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The
protein requirements for athletic populations have been the subject
of much scientific debate. Only recently has the notion that both
strength/power and endurance athletes require a greater protein
consumption than the general population become generally accepted.
In addition, high protein diets have also become quite popular in
the general population as part of many weight reduction programs.
Despite the prevalence of high protein diets in athletic and sedentary
populations, information available concerning the type of protein
(e.g. animal or vegetable) to consume is limited. The purpose of
this paper is to examine and analyze key factors responsible for
making appropriate choices on the type of protein to consume in
both athletic and general populations.
Role of Protein
Proteins are nitrogen-containing substances that are formed by amino
acids. They serve as the major structural component of muscle and
other tissues in the body. In addition, they are used to produce
hormones, enzymes and hemoglobin. Proteins can also be used as energy;
however, they are not the primary choice as an energy source. For
proteins to be used by the body they need to be metabolized into
their simplest form, amino acids. There have been 20 amino acids
identified that are needed for human growth and metabolism. Twelve
of these amino acids (eleven in children) are termed nonessential,
meaning that they can be synthesized by our body and do not need
to be consumed in the diet. The remaining amino acids cannot be
synthesized in the body and are described as essential meaning that
they need to be consumed in our diets. The absence of any of these
amino acids will compromise the ability of tissue to grow, be repaired
or be maintained.
Protein and Athletic Performance
The primary role of dietary proteins is for use in the various anabolic
processes of the body. As a result, many athletes and coaches are
under the belief that high intensity training creates a greater
protein requirement. This stems from the notion that if more protein
or amino acids were available to the exercising muscle it would
enhance protein synthesis. Research has tended to support this hypothesis.
Within four weeks of protein supplementation (3.3 versus 1.3 g·kg-1·day-1)
in subjects' resistance training, significantly greater gains were
seen in protein synthesis and body mass in the group of subjects
with the greater protein intake (Fern et al., 1991).
Similarly, Lemon et al. (1992)
also reported a greater protein synthesis in novice resistance trained
individuals with protein intakes of 2.62 versus 0.99 g·kg-1·day-1.
In studies examining strength-trained individuals, higher protein
intakes have generally been shown to have a positive effect on muscle
protein synthesis and size gains (Lemon, 1995;
Walberg et al., 1988).
Tarnapolsky and colleagues (1992)
have shown that for strength trained individuals to maintain a positive
nitrogen balance they need to consume a protein intake equivalent
to 1.8 g·kg-1·day-1.
This is consistent with other studies showing that protein intakes
between 1.4 - 2.4 g·kg-1·day-1
will maintain a positive nitrogen balance in resistance trained
athletes (Lemon, 1995).
As a result, recommendations for strength/power athletes' protein
intake are generally suggested to be between 1.4 - 1.8 g·kg-1·day-1.
Similarly, to prevent significant losses in lean tissue endurance
athletes also appear to require a greater protein consumption (Lemon,
1995). Although
the goal for endurance athletes is not necessarily to maximize muscle
size and strength, loss of lean tissue can have a significant detrimental
effect on endurance performance. Therefore, these athletes need
to maintain muscle mass to ensure adequate performance. Several
studies have determined that protein intake for endurance athletes
should be between 1.2 - 1.4 g·kg-1·day-1
to ensure a positive nitrogen balance (Freidman and Lemon, 1989;
Lemon, 1995;
Meredith et al., 1989;
Tarnopolsky et al., 1988).
Evidence is clear that athletes do benefit from increased protein
intake. The focus then becomes on what type of protein to take.
Protein Assessment
The composition of various proteins may be so unique that their
influence on physiological function in the human body could be quite
different. The quality of a protein is vital when considering the
nutritional benefits that it can provide. Determining the quality
of a protein is determined by assessing its essential amino acid
composition, digestibility and bioavailability of amino acids (FAO/WHO,
1990). There
are several measurement scales and techniques that are used to evaluate
the quality of protein.
Protein Rating Scales
Numerous methods exist to determine protein quality. These methods
have been identified as protein efficiency ratio, biological value,
net protein utilization, and protein digestibility corrected amino
acid score.
