasing production of reactive oxygen species such as superoxide (Nath et al., 1984; Ceriello et al., 1991; Wolff et al., 1991; Dandona et al., 1996) or hydrogen peroxide (Wierusz-Wysocka et al., 1995; Ruiz Munoz et al., 1997), or decreasing antioxidant defenses (Asayama et al., 1993; Tsai et al., 1994; Ceriello et al., 1997; Santini et al., 1997). These mechanisms include glucose autoxidation (Hunt et al., 1990; Wolff et al., 1991) and formation of advanced glycation endproducts (AGE) (Lyons, 1993; Schleicher et al., 1997), activation of the polyol pathway (Cameron and Cotter, 1993; Grunewald et al., 1993; De Mattia et al., 1994; Kashiwagi et al., 1994; Cameron et al., 1996; Kashiwagi et al., 1996) and altered cell and glutathione redox status (Grunewald et al., 1993; De Mattia et al., 1994; Kashiwagi et al., 1994; Kashiwagi et al., 1996) and ascorbate metabolism (Sinclair et al., 1991), antioxidant enzyme inactivation (Arai et al., 1987; Blakytny and Harding, 1992; Kawamura et al., 1992), perturbations in nitric oxide and prostaglandin metabolism (Tesfamariam, 1994) and insulin resistance (Rifici et al., 1994; Niskanen et al., 1995a; Vijayalingam et al., 1996). No consensus has been reached as to the relative importance of these mechanisms. Despite strong evidence indicating a pathogenic role of oxidative stress in the development of atherosclerosis and microvascular complications in DM, controversy exists about whether the increased oxidative stress is merely associative rather than causal, or even whether oxidative stress is increased at all in DM.

In prospective cohort studies, higher levels of physical activity have quite consistently protected against development of both CVD and type 2 diabetes mellitus (Berlin and Colditz, 1990; Helmrich et al., 1994; Lakka et al., 1994a; Lynch et al., 1996; Laukkanen et al., 2001), both of which are commonly associated with the metabolic syndrome. Although the pathogenesis of the metabolic syndrome remains unclear, the metabolic syndrome is in its early stages characterized by mild and varying degrees of abnormalities of insulin, glucose and lipid metabolism, hypertension and overweight, which if unchecked may progress over years to overt diseases such as diabetes and atherosclerosis in its various manifestations (Liese et al., 1998). Because of the current epidemic of overweight and sedentary lifestyle worldwide, the metabolic syndrome poses a serious and growing problem for clinicians and public health officials alike.

Although physical exercise favorably affects individual components of the metabolic syndrome, little evidence exists showing that physical activity prevents the metabolic syndrome itself. Such information is necessary for healthcare providers and public health policy makers seeking to prevent the consequences of the metabolic syndrome already at an early phase.

Previously, clinical and epidemiological research on the metabolic syndrome was hampered by the lack of standard definitions. To address this problem, the World Health Organization (WHO) (Alberti and Zimmet, 1998) and the NCEP have recently published definitions of the metabolic syndrome.

The purpose of this series of studies was to assess 1) the effect of aerobic exercise training on lipid and lipoprotein levels in type 1 diabetes, 2) resting and exercise induced oxidative stress and 3) the association of leisure-time physical activity and cardiorespiratory fitness with development of the metabolic syndrome.

Classification of diabetes mellitus
Diabetes mellitus is a major worldwide health problem predisposing to markedly increased cardiovascular mortality and serious morbidity and mortality related to development of nephropathy, neuropathy and retinopathy (Zimmet et al., 1997). Diabetes mellitus is characterized by derangements in carbohydrate and lipid metabolism, and is diagnosed by the presence of hyperglycemia. Diabetes has been traditionally divided mainly into type 1 and type 2 DM, with other less common forms.

Type 1 DM make up about 15% of the cases of DM in Finland, is marked by deficient or absent insulin secretion by the pancreas and tends to occur before middle age (Eriksson et al., 1992). The presence of islet cell antibodies (ICA) or glutamic acid decarboxylase antibodies (GADA), markers of autoimmune -cell destruction, are usually detected at onset. Especially in younger patients, development of symptomatic hyperglycemia is rapid, and ketoacidosis common. Features of the metabolic syndrome are not usually present. Like the general population, however, many type 1 diabetic patients develop insulin resistance and features of the metabolic syndrome, which may have adverse consequences with respect to microvascular complications and CVD (Stuhldreher et al., 1992; Koivisto et al., 1996; Idzior-Walus et al., 2001; Orchard et al., 2002). Insulin resistance may alternatively develop as a consequence of hyperglycemia (glucose toxicity) (Yki-Jarvinen, 1992). A subgroup of adult-onset diabetes with ICA or GADA and slow-onset insulin deficiency are now classified according to the most recent WHO classification as a subgroup of type 1 DM (Alberti and Zimmet, 1998; Tuomi et al., 1999; Shaw et al., 2000). In Finland, up to 10% of all diabetic patients have this form of diabetes, also called latent autoimmune diabetes in adults (LADA) (Niskanen et al., 1995b; Tuomi et al., 1999).

Type 2 diabetes is the most common form of diabetes, about 85% in Finland (Eriksson et al., 1992). Due to dietary habits and increasing obesity and sedentariness in both Western and developing countries, the prevalence of type 2 DM is growing at an exponential rate (Zimmet and Lefebvre, 1996; Ludwig and Ebbeling, 2001). Type 2 DM is characterized by insulin resistance coupled with an inability of the pancreas to sufficiently compensate by increasing insulin secretion, with onset generally in middle or old age. Onset is insidious, and ketoacidosis is rare. The prevalence of type 2 DM among adults varies from less than 5% to over 40% depending on the population in question (Zimmet et al., 1997). The pathogenesis of type 2 diabetes is still unclear, although multiple genetic and environmental factors clearly interplay to produce the disease. Although the pathophysiology is still unclear, variable defects of metabolism in skeletal muscle, fat, liver and pancreas contribute to increased insulin resistance and abnormal insulin secretion. In the Botnia study, roughly 85% of type 2 diabetic patients had the metabolic syndrome as defined by the WHO (Alberti and Zimmet, 1998).

The current WHO criteria for type 2 diabetes mellitus use a fasting plasma glucose level of 7.0 or a two-hour post-load level of 11.1 mmol·l^-1 in a 75-g oral glucose tolerance test as cutoffs for type 2 diabetes (Alberti and Zimmet, 1998). These criteria are similar to the American Diabetes Association criteria (Expert Committee on the Diagnosis and Classification of Diabetes Mellitus 1997). The American criteria differ especially from previous criteria in that an oral glucose tolerance test is recommended only when the fasting glucose level is below 7.0 mmol·l^-1 but the suspicion of diabetes is high.

Maturity-onset diabetes of the young (MODY) is a genetically, metabolically, and clinically heterogeneous type of type 2 diabetes mellitus that appears to account for less than 5% of diabetes (Velho and Froguel, 1998; Fajans et al., 2001). Gestational diabetes mellitus is carbohydrate intolerance with onset or first recognition during pregnancy (Jovanovic and Pettitt, 2001). Women with gestational diabetes also are at greater risk for developing type 2 diabetes themselves (Kahn and Williamson, 2000). There are numerous other uncommon forms of diabetes that are not included in the above classifications. Insulin-deficient diabetes can result from destruction of islet cells through acute, recurrent, or chronic pancreatitis (Malka et al., 2000). Rare mitochondrial mutations have been described in which diabetes is a manifestation (Reardon et al., 1992). Uncommon lipodystrophy syndromes are frequently associated with hyperinsulinemia and subsequent diabetes (Bhayana and Hegele, 2002).

The lipid profile in type 1 diabetes
The lipid profile is quantitatively normal in type 1 diabetic patients in good glycemic control and without microvascular complications, with only subtle adverse changes in e.g. VLDL and LDL size and HDL and LDL cholesterol triglyceride content (Verges, 1999; Perez et al., 2000). Despite a relatively normal lipid profile on average, it should be noted that similarly high proportions of type 1 diabetic patients as non-diabetic individuals have elevated LDL lipoprotein concentrations (Verges, 1999; Perez et al., 2000). In patients in poor glycemic control or who have nephropathy, elevated LDL cholesterol, apolipoprotein B and triglyceride levels are more often present (Verges, 1999; Perez et al., 2000; Chaturvedi et al., 2001). Adverse levels of HDL cholesterol and triglycerides are also associated with manifestations of the metabolic syndrome in type 1 diabetes (Idzior-Walus et al., 2001).

Lipoproteins, apolipoproteins and lipids as risk factors in type 1 diabetes
Decreased HDL and high LDL cholesterol and triglyceride levels are established cardiovascular risk factors in non-diabetic (Kannel et al., 1971) and type 2 (non-insulin-dependent) DM individuals (Uusitupa et al., 1993). The role of the HDL subfractions HDL₂ and HDL₃ are more controversial, with some (Salonen et al., 1991), but not all (Stampfer et al., 1991) studies suggesting that HDL₂ cholesterol may be more important in reducing cardiovascular risk because of its role in reverse cholesterol transport (Eisenberg, 1984). Low apoliprotein (apo) A-I and high apo B levels are also associated with increased risk for cardiovascular death (Stampfer et al., 1991). Much less, however, is known of the role of lipoprotein and apolipoprotein levels in the pathogenesis of the accelerated atherosclerosis (Krolewski et al., 1987) in type 1 DM. Even so, results from cross-sectional studies suggest that lipoprotein and apolipoprotein levels are also important cardiovascular risk factors in type 1 DM (Maser et al., 1991; Winocour et al., 1992; Koivisto et al., 1996).

Aerobic exercise and the lipid profile in type 1 diabetes mellitus
Regular exercise in non-diabetic subjects is best known to increase HDL cholesterol and the HDL/total cholesterol ratio (e.g., (Williams, 1996; 1997); reviewed in (Stefanick and Wood, 1994; U.S. Department of Health and Human Services, 1996)). Many studies have also shown that endurance training decreases LDL cholesterol and less frequently triglyceride levels (Stefanick et al., 1998). The role of weight loss or body composition changes in these lipid changes is still controversial (Thompson, 1990a; Williams et al., 1990), although many studies have shown favorable effects of regular exercise on the lipid profile independent of weight loss (Thompson et al., 1997). Antiatherogenic effects of physical exercise on apolipoproteins B (apo B) and A-I (apo A-I) in non-diabetic individuals have been less consistently observed, but appear to have been related mainly to weight loss (Schwartz, 1987; 1988; Despres et al., 1991; Williams et al., 1992; Crouse et al., 1997).

Results from mainly small and uncontrolled studies testing the effects of regular aerobic exercise on the lipid profile in type 1 DM individuals have been variable. In a small controlled but not randomized study Yki-Jarvinen et al. (Yki-Jarvinen et al., 1984) also found increases in the HDL/total cholesterol ratio, without significant changes in HDL- or total cholesterol, body mass index (BMI) or glycemic control after six weeks of ergometer cycling exercise for 60 min 4 days a week. The relative change did not differ significantly between the training and control groups, however. In an uncontrolled study investigating the effect of three months of regular exercise in 20 type 1 DM men and women 22-48 years old, LDL decreased by 14% and HDL increased by 10%, with concomitant weight loss and decreased percent body fat (Lehmann et al., 1997). Corresponding changes in apo B and apo A-I were also found. A 16-week program of 60 min mixed and aerobic exercise three times a week decreased total cholesterol without effects on HDL, triglycerides, body weight or glycemic control in nine 25-46 year old men with type 1 DM in an uncontrolled study (Wallberg-Henriksson et al., 1982). Twenty minutes of daily bicycle exercise had no effect on major lipid profile indices after five months in 25-45 year old women with type 1 DM, although a small improvement in maximal oxygen consumption (VO_2max) was noted (Wallberg-Henriksson et al., 1986). Reasons for conflicting results may be differences in the number, age and gender of the subjects, the type of training protocol, glycemic status and baseline lipid status or seasonal variation in lipids.

Oxidative stress and antioxidant defenses
Oxidative stress has been defined as the imbalance of pro-oxidant and antioxidant forces in favor of the former (Steinberg et al., 1989; 2002). Oxidative stress can result in widespread lipid, protein and DNA damage (Halliwell, 1994), including oxidative modification of LDL cholesterol, believed to be central in the pathogenesis of atherosclerois, and endothelial dysfunction (Haberland et al., 1988; Steinberg et al., 1989; Lyons, 1993; Tesfamariam, 1994; Witztum, 1994). Oxidized LDL cholesterol is found in high concentrations in atherosclerotic lesions, and at least in vitro, uptake of LDL by mononuclear cells and macrophages does not occur without oxidation of the LDL (Haberland et al., 1988; Steinberg et al., 1989; Yla-Herttuala et al., 1989; Lyons, 1993; Tesfamariam, 1994; Witztum, 1994). The apparent increased oxidative stress in diabetes mellitus has been implicated in the accelerated atherosclerosis and microvascular complications of diabetes (Cameron and Cotter, 1993; Lyons, 1993; Tesfamariam, 1994; Cameron et al., 1996).

The mechanisms underlying the increased oxidative stress in diabetes are not entirely clear. Accumulating evidence points to many, often interrelated mechanisms (Cameron and Cotter, 1993; Lyons, 1993; Tesfamariam, 1994; Cameron et al., 1996), increasing production of reactive oxygen species (ROS) such as superoxide (Nath et al., 1984; Ceriello et al., 1991; Wolff et al., 1991; Dandona et al., 1996) or hydrogen peroxide (Wierusz-Wysocka et al., 1995; Ruiz Munoz et al., 1997), or decreasing antioxidant defenses (Figure 1, Asayama et al., 1993; Tsai et al., 1994; Ceriello et al., 1997; Santini et al., 1997)). Glucose autoxidation and formation of advanced glycation endproducts (AGE) not only generate ROS, but also may activate nuclear factor B and adhesion molecules and induce lipid peroxidation (Lyons, 1993; Schleicher et al., 1997; Arnalich et al., 2001). Activation of the polyol pathway may decrease the NADPH/NADP+ ratio, resulting in reductive stress and possibly adversely affecting NADPH-dependent antioxidant enzyme activity (Cameron and Cotter, 1993; Grunewald et al., 1993; De Mattia et al., 1994; Kashiwagi et al., 1994; Cameron et al., 1996; Kashiwagi et al., 1996). Increased reductive and oxidative stress may also alter cell and glutathione redox status (Grunewald et al., 1993; De Mattia et al., 1994; Kashiwagi et al., 1994; Kashiwagi et al., 1996) and ascorbate metabolism (Sinclair et al., 1991; Maxwell et al., 1997; Cunningham, 1998; Seghieri et al., 1998), although plasma vitamin E levels are not decreased (Vessby et al., 2002). Glycation may also inactivate antioxidant enzymes like glutathione reductase and superoxide dismutase (Arai et al., 1987; Blakytny and Harding, 1992; Kawamura et al., 1992). Endothelial dysfunction and injury may occur as a result of increased oxidative stress (Tesfamariam, 1994; Soriano et al., 2001). Nitric oxide, itself an ROS, may react with superoxide to form the highly toxic peroxynitrite radical (Soriano et al., 2001). Increased prostaglandin synthesis and alterations in the balance of opposing prostaglandins may also contribute to endothelial dysfunction and platelet activation (Tesfamariam, 1994). High insulin and insulin-like growth factor-1 concentrations may increase superoxide production in mononuclear cells (Rifici et al., 1994). Insulin resistance has also been linked to lipid peroxidation and impaired antioxidant defenses (Rifici et al., 1994; Niskanen et al., 1995a; Vijayalingam et al., 1996). No consensus has been reached as to the relative importance of these mechanisms.