Protein Efficiency Ratio
The protein efficiency ratio (PER) determines the effectiveness
of a protein through the measurement of animal growth. This technique
requires feeding rats a test protein and then measuring the weight
gain in grams per gram of protein consumed. The computed value is
then compared to a standard value of 2.7, which is the standard
value of casein protein. Any value that exceeds 2.7 is considered
to be an excellent protein source. However, this calculation provides
a measure of growth in rats and does not provide a strong correlation
to the growth needs of humans.
Biological Value
Biological value measures protein quality by calculating the nitrogen
used for tissue formation divided by the nitrogen absorbed from
food. This product is multiplied by 100 and expressed as a percentage
of nitrogen utilized. The biological value provides a measurement
of how efficient the body utilizes protein consumed in the diet.
A food with a high value correlates to a high supply of the essential
amino acids. Animal sources typically possess a higher biological
value than vegetable sources due to the vegetable source's lack
of one or more of the essential amino acids. There are, however,
some inherent problems with this rating system. The biological value
does not take into consideration several key factors that influence
the digestion of protein and interaction with other foods before
absorption. The biological value also measures a protein's maximal
potential quality and not its estimate at requirement levels.
Net Protein Utilization
Net protein utilization is similar to the biological value except
that it involves a direct measure of retention of absorbed nitrogen.
Net protein utilization and biological value both measure the same
parameter of nitrogen retention, however, the difference lies in
that the biological value is calculated from nitrogen absorbed whereas
net protein utilization is from nitrogen ingested.
Protein Digestibility Corrected Amino Acid Score
In 1989, the Food & Agriculture Organization and World Health
Organization (FAO/WHO) in a joint position stand stated that protein
quality could be determined by expressing the content of the first
limiting essential amino acid of the test protein as a percentage
of the content of the same amino acid content in a reference pattern
of essential amino acids (FAO/WHO, 1990).
The reference values used were based upon the essential amino acids
requirements of preschool-age children. The recommendation of the
joint FAO/WHO statement was to take this reference value and correct
it for true fecal digestibility of the test protein. The value obtained
was referred to as the protein digestibility corrected amino acid
score (PDCAAS). This method has been adopted as the preferred method
for measurement of the protein value in human nutrition (Schaafsma,
2000). Table
1 provides a measure of the quantity of various proteins using
these protein rating scales.
Although the PDCAAS is currently the most accepted and widely used
method, limitations still exist relating to overestimation in the
elderly (likely related to references values based on young individuals),
influence of ileal digestibility, and antinutritional factors (Sarwar,
1997).
Amino acids that move past the terminal ileum may be an important
route for bacterial consumption of amino acids, and any amino acids
that reach the colon would not likely be utilized for protein synthesis,
even though they do not appear in the feces (Schaarfsma, 2000). Thus, to get truly valid
measure of fecal digestibility the location at which protein synthesis
is determined is important in making a more accurate determination.
Thus, ileal digestibility would provide a more accurate measure
of digestibility. PDCAAS, however, does not factor ileal digestibility
into its equation. This is considered to be one of the shortcomings
of the PDCAAS (Schaafsma 2000).
Antinutritional factors such as trypsin inhibitors, lectins, and
tannins present in certain protein sources such as soybean meal,
peas and fava beans have been reported to increase losses of endogenous
proteins at the terminal ileum (Salgado et al., 2002).
These antinutritional factors may cause reduced protein hydrolysis
and amino acid absorption. This may also be more effected by age,
as the ability of the gut to adapt to dietary nutritional insults
may be reduced as part of the aging process (Sarwar, 1997).
Protein Sources
Protein is available in a variety of dietary sources. These include
foods of animal and plant origins as well as the highly marketed
sport supplement industry. In the following section proteins from
both vegetable and animal sources, including whey, casein, and soy
will be explored. Determining the effectiveness of a protein is
accomplished by determining its quality and digestibility. Quality
refers to the availability of amino acids that it supplies, and
digestibility considers how the protein is best utilized. Typically,
all dietary animal protein sources are considered to be complete
proteins. That is, a protein that contains all of the essential
amino acids. Proteins from vegetable sources are incomplete in that
they are generally lacking one or two essential amino acids. Thus,
someone who desires to get their protein from vegetable sources
(i.e. vegetarian) will need to consume a variety of vegetables,
fruits, grains, and legumes to ensure consumption of all essential
amino acids. As such, individuals are able to achieve necessary
protein requirements without consuming beef, poultry, or dairy.