Alterations in glutathione metabolism in type 1 diabetes
Tissue glutathione plays a central role in antioxidant defenses (Meister, 1995; Sen et al., 2000). Reduced glutathione (GSH) detoxifies reactive oxygen species such as hydrogen peroxide and lipid peroxides directly or in a glutathione peroxidase (GPX) -catalyzed mechanism. Glutathione also regenerates the major aqueous and lipid phase antioxidants ascorbate and a-tocopherol. Glutathione reductase (GRD) catalyzes the NADPH-dependent reduction of oxidized glutathione, serving to maintain intracellular glutathione stores and a favorable redox status. Glutathione-S-transferase (GST) catalyzes the reaction between the -SH group and potential alkylating agents, rendering them more water soluble and suitable for transport out of the cell. GST can also use peroxides as a substrate (Mannervik and Danielson, 1988).

Platelet GSH content were ten-fold lower in type 1 DM patients with glycated Hb greater than 7%, but no further decrease was found when glycated Hb was greater than 11% (Muruganandam et al., 1992). Di Simplicio et al. (1995) found normal GSH levels, increased GRD activity and decreased thiol transferase activity in platelets of 46 type 1 DM patients. Platelets from the DM patients also had a lower level of threshold for aggregation induced by arachidonic acid. Children with type 1 DM also had lower erythrocyte GSH than control subjects (Jain and McVie, 1994). Hemoglobin A_1c (Hb_A1c) was inversely correlated with red cell GSH content. Thornalley et al. (Thornalley et al., 1996) found an inverse correlation between erythrocyte GSH levels and the presence of DM complications in type 1 patients. Normal blood GSH levels were found in 43 patients with type 1 DM compared to 21 non-diabetic subjects (McLellan et al., 1994).

Most studies have found decreased blood or red cell glutathione levels in type 2 DM patients (Thomas et al., 1985; Murakami et al., 1989; De Mattia et al., 1994; Yoshida et al., 1995; Ciuchi et al., 1997). Less firm conclusions can be drawn in type 1 DM patients. Further information is also needed about whether levels are decreased in patients without complications and whether patients with complications have even lower levels, although some studies do suggest this. The pathophysiological significance of decreased glutathione levels in diabetes remains to be shown.

Glutathione-dependent enzymes in type 1 diabetes
Blood GRD activity was lower in 11 children with type 1 DM compared to 49 healthy children (Stahlberg and Hietanen, 1991). On the other hand, normal red cell GRD activity has been found (Walter et al., 1991; Muruganandam et al., 1992) In type 1 DM red cell selenium content and GPX activity were decreased (Osterode et al., 1996). Walter et al. (1991) found no difference in whole blood GPX activity in 57 type 1 and type 2 DM patients compared to 28 non-diabetic control patients, a finding supported by Leonard et al. (Leonard et al., 1995). Normal red cell GST enzyme kinetics have also been found in type 1 DM patients (Muruganandam et al., 1992).

Changes in glutathione-dependent enzymes in diabetic patients are inconsistent. Differences in results cannot be completely explained by study methodology.

Impairment of superoxide dismutase and catalase activity in type 1 diabetes
Superoxide dismutase and catalase are major antioxidant enzymes (Michiels et al., 1994). SOD exists in three different isoforms. Cu,Zn-SOD is mostly in the cytosol and dismutates superoxide to hydrogen peroxide. Extracellular (EC) SOD is found in the plasma and extracellular space. Mn-SOD is located in mitochondria. Catalase is a hydrogen peroxide decomposing enzyme mainly localized to peroxisomes or microperoxisomes. Decreased Cu,Zn-SOD activity coupled with the increased superoxide or H₂O₂ production that may occur in DM (Ceriello et al., 1991; Wolff et al., 1991) could predispose to increased oxidative stress, especially if not compensated with increased catalase or Se-GPX activity. Superoxide may react with other reactive oxygen species such as nitric oxide to form highly toxic species such as peroxynitrite, in addition to having direct toxic effects (Tesfamariam, 1994). Alternatively, superoxide can be dismutated to much more reactive hydrogen peroxide, which through the Fenton reaction can then lead to highly toxic hydroxyl radical formation (Wolff et al., 1991).

Red cell Cu,Zn/SOD activity has also been found to be decreased in type 1 DM patients (Kawamura et al., 1992; Skrha et al., 1996). Red cell glycosylated Cu,Zn-SOD levels were elevated in type 1 DM patients (Kawamura et al., 1992). Glycation appears to decrease Cu,Zn-SOD activity, which could predispose to oxidative damage (Kawamura et al., 1992). Decreased red cell Cu,Zn-SOD activity has been found in type 1 DM patients with retinopathy compared to type 1 DM patients without microvascular complications (Jennings et al., 1991; Skrha et al., 1994), although no difference was found between patients without retinopathy and healthy individuals (Jennings et al., 1991). Yaquoob et al. (1994) reported increased red cell superoxide dismutase and serum malondialdehyde (MDA) in patients with type 1 DM and normo- and microalbuminuria compared to healthy subjects. There was no difference, however, between DM patients with normo- or microalbuminuria, in agreement with another study (Leonard et al., 1995). In contrast, red cell Cu,Zn-SOD activity has been found to be similar in Type 1 DM patients and healthy individuals, irrespective of microvascular complications (Walter et al., 1991). EC-SOD can also be glycated, although glycation does not affect enzyme activity (Adachi et al., 1994). EC-SOD activity was found to be similar in 23 children with type 1 DM of varying duration and healthy children (Marklund and Hagglof, 1984).

The wide variability among studies does not allow conclusions to be drawn as to whether SOD isoform activity is abnormal in diabetic patients. Again, differences in methodology or study design do not completely explain the conflicting findings among studies. Less information is available about catalase activity in type 1 DM. Normal red blood cell catalase activity has been reported (Seghieri et al., 2001).

Lipid peroxidation in type 1 diabetes
Use of thiobarbituric acid reactive substances (TBARS) as an index of lipid peroxidation was pioneered by Yagi (1976), whose group also showed increased plasma TBARS levels in diabetes (Sato et al., 1979). Walter et al. (Walter et al., 1991) found increased plasma peroxide concentrations in 57 Type 1 and Type 2 DM patients compared to 28 non-diabetic control patients. Higher plasma MDA levels were found in 67 middle aged diabetic patients (20 type 1, 47 type 2) than in 40 healthy subjects (Noberasco et al., 1991). MDA levels showed a significant correlation with glycosylated Hb. Women with well controlled type 1 DM had higher levels of lipid peroxidation during pregnancy than healthy women (Carone et al., 1993).