Protein digestibility ratings usually involve measuring how the
body can efficiently utilize dietary sources of protein. Typically,
vegetable protein sources do not score as high in ratings of biological
value, net protein utilization, PDCAAS, and protein efficiency ratio
as animal proteins.
Animal Protein
Proteins from animal sources (i.e. eggs, milk, meat, fish and poultry)
provide the highest quality rating of food sources. This is primarily
due to the 'completeness' of proteins from these sources. Although
protein from these sources are also associated with high intakes
of saturated fats and cholesterol, there have been a number of studies
that have demonstrated positive benefits of animal proteins in various
population groups (Campbell et al., 1999;
Godfrey et al., 1996;
Pannemans et al., 1998).
Protein from animal sources during late pregnancy is believed to
have an important role in infants born with normal body weights.
Godfrey et al. (1996)
examined the nutrition behavior of more than 500 pregnant women
to determine the effect of nutritional intake on placental and fetal
growth. They reported that a low intake of protein from dairy and
meat sources during late pregnancy was associated with low birth
weights.
In addition to the benefits from total protein consumption, elderly
subjects have also benefited from consuming animal sources of protein.
Diets consisting of meat resulted in greater gains in lean body
mass compared to subjects on a lactoovovegetarian diet (Campbell
et al., 1999).
High animal protein diets have also been shown to cause a significantly
greater net protein synthesis than a high vegetable protein diet
(Pannemans et al., 1998).
This was suggested to be a function of reduced protein breakdown
occurring during the high animal protein diet.
There have been a number of health concerns raised concerning the
risks associated with protein emanating primarily from animal sources.
Primarily, these health risks have focused on cardiovascular disease
(due to the high saturated fat and cholesterol consumption), bone
health (from bone resorption due to sulfur-containing amino acids
associated with animal protein) and other physiological system disease
that will be addressed in the section on high protein diets.
Whey
Whey is a general term that typically denotes the translucent liquid
part of milk that remains following the process (coagulation and
curd removal) of cheese manufacturing. From this liquid, whey proteins
are separated and purified using various techniques yielding different
concentrations of whey proteins. Whey is one of the two major protein
groups of bovine milk, accounting for 20% of the milk while casein
accounts for the remainder. All of the constituents of whey protein
provide high levels of the essential and branched chain amino acids.
The bioactivities of these proteins possess many beneficial properties
as well. Additionally, whey is also rich in vitamins and minerals.
Whey protein is most recognized for its applicability in sports
nutrition. Additionally, whey products are also evident in baked
goods, salad dressings, emulsifiers, infant formulas, and medical
nutritional formulas.
Varieties of Whey Protein
There are three main forms of whey protein that result from various
processing techniques used to separate whey protein. They are whey
powder, whey concentrate, and whey isolate. Table
2 provides the composition of Whey Proteins.
Whey Protein Powder
Whey protein powder has many applications throughout the food industry.
As an additive it is seen in food products for beef, dairy, bakery,
confectionery, and snack products. Whey powder itself has several
different varieties including sweet whey, acid whey (seen in salad
dressings), demineralized (seen primarily as a food additive including
infant formulas), and reduced forms. The demineralized and reduced
forms are used in products other than sports supplements.
Whey
Protein Concentrate
The processing of whey concentrate removes the water, lactose, ash,
and some minerals. In addition, compared to whey isolates whey concentrate
typically contains more biologically active components and proteins
that make them a very attractive supplement for the athlete.
Whey Protein Isolate (WPI)
Isolates are the purest protein source available. Whey protein isolates
contain protein concentrations of 90% or higher. During the processing
of whey protein isolate there is a significant removal of fat and
lactose. As a result, individuals who are lactose-intolerant can
often safely take these products (Geiser, 2003).
Although the concentration of protein in this form of whey protein
is the highest, it often contain proteins that have become denatured
due to the manufacturing process. The denaturation of proteins involves
breaking down their structure and losing peptide bonds and reducing
the effectiveness of the protein.
Whey is a complete protein whose biologically active components
provide additional benefits to enhance human function. Whey protein
contains an ample supply of the amino acid cysteine. Cysteine appears
to enhance glutathione levels, which has been shown to have strong
antioxidant properties that can assist the body in combating various
diseases (Counous, 2000).
In addition, whey protein contains a number of other proteins that
positively effect immune function such as antimicrobial activity
(Ha and Zemel, 2003).
Whey protein also contains a high concentration of branched chain
amino acids (BCAA) that are important for their role in the maintenance
of tissue and prevention of catabolic actions during exercise. (MacLean
et al., 1994).
Casein
Casein is the major component of protein found in bovine milk accounting
for nearly 70-80% of its total protein and is responsible for the
white color of milk. It is the most commonly used milk protein in
the industry today. Milk proteins are of significant physiological
importance to the body for functions relating to the uptake of nutrients
and vitamins and they are a source of biologically active peptides.
Similar to whey, casein is a complete protein and also contains
the minerals calcium and phosphorous. Casein has a PDCAAS rating
of 1.23 (generally reported as a truncated value of 1.0) (Deutz
et al. 1998).
Casein exists in milk in the form of a micelle, which is a large
colloidal particle. An attractive property of the casein micelle
is its ability to form a gel or clot in the stomach. The ability
to form this clot makes it very efficient in nutrient supply. The
clot is able to provide a sustained slow release of amino acids
into the blood stream, sometimes lasting for several hours (Boirie
et al. 1997).
This provides better nitrogen retention and utilization by the body.
Bovine Colostrum
Bovine colostrum is the "pre" milk liquid secreted by
female mammals the first few days following birth. This nutrient-dense
fluid is important for the newborn for its ability to provide immunities
and assist in the growth of developing tissues in the initial stages
of life. Evidence exists that bovine colostrum contains growth factors
that stimulate cellular growth and DNA synthesis (Kishikawa et al.,
1996), and as
might be expected with such properties, it makes for interesting
choice as a potential sports supplement.
Although bovine colostrum is not typically thought of as a food
supplement, the use by strength/power athletes of this protein supplement
as an ergogenic aid has become common. Oral supplementation of bovine
colostrum has been demonstrated to significantly elevate insulin-like-growth
factor 1 (IGF-1) (Mero et al., 1997)
and enhance lean tissue accruement (Antonio et al., 2001;
Brinkworth et al., 2004).
However, the results on athletic performance improvement are less
conclusive. Mero and colleagues (1997)
reported no changes in vertical jump performance following 2-weeks
of supplementation, and Brinkworth and colleagues (2004)
saw no significant differences in strength following 8-weeks of
training and supplementation in both trained and untrained subjects.
In contrast, following 8-weeks of supplementation significant improvements
in sprint performance were seen in elite hockey players (Hofman
et al., 2002).
Further research concerning bovine colostrum supplementation is
still warranted.
Vegetable
Protein
Vegetable proteins, when combined to provide for all of the essential
amino acids, provide an excellent source for protein considering
that they will likely result in a reduction in the intake of saturated
fat and cholesterol. Popular sources include legumes, nuts and soy.
Aside from these products, vegetable protein can also be found in
a fibrous form called textured vegetable protein (TVP). TVP is produced
from soy flour in which proteins are isolated. TVP is mainly a meat
alternative and functions as a meat analog in vegetarian hot dogs,
hamburgers, chicken patties, etc. It is also a low-calorie and low-fat
source of vegetable protein. Vegetable sources of protein also provide
numerous other nutrients such as phytochemicals and fiber that are
also highly regarded in the diet diet.
Soy
Soy is the most widely used vegetable protein source. The soybean,
from the legume family, was first chronicled in China in the year
2838 B.C. and was considered to be as valuable as wheat, barley,
and rice as a nutritional staple. Soy's popularity spanned several
other countries, but did not gain notoriety for its nutritional
value in The United States until the 1920s. The American population
consumes a relatively low intake of soy protein (5g·day-1)
compared to Asian countries (Hasler, 2002).