Plasma TBARS levels were higher in 117 type 1 and 2 DM patients than in 53 control subjects, independently of metabolic control (Gallou et al., 1993). There were no differences between type 1 and type 2 patients. Plasma MDA and lipid hydroperoxide levels were elevated in hospitalized ketotic type 1 DM patients (Faure et al., 1993). One week after achieving glycemic control with insulin treatment, MDA levels approached reference values. Plasma TBARS were elevated in women but not men in a study investigating lipid peroxidation in 56 young adult type 1 DM and 56 matched non-diabetic control subjects (Evans and Orchard, 1994). TBARS levels were elevated in 158 DM patients compared to control subjects (Griesmacher et al., 1995). TBARS levels were increased in 18 type 1 DM patients with no or mild retinopathy compared to previously established reference values (Faure et al., 1995). The initial plasma H₂O₂ and MDA levels in 15 patients with Type 1 and 15 with Type 2 diabetes before and after 2 weeks of intensive treatment were higher than in control subjects (Wierusz-Wysocka et al., 1995). After 2 weeks of treatment, the values for both parameters were lower; although still higher than in the control group. Lipid hydroperoxides and conjugated dienes were elevated and total antioxidan capacity decreased in 72 patients with well-controlled type 1 DM and without complications, independently of metabolic control or diabetes duration (Santini et al., 1997). In a later study by the same goup, these basic findings were repeated in 37 patients with uncomplicated type 1 diabetes and 29 non-diabetic men and women. Compared with the diabetic men, diabetic women had even higher levels of lipid hydroperoxides and lower antioxidant capacity (Marra et al., 2002).

On the other hand, serum levels of a conjugated diene isomer of linoleic acid was lower in type 1DM patients than control subjects (Collier et al., 1988). No difference in serum conjugated diene levels between otherwise healthy diabetic patients and healthy control subjects were noted, although conjugated diene levels were increased in 26 diabetic patients with microangiopathy compared to 36 diabetic patients without microangiopathy and 36 control subjects (Jennings et al., 1991). Plasma TBARS levels were similar in 17-40 year old type 1 DM patients as in control subjects, and were also similar in smokers (Leonard et al., 1995). Zoppini et al. (Zoppini et al., 1996) also found similar plasma TBARS levels in 56 type 1 DM patients as in 32 age- and sex-matched control subjects, but TBARS were higher in type 1 DM smokers. No differences in plasma MDA and 8-iso-prostaglandin F2 levels were found between 38 type 1 diabetic patients and 41 control subjects, despite a lower total antioxidant capacity (Vessby et al., 2002).

Whether lipid peroxidation is increased in DM even before development of micro- and macrovascular disease is unclear. Many published studies have found increased lipid peroxidation in type 1 DM patients, but conflicting results have also been found. Inconsistent evidence also suggests that increased lipid peroxidation and impaired antioxidant defenses may be more pronounced in women with type 1 DM. The differing findings cannot be explained simply based on study design or methodology. A causal role for lipid peroxidation in the development of diabetic macro- and microvascular complications is far from established.

Lipid peroxidation and type 1 diabetic complications
Jennings et al. (1987) reported increased serum conjugated diene levels in 26 diabetic patients with microangiopathy compared to 36 diabetic patients without microangiopathy. Lipid peroxides were also significantly elevated in 15 type 1 patients with retinopathy compared to type 1 DM patients without microvascular complications (Jennings et al., 1991). Plasma TBARS levels correlated with albumin excretion in 64 type 1 and type 2 DM patients (Knobl et al., 1993). Twenty-one normotensive type 1 diabetic patients without microalbuminuria but with evidence of endothelial injury (elevated levels of plasma von Willebrand factor, soluble thrombomodulin content and angiotensin converting enzyme activity) had elevated levels of serum MDA compared to patients without evidence of endothelial injury (Yaqoob et al., 1993). Type 1 and 2 DM patients in poor metabolic control or with angiopathy had higher levels of TBARS than those in good control or without angiopathy, independently of lipid levels (Griesmacher et al., 1995). In type 1 DM patients with microangiopathy, the oxidized LDL/normal LDL antibody ratio was paradoxically lower than in patients without complications, most likely due to oxidized LDL specific immune complexes found exclusively in antibody-negative patients (Festa et al., 1998).

In contrast, levels of serum MDA were similar between 33 type 1 DM patients with microalbuminuria and 49 patients without microalbuminuria (Yaqoob et al., 1994). TBARS levels were similar in 16 patients with micro-or macroalbuminuria compared to 69 normoalbuminuric patients in young type 1 DM patients (Leonard et al., 1995).

There seems to be no clear consensus as to whether patients who have developed diabetic complications have increased lipid peroxidation compared to patients without complications, although more studies have reported higher levels of lipid peroxidation in DM patients with complications than in patients without complications. Further studies are needed to clarify this issue and also whether such increased oxidative stress is pathologically important or merely a marker of micro- or macrovascular damage.

Susceptibility of LDL cholesterol to oxidation in type 1 diabetes
Susceptibility of LDL to oxidation was strongly correlated with degree of LDL glycosylation. LDL and red blood cell (RBC) membranes in 11 normolipidemic type 1 and 18 type 2 DM patients were more susceptible to oxidation than in normal subjects (Rabini et al., 1994). The susceptibility of LDL to copper-catalyzed oxidation was greatest in 22 familial hypertriglyceridemic patients while intermediate values were found in 24 type 1, 16 type 2 and 14 abdominally obese patients compared to gluteal-femoral obese subjects and controls (Cominacini et al., 1994). The different susceptibility to oxidation found in the different groups of patients was only partially explained by plasma triglyceride values. Plasma TRAP (total peroxyl radical trapping potential) was less and susceptibility of LDL to oxidation as measured by the lag phase of conjugated diene formation after initiation of LDL oxidation by the addition of copper was greater in poorly controlled type 1 diabetic subjects than in normal control subjects (Tsai et al., 1994). This could not be attributed to the presence of oxidation-susceptible, small, dense LDL particles in the diabetic subjects, whose lipoprotein particle distribution did not differ from the control subjects. LDL from both type 1 (n=20) and type 2 (n=20) diabetic patients exhibited a shorter lag phase duration for conjugated diene formation, regardless of the presence of vascular complications (Beaudeux et al., 1995). LDL exhibited a shorter lagtime and a lower -tocopherol/LDL ratio for 10 type 1 and 53 type 2 diabetic patients than for sex and age-matched control subjects (Leonhardt et al., 1996). The lagtime was positively correlated to the LDL a-tocopherol/LDL and inversely correlated to HbA_1c. Recently diagnosed type 1 DM patients (n=25) with poor glycemic control showed higher electronegative LDL (suggesting a higher degree of oxidaton), similar LDL subfraction phenotype and lower susceptibility to oxidation compared to 25 matched healthy control subjects (Sanchez Quesada et al., 1996). After three months of intensive insulin therapy, HbA_1c and LDL electronegativity decreased, but no changes in LDL susceptibility to oxidation or LDL subfraction phenotype were observed.