Although cultural differences may be partly responsible, the low
protein quality rating from the PER scale may also have influenced
protein consumption tendencies. However, when the more accurate
PDCAAS scale is used, soy protein was reported to be equivalent
to animal protein with a score of 1.0, the highest possible rating
(Hasler, 2002).
Soy's quality makes it a very attractive alternative for those seeking
non-animal sources of protein in their diet and those who are lactose
intolerant. Soy is a complete protein with a high concentration
of BCAA's. There have been many reported benefits related to soy
proteins relating to health and performance (including reducing
plasma lipid profiles, increasing LDL-cholesterol oxidation and
reducing blood pressure), however further research still needs to
be performed on these claims.
Soy
Protein Types
The soybean can be separated into three distinct categories; flour,
concentrates, and isolates. Soy flour can be further divided into
natural or full-fat (contains natural oils), defatted (oils removed),
and lecithinated (lecithin added) forms (Hasler, 2002).
Of the three different categories of soy protein products, soy flour
is the least refined form. It is commonly found in baked goods.
Another product of soy flour is called textured soy flour.
This is primarily used for processing as a meat extender. See Table
3 for protein composition of soy flour, concentrates, and isolates.
Soy concentrate was developed in the late 1960s and early 1970s
and is made from defatted soybeans. While retaining most of the
bean's protein content, concentrates do not contain as much soluble
carbohydrates as flour, making it more palatable. Soy concentrate
has a high digestibility and is found in nutrition bars, cereals,
and yogurts.
Isolates are the most refined soy protein product containing the
greatest concentration of protein, but unlike flour and concentrates,
contain no dietary fiber. Isolates originated around the 1950s in
The United States. They are very digestible and easily introduced
into foods such as sports drinks and health beverages as well as
infant formulas.
Nutritional Benefits
For centuries, soy has been part of a human diet. Epidemiologists
were most likely the first to recognize soy's benefits to overall
health when considering populations with a high intake of soy. These
populations shared lower incidences in certain cancers, decreased
cardiac conditions, and improvements in menopausal symptoms and
osteoporosis in women (Hasler, 2002).
Based upon a multitude of studies examining the health benefits
of soy protein the American Heart Association issued a statement
that recommended soy protein foods in a diet low in saturated fat
and cholesterol to promote heart health (Erdman, 2000).
The health benefits associated with soy protein are related to the
physiologically active components that are part of soy, such as
protease inhibitors, phytosterols, saponins, and isoflavones (Potter,
2000). These
components have been noted to demonstrate lipid-lowering effects,
increase LDL-cholesterol oxidation, and have beneficial effects
on lowering blood pressure.
Isoflavones
Of the many active components in soy products, isoflavones have
been given considerably more attention than others. Isoflavones
are thought to be beneficial for cardiovascular health, possibly
by lowering LDL concentrations (Crouse et al., 1999)
increasing LDL oxidation (Tikkanen et al., 1998)
and improving vessel elasticity (Nestel et al., 1999).
However, these studies have not met without conflicting results
and further research is still warranted concerning the benefits
of isoflavones.
Soy Benefits for Women
An additional focus of studies investigating soy supplementation
has been on women's health issues. It has been hypothesized that
considering that isoflavones are considered phytoestrogens (exhibit
estrogen- like effects and bind to estrogen receptors) they compete
for estrogen receptor sites in breast tissue with endogenous estrogen,
potentially reducing the risk for breast cancer risk (Wu et al.
1998). Still,
the association between soy intake and breast cancer risk remains
inconclusive. However, other studies have demonstrated positive
effects of soy protein supplementation on maintaining bone mineral
content (Ho et al., 2003)
and reducing the severity of menopausal symptoms (Murkies et al.,
1995).
High Protein Diets
Increased protein intakes and supplementation have generally been
focused on athletic populations. However, over the past few years
high protein diets have become a method used by the general population
to enhance weight reduction. The low-carbohydrate, high protein,
high fat diet promoted by Atkins may be the most popular diet used
today for weight loss in the United States (Johnston et al., 2004).
The basis behind this diet is that protein is associated with feelings
of satiety and voluntary reductions in caloric consumption (Araya
et al., 2000;
Eisenstein et al., 2002).
A recent study has shown that the Atkins diet can produce greater
weight reduction at 3 and 6 months than a low-fat, high carbohydrate
diet based upon U.S. dietary guidelines (Foster et al., 2003).
However, potential health concerns have arisen concerning the safety
of high protein diets. In 2001, the American Heart Association published
a statement on dietary protein and weight reduction and suggested
that individuals following such a diet may be at potential risk
for metabolic, cardiac, renal, bone and liver diseases (St. Jeor
et al., 2001).
Protein
Intake and Metabolic Disease Risk
One of the major concerns for individuals on high protein, low carbohydrate
diets is the potential for the development of metabolic ketosis.
As carbohydrate stores are reduced the body relies more upon fat
as its primary energy source. The greater amount of free fatty acids
that are utilized by the liver for energy will result in a greater
production and release of ketone bodies in the circulation. This
will increase the risk for metabolic acidosis and can potentially
lead to a coma and death. A recent multi-site clinical study (Foster
et al., 2003)
examined the effects of low-carbohydrate, high protein diets and
reported significant elevation in ketone bodies during the first
three months of the study. However, as the study duration continued
the percentage of subjects with positive urinary ketone concentrations
became reduced, and by six months urinary ketones were not present
in any of the subjects.
Dietary
Protein and Cardiovascular Disease Risk
High protein diets have also been suggested to have negative effects
on blood lipid profiles and blood pressure, causing an increase
risk for cardiovascular disease. This is primarily due to the higher
fat intakes associated with these diets. However, this has not been
proven in any scientifically controlled studies. Hu et al., (1999)
have reported an inverse relationship between dietary protein (animal
and vegetable) and risk of cardiovascular disease in women, and
Jenkins and colleagues (2001)
reported a decrease in lipid profiles in individuals consuming a
high protein diet. Furthermore, protein intake has been shown to
often have a negative relationship with blood pressure (Obarzanek
et al., 1996).
Thus, the concern for elevated risk for cardiovascular disease from
high protein diets appears to be without merit. Likely, the reduced
body weight associated with this type of diet is facilitating these
changes.
In strength/power athletes who consume high protein diets, a major
concern was the amount of food being consumed that was high in saturated
fats. However, through better awareness and nutritional education
many of these athletes are able to obtain their protein from sources
that minimizes the amount of fat consumed. For instance, removing
the skin from chicken breast, consuming fish and lean beef, and
egg whites. In addition, many protein supplements are available
that contain little to no fat. It should be acknowledged though
that if elevated protein does come primarily from meats, dairy products
and eggs, without regard to fat intake, there likely would be an
increase in the consumption of saturated fat and cholesterol.
Dietary Protein and Renal Function
The major concern associated with renal function was the role that
the kidneys have in nitrogen excretion and the potential for a high
protein diet to over-stress the kidneys. In healthy individuals
there does not appear to be any adverse effects of a high protein
diet. In a study on bodybuilders consuming a high protein (2.8 g·kg-1)
diet no negative changes were seen in any kidney function tests
(Poortsman and Dellalieux, 2000).
However, in individuals with existing kidney disease it is recommended
that they limit their protein intake to approximately half of the
normal RDA level for daily protein intake (0.8 g·kg-1·day-1).
Lowering protein intake is thought to reduce the progression of
renal disease by decreasing hyperfiltration (Brenner et al., 1996).
Dietary Protein and Bone
High protein diets are associated with an increase in calcium excretion.
This is apparently due to a consumption of animal protein, which
is higher in sulfur-based amino acids than vegetable proteins (Remer
and Manz, 1994;
Barzel and Massey, 1998).
Sulfur-based amino acids are thought to be the primary cause of
calciuria (calcium loss). The mechanism behind this is likely related
to the increase in acid secretion due to the elevated protein consumption.
If the kidneys are unable to buffer the high endogenous acid levels,
other physiological systems will need to compensate, such as bone.