In contrast, there was no difference between 20 type 1 diabetic patients in moderate glycemic control and non-diabetic subjects in the susceptibility of LDL cholesterol to either copper--dependent or non-transition metal-dependent oxidation (O-Brien et al., 1995). Furthermore, there was no difference between the groups for LDL vitamin E content, LDL fatty acid composition in cholesterol esters or triglycerides, but LDL glycation was elevated in the type 1 DM subjects. There was no difference between 34 type 1 DM patients without clinical signs of vascular disease and 22 healthy control patients in the oxidizability of LDL and very-low-density lipoprotein (VLDL) (Jain et al., 1998). There was no difference in the susceptibility to in vitro oxidation of LDL isolated from 15 type 1 DM patients in good glycemic control and with no evidence of macrovascular disease or proteinuria compared with control subjects (Jenkins et al., 1996). The particle size, lipid composition, fatty acid content, antioxidant content, and glycation were similar for LDL isolated from both groups. LDL size was smaller in 31 type 1 diabetic patients than in 45 control subjects, but susceptibility of LDL cholesterol to oxidation was similar (Skyrme-Jones et al., 2000).

Most studies have found increased susceptibility of LDL cholesterol to oxidation in DM patients, although some studies have had conflicting results. Studies carried out to date do not allow firm conclusions to be drawn about whether LDL is more susceptible to oxidation in DM patients without complications than in healthy subjects, or about what effect complications and glycemic control have on the susceptibility of LDL to oxidation.

Autoantibodies to oxidized cholesterol in type 1 diabetes
Levels of anti-oxidized LDL antibodies and anti-MDA-modified LDL antibodies were similar in 16 type 1 diabetes mellitus patients free of macrovascular complications and 16 control subjects (Mironova et al., 1997). In 101 type 1 DM normo- and macroalbuminuric patients with a long duration of diabetes and 54 healthy subjects, antibodies against MDA-modified LDL did not differ among normoalbuminuric DM, albuminuric DM and control subjects (Korpinen et al., 1997). In contrast, antibodies to oxidized LDL cholesterol were 1.5-fold higher in 38 type 1 diabetic patients free of macrovascular disease than in 33 normal subjects (Makimattila et al., 1999). Antibodies to oxidized LDL were correlated with age in normal subjects, but not with age, duration of disease, LDL-cholesterol, HbA_1c or degree of microvascular complications in patients with type 1 diabetes.

Relatively few studies have examined the association of type 1 DM with autoantibodies to oxidized LDL, but no clear consensus suggesting increased oxidized LDL antibodies in type 1 diabetes has been found. The fact that type 1 diabetes has an autoimmune basis could explain some of the variation in results.

Oxidative stress and antioxidant defenses in physical exercise
Many recent studies have shown that even moderate exercise may increase free radical production beyond the capacity of antioxidant defenses, resulting in oxidative stress (Davies et al., 1982; Alessio, 1993; Sen et al., 1994b; Ji, 1995; Sen, 1995; Liu et al., 1999). In animals, exercise training may strengthen antioxidant defenses and may reduce resting and acute exercise-induced oxidative stress (Alessio and Goldfarb, 1988; Sen et al., 1992; Sen, 1995; Kim et al., 1996a; Kim et al., 1996b). Several theses and reviews on the topic have been published by members of our research team (Sen, 1994; Sen, 1995; Atalay, 1998; Khanna, 1998; Sen et al., 2000). Therefore review of oxidative stress and antioxidant defenses in physical exercise here will be limited briefly to exercise intervention studies and oxidative stress in humans.

Relatively few studies on the effect of exercise training on indices of oxidative stress or antioxidant defenses in humans have been published. A 10-month exercise program that increased VO_2max by 19% also decreased LDL oxidation and other lipid risk factors in an uncontrolled study in 34 sedentary men and 70 women (Vasankari et al., 1998). On the other hand, three months of relatively intense running training in nine fit men decreased circulating antioxidants (uric acid, SH-groups, -tocopherol, beta-carotene, retinol) except ascorbate, without affecting the lag time for the susceptibility of plasma LDL to oxidation in vitro (Bergholm et al., 1999). Normoxic and especially intermittent hypoxic training attenuated the increases in lipid hydroperoxides and MDA induced by acute normoxic exercise after four weeks of aerobic training in a trial in which the normoxic training group served as the control group (Bailey et al., 2001). Twelve weeks of high-intensity endurance training increased erythrocyte SOD and GPX antioxidant enzyme activities and decreased neutrophil superoxide production in response to exhausting exercise in an uncontrolled study (Miyazaki et al., 2001). A reduction in exercise-induced lipid peroxidation in erythrocyte membrane was also observed. Reduced glutathione levels increased in five age-matched control subjects with high-intensity aerobic training, whereas only oxidized glutathione levels increased in 17 patients with COPD (Rabinovich et al., 2001). Immediately after acute treadmill exercise, 46 claudicants developed significant neutrophil activation and degranulation with free radical damage, an effect that decreased after three months of exercise training. No effect was seen in 22 control subjects (Turton et al., 2002).

No randomized controlled trials of aerobic training on indices of oxidative stress or antioxidant defenses have yet been published. Nonetheless, some evidence suggests that exercise training may favorably affect indices of oxidative stress and antioxidant protection in some diseases and in healthy persons, although contradictory findings exist (Bergholm et al., 1999).

The metabolic syndrome
The concurrence of disturbed glucose and insulin metabolism, overweight and abdominal fat distribution, mild dyslipidemia and hypertension, has given rise to the concept of the metabolic syndrome, also known as Syndrome X, the Deadly Quartet, and the insulin resistance syndrome (Reaven, 1988; Kaplan, 1989; DeFronzo and Ferrannini, 1991; Kaplan, 1996; Liese et al., 1998). Although the metabolic syndrome has been in the scientific limelight only since being re-introduced as Syndrome X in 1988 (Reaven, 1988; Liese et al., 1998), clustering of hypertension, hyperglycemia and gout was described already in 1923 (Kylin, 1923). Insulin resistance has been considered to be the underlying abnormality of this syndrome. The pathogenesis of this syndrome has multiple origins, but obesity and sedentary lifestyle coupled with diet and still largely unknown genetic factors clearly interact to produce it (Reaven, 1988; Kaplan, 1989; DeFronzo and Ferrannini, 1991; Bouchard, 1995; Kaplan, 1996; Liese et al., 1998). The metabolic syndrome is from a clinical and public health standpoint most important because of subsequent high morbidity and mortality from diseases such as type 2 diabetes and CVD (Reaven, 1988; Kaplan, 1989; DeFronzo and Ferrannini, 1991; Kaplan, 1996; Liese et al., 1998; Lempiäinen et al., 1999; Pyörälä et al., 2000). Patients with type 1 diabetes are also not immune from the metabolic syndrome and its consequences, including CVD and microvascular disease (Stuhldreher et al., 1992; Koivisto et al., 1996; Idzior-Walus et al., 2001; Orchard et al., 2002). Overweight and physical inactivity also appear to bring about the metabolic syndrome in type 1 diabetes (Idzior-Walus et al., 2001).

As the epidemic of obesity and sedentary lifestyle continues worldwide, the metabolic syndrome and its consequences, especially diabetes, can be expected to become increasingly common at younger ages. In the US type 2 diabetes is indeed becoming alarmingly common in particularly Hispanic, black and American Indian children (Ludwig and Ebbeling, 2001). Although the metabolic syndrome has been less closely associated with coronary heart disease than with type 2 diabetes, the obesity epidemic and its associated metabolic syndrome may explain the plateauing in the decline in the incidence of myocardial infarction over the past ten years in the United States (Rosamond et al., 1998).