Bone acts as a reservoir of alkali, and as a result calcium is liberated
from bone to buffer high acidic levels and restore acid-base balance.
The calcium released by bone is accomplished through osteoclast-mediated
bone resorption (Arnett and Spowage, 1996).
Bone resorption (loss or removal of bone) will cause a decline in
bone mineral content and bone mass (Barzel, 1976),
increasing the risk for bone fracture and osteoporosis.
The effect of the type of protein consumed on bone resorption has
been examined in a number of studies. Sellmeyer and colleagues (2001)
examined the effects of various animal-to- vegetable protein ratio
intakes in elderly women (> 65 y). They showed that the women
consuming the highest animal to vegetable protein ratio had nearly
a 4-fold greater risk of hip fractures compared with women consuming
a lower animal to vegetable protein ratio. Interestingly, they did
not report any significant association between the animal to vegetable
protein ratio and bone mineral density. Similar results were shown
by Feskanich et al (1996),
but in a younger female population (age range = 35 - 59 mean 46).
In contrast, other studies examining older female populations have
shown that elevated animal protein will increase bone mineral density,
while increases in vegetable protein will have a lowering effect
on bone mineral density (Munger et al., 1999;
Promislow et al., 2002).
Munger and colleagues (1999)
also reported a 69% lower risk of hip fracture as animal protein
intake increased in a large (32,000) postmenopausal population.
Other large epidemiological studies have also confirmed elevated
bone density following high protein diets in both elderly men and
women (Dawson-Hughes et al., 2002;
Hannan et al., 2000).
Hannon and colleagues (2000)
demonstrated that animal protein intake in an older population,
several times greater than the RDA requirement, results in a bone
density accruement and significant decrease in fracture risk. Dawson-Hughes
et al (2002),
not only showed that animal protein will not increase urinary calcium
excretion, but was also associated with higher levels of IGF-I and
lower concentrations of the bone resorption marker N-telopeptide.
These conflicting results have contributed to the confusion regarding
protein intake and bone. It is likely that other factors play an
important role in further understanding the influence that dietary
proteins have on bone loss or gain. For instance, the intake of
calcium may have an essential function in maintaining bone. A higher
calcium intake results in more absorbed calcium and may offset the
losses induced by dietary protein and reduce the adverse effect
of the endogenous acidosis on bone resorption (Dawson-Hughes, 2003).
Furthermore, it is commonly assumed that animal proteins have a
higher content of sulfur-containing amino acids per g of protein.
However, examination of Table 4
shows that this may not entirely correct. If protein came from wheat
sources it would have a mEq of 0.69 per g of protein, while protein
from milk contains 0.55 mEq per g of protein. Thus, some plant proteins
may have a greater potential to produce more mEq of sulfuric acid
per g of protein than some animal proteins (Massey, 2003).
Finally, bone resorption may be related to the presence or absence
of a vitamin D receptor allele. In subjects that had this specific
allele a significant elevation in bone resorption markers were present
in the urine following 4-weeks of protein supplementation, while
in subjects without this specific allele had no increase in N-telopeptide
(Harrington et al., 2004).
The effect of protein on bone health is still unclear, but it does
appear to be prudent to monitor the amount of animal protein in
the diet for susceptible individuals. This may be more pronounced
in individuals that may have a genetic endowment for this. However,
if animal protein consumption is modified by other nutrients (e.g.
calcium) the effects on bone health may be lessened.
Protein Intake and Liver Disease Risk
The American Heart Association has suggested that high protein diets
may have detrimental effects on liver function (St. Jeor et al.,
2001). This
is primarily the result of a concern that the liver will be stressed
through metabolizing the greater protein intakes. However, there
is no scientific evidence to support this contention. Jorda and
colleagues (1988)
did show that high protein intakes in rats produce morphological
changes in liver mitochondria. However, they also suggested that
these changes were not pathological, but represented a positive
hepatocyte adaptation to a metabolic stress.
Protein is important for the liver not only in promoting tissue
repair, but to provide lipotropic agents such as methionine and
choline for the conversion of fats to lipoprotein for removal form
the liver (Navder and Leiber, 2003a).