Pathophysiology of the metabolic syndrome
The pathogensis of the metabolic syndrome is poorly understood, and will be discussed here only briefly. An abdominal distribution of fat appears to be particularly deleterious (Figure 2, Larsson et al., 1984; Folsom et al., 1993; Rexrode et al., 1998; Folsom et al., 2000). Abdominal fat can also be divided into subcutaneous and visceral compartments that can be assessed with computed tomography or magnetic resonance imaging. Mainly experimental evidence suggests that abdominal obesity may mediate its deleterious effects on carbohydrate and lipid metabolism through the increased lipolytic activity of especially omental fat, which drains directly into the portal-venous system (Bjorntorp, 1991). This in turn results in higher non-esterified fatty acid concentrations, with consequent insulin resistance in the liver and skeletal muscle, and dyslipidemia. According to this "portal hypothesis", because of the higher lipolytic activity of visceral than subcutaneous abdominal fat, visceral fat should be more closely associated with insulin resistance and its associated metabolic derangements (Bjorntorp, 1991). The pathophysiological significance of these subdivisions are unclear, however (Despres et al., 1989; Abate et al., 1995; Goodpaster et al., 1997; Brochu et al., 2000; Kelley et al., 2000; Ross et al., 2000; Sardinha et al., 2000; DeNino et al., 2001; Smith et al., 2001; Cnop et al., 2002; Ross et al., 2002).

More recently, the concept of ectopic fat deposition has been developed (Ginsberg, 2000; Kahn and Flier, 2000; Kelley and Mandarino, 2000; Shulman, 2000). In addition to the quantity of abdominal subcutaneous and visceral fat, the degree of lipid storage in skeletal muscle and liver has also been shown to be powerful determinants of insulin sensitivity. Peripheral adipocytes have limited reserves for storing fat. Those reserves in turn depend in part on genetic and environmental factors. As the ability of the peripheral adipocyte to store fat is exceeded, the fat cells become insulin resistant, reulting in increased lipolysis and release of fatty acids into the blood stream, and decreased uptake of fatty acids. This in turn results in not only abdominal subcutaneous and visceral fat deposition, but also storage of lipids in liver and skeletal muscle.

Triglyceride accumulation in the liver results in decreased hepatic insulin sensitivity and increased VLDL production, which results in increased transfer of cholesterol esters from HDL and LDL cholesterol to VLDL cholesterol in exchange for triglyceride (Eisenberg, 1984; Ginsberg, 2000; Kahn and Flier, 2000). This in turn impairs reverse cholesterol transport and results in a decrease HDL levels, a shift in balance to HDL₃ cholesterol, and a shift from large buoyant LDL particles to small dense LDL particles. Increased hepatic insulin resistance also results in inappropriate gluconeogenesis postprandially.

Skeletal muscle is a major determinant of whole-body glucose disposal (Kahn and Flier, 2000; Kelley and Mandarino, 2000). More recent evidence suggests that intramuscular lipid deposits play a major role in decreasing glucose uptake in skeletal muscle (Ginsberg, 2000; Kahn and Flier, 2000; Kelley and Mandarino, 2000; Shulman, 2000). Intramuscular lipids appear to decrease glycogen syntheis and impair glucose transport by activating protein kinase CӨ, which results in a cascade that phosphorylates insulin substrates 1 and 2, impairing the insulin reseptor's ability to activate phosphatidylisositol kinase 3 and ultimately impairing glucose transport into the cell. Paradoxically, the ability to utilize fatty acids as an energy source in the resting state is impaired in insulin resistance, whereas in insulin-stimulated states, glucose oxidation is impaired (Kelley and Mandarino, 2000).

As the metabolic syndrome becomes more severe, interplay between genetic susceptibility, insulin resistance and diet may lead to progressive -cell failure and impaired insulin secretory capacity (Nijpels, 1998; Cavaghan et al., 2000; Hu et al., 2001; Kahn et al., 2001; Trayhurn and Beattie, 2001). As -cell secretory capacity declines, impaired glucose tolerance (IGT) develops. IGT is common in older persons, up to 25% of individuals of European descent. Roughly 5-10% of persons with IGT convert to frank diabetes yearly, again with weight gain, diet, genetic susceptibility and insulin resistance contributing to the progressive -cell failure. The manifestations of cardiovascular risk factors such as dyslipidemia, hypertension, endothelial dysfunction, inflammation, hypercoagulability and impaired fibrinolysis, obesity and abnormal insulin and glucose metabolism predispose persons with the metabolic syndrome to development of another important end-stage consequence of the metabolic syndrome, cardiovascular disease (Reaven, 1988; Kaplan, 1989; DeFronzo and Ferrannini, 1991; Kaplan, 1996; Liese et al., 1998; Lempiäinen et al., 1999; Pyörälä et al., 2000).

Disturbances in the adrenal-pituitary axis (Bjorntorp and Rosmond, 2000), inflammation (Pradhan and Ridker, 2002) and abnormal sex steroid metabolism (Livingstone and Collison, 2002) have all been proposed to contribute to or exacerbate the development of the metabolic syndrome, but evidence for these abnormalities as the primary mechanism for the pathogenisis of the metabolic syndrome is insufficient. Adipose tissue also produces hormones, cytokines and other peptides such as angiotensinogen, adipsin, acylation-stimulating protein, adiponectin, retinol-binding protein, leptin, resistin, tumor neorosis factor , interleukin 6, plasminogen activator inhibitor-1 that may play a role in insulin resistance, inflammation and the development of diabetes and CVD (Fruhbeck et al., 2001; Trayhurn and Beattie, 2001; Pradhan and Ridker, 2002).

The pathophysiology behind the association of obesity and insulin resistance with hypertension is also poorly understood. Contributing mechanisms include resistance to insulin-mediated vasodilation and endothelial dysfunction (McFarlane et al., 2001; Steinberg and Baron, 2002), hyperinsulinemia-mediated increased sodium and water absorption (Esler et al., 2001; McFarlane et al., 2001; Montani et al., 2002) and activation of the sympathetic nervous system (Esler et al., 2001; McFarlane et al., 2001; Montani et al., 2002).

Environmental and genetic (Groop and Orho-Melander, 2001; Ukkola and Bouchard, 2001) factors contribute to both the development of overweight and the propensity for insulin resistance and ectopic fat deposition and other manifestations of the metabolic syndrome (Figure 2). Environmental factors include sedentary lifestyle and poor physical fitness (U.S. Department of Health and Human Services, 1996; World Health Organization, 2000; Uusitupa, 2001), diet (Hu et al. 2001; Uusitupa 2001; Bray et al., 2002), low childhood and adult socioeconomic status (Brunner et al., 1997; Davey Smith and Hart, 1997; Lawlor et al., 2002) and low birthweight and rapid childhood growth (Forsen et al., 2000; Eriksson et al., 2001; Eriksson et al., 2002).

Definitions of the metabolic syndrome
Despite the abundant epidemiological and experimental research that has been published on the metabolic syndrome, definitions of the metabolic syndrome and the various cut-offs for its components have varied widely (Liese et al., 1998). The World Health Organization (WHO) consultation for the classification of diabetes and its complications (Alberti and Zimmet 1998) and the National Cholesterol Education Program (NCEP) Expert Panel have recently published definitions of the metabolic syndrome.

The WHO published a working definition of the metabolic syndrome meant to facilitate research on the metabolic syndrome and aid comparability between studies, rather than serve as a strict definition (Alberti and Zimmet, 1998). The metabolic syndrome was defined (without assumptions of causality) for men as: insulin resistance in the top 25% of the population as measured by the euglycemic hyperinsulinemic clamp or presence of impaired glucose tolerance (IGT) or type 2 diabetes and the presence of at least two of the following: abdominal obesity (waist-hip ratio >0.90 or BMI 30 kg·m^-2), dyslipidemia (serum triglycerides 1.70 mmol·l^-1 or HDL cholesterol <0.9 mmol·l^-1), hypertension (160/90), or microalbuminuria. These core components were considered most suitable for a general definition (Liese et al., 1998), although many other disturbances, e.g. disorders of coagulation and endothelial function, hyperuricemia and elevated leptin levels, have been associated with the metabolic syndrome (Figure 2).

This working definition has not been without criticism. Inclusion of microalbuminuria as a core component is controversial, and microalbuminuria in non-diabetic individuals is uncommon (Hodge et al., 1996; Zavaroni et al., 1996; Jager et al., 1998; Balkau and Charles, 1999). The most appropriate measure of abdominal obesity is also in dispute. Although waist-hip ratio may carry information relevant to disease endpoints independently of waist girth or BMI (Folsom et al., 2000), waist circumference correlates better with visceral fat deposits as measured by computerized tomography (Seidell et al., 1988). Defining adiposity as waist girth 94 cm has been proposed by the European Group for the Study of Insulin Resistance (EGIR) (Balkau and Charles, 1999). Furthermore, the euglycemic hyperinsulinemic clamp is not practical for epidemiological research. The EGIR recommended use of fasting insulin levels to estimate insulin resistance and IFG as a substitute for IGT in epidemiological studies (Balkau and Charles, 1999). The EGIR also proposed lower cut-offs for hypertension (140/90) (Balkau and Charles 1999) that are in accordance with current WHO-ISH (International Society of Hypertension) and Sixth Joint National Committee recommendations (Balkau and Charles 1999).

The NCEP Expert Panel has also recently published a definition of the metabolic syndrome for clinical use (NCEP, 2001). The metabolic syndrome was defined as three or more of the following: fasting plasma glucose levels 6.1 mmol·l^-1, serum triglycerides 1.7 mmol·l^-1, serum HDL <1.0 mmol·l^-1, blood pressure 130/85 mmHg, waist girth >102 cm. Use of waist circumference >94 cm was suggested for some men who may be genetically susceptible to insulin resistance (NCEP, 2001). Over 30% of middle-aged persons in the US have been reported to have the metabolic syndrome as defined by the NCEP (Ford et al., 2002).

Components of the metabolic syndrome

Hyperinsulinemia and insulin resistance
Hyperinsulinemia and insulin resistance have consistently predicted type 2 diabetes, even when adjusted for other components of the metabolic syndrome (Charles et al., 1991; Martin et al., 1992; Lillioja et al., 1993; Haffner et al., 1995). Hyperinsulinemia has also predicted hypertension independently of obesity (Skarfors et al., 1991; Lissner et al., 1992; Salonen et al., 1998), although in some studies only in subgroups, such as non-diabetic non-Hispanic whites (Shetterly et al., 1994) and lean normoglycemic individuals (Haffner et al., 1992). Hyperinsulinemia has also predicted dyslipidemia independently of obesity in some (Haffner et al., 1992; Salonen et al., 1998), but not all (Mykkanen et al., 1994a) studies. These findings suggest that insulin resistance may precede development of hypertension and dyslipidemia in the early stages of the metabolic syndrome. Hyperinsulinemia has also predicted CVD incidence or mortality (Casassus et al., 1992; Yarnell et al., 1994; Despres et al., 1996; Lakka et al., 1996; Perry et al., 1996; Lakka et al., 2000; Pyorala et al., 2000), although often not independently of other cardiovascular risk factors (Casassus et al., 1992; Yarnell et al., 1994; Lakka et al., 1996; Lakka et al., 2000).

The gold standard for measuring whole-body insulin resistance is the euglycemic hyperinsulinemic clamp (Ferrannini and Mari, 1998). The procedure is time- and labor-intensive, however, and not practical for especially epidemiological studies or routine clinical use. As a substitute, use of fasting insulin levels has been recommended (Balkau and Charles, 1999). Indeed, fasting insulin levels have a correlation of at least 0.6 in non-diabetic individuals (Laakso, 1993). Although no internationally agreed cut-offs are available, the top quarter of insulin resistance as measured by the clamp (Alberti and Zimmet, 1998) or as estimated by fasting insulin levels (Balkau and Charles, 1999) has been recommended. The homeostasis model assessment (HOMA) (Matthews et al., 1985) is a common method of estimating insulin resistance based on fasting insulin and glucose levels. The recently validated quantitative insulin sensitivity check index (QUICKI) is also based on fasting insulin and glucose concentrations and is closely (inversely) correlated with HOMA, differing mainly in being normally distributed (Katz et al., 2000). The correlation of insulin sensitivity as estimated by QUICKI and the euglycemic clamp was 0.75, better than the minimal model intravenous glucose tolerance test (Katz et al., 2000). Some controversy still exists, however, about whether these measures predict insulin resistance better than fasting insulin levels (Yeni-Komshian et al., 2000).

Factor analysis has been used to reduce intercorrelated variables into a smaller set of underlying uncorrelated factors that can be used to explain complex underlying physiological phenomena, and is particularly well suited for analysis with components of or related to the metabolic syndrome (Edwards et al., 1994; Meigs, 2000; Pyörälä et al., 2000). Although previous studies sometimes have generated separate lipid (Lempiäinen et al., 1999; Chen et al., 2000; Pyörälä et al., 2000) or blood pressure factors (Meigs et al., 1997; Chen et al., 1999; Lempiäinen et al., 1999; Chen et al., 2000; Hodge et al., 2001; Lindblad et al., 2001), with differences at least in part related to the variables entered into the analyses, the factor explaining the greatest variance has consistently had heavy loadings by measures of adiposity and fat distribution, insulin and glucose (Edwards et al., 1994; Meigs et al., 1997; Gray et al., 1998; Chen et al., 1999; Lempiäinen et al., 1999; Chen et al., 2000; Pyörälä et al., 2000; Snehalatha et al., 2000; Hodge et al., 2001; Lindblad et al., 2001), all components of the metabolic syndrome.

Hyperglycemia
In epidemiological studies employing factor analysis, fasting glucose and two-hour post-load glucose levels have also consistently associated with the factor explaining the greatest variance and having heavy loadings by measures of adiposity and fat distribution and insulin (Edwards et al., 1994; Meigs et al., 1997; Gray et al., 1998; Chen et al., 1999; Lempiäinen et al., 1999; Chen et al., 2000; Pyörälä et al., 2000; Snehalatha et al., 2000; Hodge et al., 2001; Lindblad et al., 2001). Both fasting and two-hour post-load glucose levels can therefore be considered a core component of the metabolic syndrome.

Type 1 and type 2 diabetes mellitus have a well-characterized 2-4-fold increased risk for CVD that is independent of known cardiovascular risk factors (Krolewski et al., 1987; Marks and Raskin, 2000; Laakso, 2001). IFG and IGT also predict cardiovascular mortality (Gabir et al., 2000a; Eschwege et al., 2001; Rajala et al., 2001). There is a graded increase in the cardiovascular risk of fasting and two-hour post-load glucose levels even in the normal range (Coutinho et al., 1999). Both IFG and IGT are strong predictors of future diabetes (Edelstein et al., 1997; Gabir et al., 2000b; de Vegt et al., 2001).

Overweight and an abdominal fat distribution
The most widely used measure of adiposity is the BMI (kg·m^-2), which is independent of height. Despite its crudeness, BMI provides a good index of overall adiposity at the population level (World Health Organization, 2000). Somewhat more accurate calculations of percent body fat may be obtained from skinfold measures and bioelectrical impedance, but these measures require sex- and age-dependent norms that may vary from population to population (Heymsfield et al., 1997; Ellis, 2000). The most accurate and widely used measurements of adiposity are currently obtained through underwater weighing and dual-energy X-ray absorptiometry (Heymsfield et al. 1997; Ellis 2000), although these methods are not practical for most epidemiological studies. The WHO and the National Institute of Health have defined overweight as BMI 25 kg·m^-2, and obesity as BMI30 kg·m^-2(National Institutes of Health. National Heart, 1998; World Health Organization, 2000).

An abdominal distribution of fat appears to be particularly deleterious (Larsson et al., 1984; Folsom et al., 1993; Rexrode et al., 1998; Folsom et al., 2000). Waist and the waist-hip ratio are the most common anthropometric measures of abdominal fat distribution. Waist girth and even BMI correlate better than the waist-hip ratio with CT or MRI measures of abdominal obesity (Seidell et al., 1987). It has been suggested that the use of waist circumference should be preferred over waist-hip ratio (National Institutes of Health. National Heart, 1998; World Health Organization, 2000), although the waist-hip ratio may offer additional information affecting health outcomes not related to abdominal fat distribution (Han et al., 1998). It should be noted, however, that as obesity increases, abdominal obesity also generally increases. Even BMI alone correlates nearly as well as waist circumference with abdominal fat as measured by computed tomography (Seidell et al., 1987). Cut-offs of 94 cm and 102 cm for waist circumference have been suggested as action levels for intervention in men. These cut-offs are based on a large cross-sectional population-based study in the Netherlands, in which those cut-offs corresponded to BMIs of25 and 30 kg·m^-2, and were associated with increased prevalence of chronic diseases and cardiovascular risk factors (Lean et al., 1998).

Visceral abdominal fat has been reported to be associated with insulin resistance independently of total body fat or subcutaneous abdominal fat (Despres et al., 1989; Brochu et al., 2000; Ross et al., 2000; DeNino et al., 2001; Cnop et al., 2002; Ross et al., 2002), but many other studies have found that subcutaneous abdominal adipose tissue is as strong or stronger correlate of insulin resistance (Abate et al., 1995; Goodpaster et al., 1997; Kelley et al., 2000; Sardinha et al., 2000; Smith et al., 2001). Subcutaneous fat tissue can be further divided into deep and superficial compartments, and visceral fat tissue can be divided into retroperitoneal and intraperitoneal compartments (Kelley et al., 2000; Smith et al., 2001; Janssen et al., 2002), although the pathophysiological significance of these subdivisions are unclear (Ross et al., 2002).

Adiposity and an abdominal fat distribution have also consistently loaded onto the factor explaining the greatest variance and having heavy loadings by measures of insulin and glucose metabolism in epidemiological studies employing factor analysis (Edwards et al., 1994; Meigs et al., 1997; Gray et al., 1998; Chen et al., 1999; Lempiäinen et al., 1999; Chen et al., 2000; Pyörälä et al., 2000; Snehalatha et al., 2000; Hodge et al., 2001; Lindblad et al., 2001). Although insulin resistance has been considered to be the underlying abnormality of the metabolic syndrome, overweight and obesity are clearly the main triggering factors (Liese et al., 1998).

An abdominal distribution of fat as measured by waist girth or waist-hip ratio has predicted cardiovascular endpoints even after adjustment for BMI (Larsson et al., 1984; Folsom et al., 1993; Rexrode et al., 1998; Folsom et al., 2000). Interestingly, the independent contribution of waist circumference or waist-hip ratio over BMI to the development of diabetes is not as clear (Ohlson et al., 1985; Chan et al., 1994; Wei et al., 1997).

Dyslipidemia
Low fasting serum HDL cholesterol levels and hypertriglyceridemia are consistently associated with the other components of the metabolic syndrome (Reaven, 1988; Kaplan, 1989; DeFronzo and Ferrannini, 1991; Mykkanen et al., 1994a; Kaplan, 1996; Mykkanen et al., 1997; Liese et al., 1998). Other lipid subfractions such as apolipoprotein A1 and B levels, small dense LDL cholestrerol and HDL subfractions are associated with the metabolic syndrome as well (Mykkanen et al., 1994a; Mykkanen et al., 1997; Liese et al., 1998).

Dyslipidemia has predicted the incidence of type 2 diabetes mellitus in several studies (Ohlson et al., 1988; Haffner et al., 1990; McPhillips et al., 1990; Perry et al., 1995). Low HDL cholesterol levels are a well-established risk factor for CVD (Boden, 2000). The independent role of triglycerides as a cardiovascular risk factor is more controversial, although a meta-analysis suggests that triglycerides are an independent risk factor (Hokanson and Austin, 1996). The Veterans Affairs High-Density Lipoprotein Cholesterol Intervention Trial showed a decrease in cardiovascular events in men with low HDL cholesterol levels but normal LDL cholesterol levels who treated with gemfibrozil. Because gemfibrozil is an HDL-elevating and triglyceride-lowering drug, this study offers additional support for the importance of triglyceride and HDL levels as cardiovascular risk factors (Rubins et al., 1999).

Blood pressure
Hyperinsulinemia was associated with the incidence of hypertension and dyslipidemia in the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD) cohort of middle-aged men (Salonen et al., 1998). Obesity and abdominal fat distribution also have a well-described association with hypertension (Cassano et al., 1990; Jousilahti et al., 1995; Curhan et al., 1996; Haffner et al., 1996; Kannel, 1996; Srinivasan et al., 1996; Harris et al., 2000; Juhaeri et al., 2002).

Hypertension is a classic cardiovascular risk factor, as has been demonstrated by both longitudinal cohort studies and blood pressure medication trials (Psaty et al., 1997; Kannel, 2000). The magnitude of the decrease in coronary morbidity and mortality is less than what would be predicted by epidemiological studies, however. This has been speculated to be due in part to adverse effects of (high-dose) diuretics and (non-selective) beta-blockers on insulin resistance, dyslipidemia and other factors related to the metabolic syndrome, or alternatively, that only part of the mortality associated with hypertension is due to blood pressure itself (Thompson, 1990b; Black, 1996; Brook, 2000; Reyes, 2002). Hypertension is also an independent risk factor for type 2 diabetes (Ohlson et al., 1988; Haffner et al., 1990; Mykkanen et al., 1994b; Perry et al., 1995).

Physical activity and cardiorespiratory fitness
In intervention studies in non-diabetic persons, aerobic physical exercise has in variable degrees and at least in the short term decreased weight and visceral fat accumulation (Ivy, 1997; Rice et al., 1999; Ross et al., 2000), improved insulin sensitivity (Ivy, 1997; Rice et al., 1999; Ross et al., 2000) increased HDL cholesterol and decreased triglyceride levels (Tran et al., 1983; Haskell, 1984), and decreased blood pressure (Arroll and Beaglehole, 1992) in addition to increasing cardiorespiratory fitness. These changes have often occurred independently of weight loss, although it is not completely clear how much of these favorable effects are independent of weight loss and changes in body composition.

The mechanisms by which exercise may increase insulin sensitivity independently of weight loss are only partly understood. Exercise appears to acutely increase glucose uptake in part through the mechanistic action of contraction, perhaps partially mediated by increased translocation of glucose transporter