The importance of high protein diets has also been acknowledged
for individuals with liver disease and who are alcoholics. High
protein diets may offset the elevated protein catabolism seen with
liver disease (Navder and Leiber, 2003b),
while a high protein diet has been shown to improve hepatic function
in individuals suffering from alcoholic liver disease (Mendellhall
et al., 1993).
Comparisons between Different Protein Sources
on Human Performance
Earlier discussions on protein supplementation and athletic performance
have shown positive effects from proteins of various sources. However,
only limited research is available on comparisons between various
protein sources and changes in human performance. Recently, there
have been a number of comparisons between bovine colostrum and whey
protein. The primary reason for this comparison is the use by these
investigators of whey protein as the placebo group in many of the
studies examining bovine colostrum (Antonio et al., 2001;
Brinkworth et al., 2004;
Brinkworth and Buckley, 2002;
Coombes et al., 2002;
Hofman et al., 2002).
The reason being that whey protein is similar in taste and texture
as bovine colostrum protein.
Studies performed in non-elite athletes have been inconclusive concerning
the benefits of bovine colostrum compared to whey protein. Several
studies have demonstrated greater gains in lean body mass in individuals
supplementing with bovine colostrum than whey, but no changes in
endurance or strength performance (Antonio et al., 2001;
Brinkworth et al., 2004).
However, when performance was measured following prolonged exercise
(time to complete 2.8 kJ·kg-1
of work following a 2-hour ride) supplement dosages of 20 g·day-1
and 60 g·day-1
were shown to significantly improve time trial performance in competitive
cyclists (Coombes et al., 2002).
These results may be related to an improved buffering capacity following
colostrum supplementation. Brinkworth and colleagues (2002)
reported that although no performance changes were seen in rowing
performance, the elite rowers that were studied did demonstrate
an improved buffering capacity following 9-weeks of supplementation
with 60 g·day-1
of bovine colostrum when compared to supplementing with whey protein.
The improved buffering capacity subsequent to colostrum supplementation
may have also influenced the results reported by Hofman et al.,
(2002). In that
study elite field hockey players supplemented with either 60 g·day-1
of either colostrum or whey protein for 8-weeks. A significantly
greater improvement was seen in repeated sprint performance in the
group supplementing with colostrum compared to the group supplementing
with whey protein. However, a recent study has suggested that the
improved buffering system seen following colostrum supplementation
is not related to an improved plasma buffering system, and that
any improved buffering capacity occurs within the tissue (Brinkworth
et al., 2004).
In a comparison between casein and whey protein supplementation,
Boirie and colleagues (1997)
showed that a 30-g feeding of casein versus whey had significantly
different effects on postprandial protein gain. They showed that
following whey protein ingestion the plasma appearance of amino
acids is fast, high and transient. In contrast, casein is absorbed
more slowly producing a much less dramatic rise in plasma amino
acid concentrations. Whey protein ingestion stimulated protein synthesis
by 68%, while casein ingestion stimulated protein synthesis by 31%.
When the investigators compared postprandial leucine balance after
7-hours post ingestion, casein consumption resulted in a significantly
higher leucine balance, whereas no change from baseline was seen
7-hours following whey consumption. These results suggest that whey
protein stimulates a rapid synthesis of protein, but a large part
of this protein is oxidized (used as fuel), while casein may result
in a greater protein accretion over a longer duration of time. A
subsequent study showed that repeated ingestions of whey protein
(an equal amount of protein but consumed over a prolonged period
of time [4 hours] compared to a single ingestion) produced a greater
net leucine oxidation than either a single meal of casein or whey
(Dangin et al., 2001).
Interestingly, both casein and whey are complete proteins but their
amino acid composition is different. Glutamine and leucine have
important roles in muscle protein metabolism, yet casein contains
11.6 and 8.9 g of these amino acids, respectively while whey contains
21.9 and 11.1 g of these amino acids, respectively. Thus, the digestion
rate of the protein may be more important than the amino acid composition
of the protein.
In a study examining the effects of casein and whey on body composition
and strength measures, 12 weeks of supplementation on overweight
police officers showed significantly greater strength and lean tissue
accruement in the subjects ingesting casein compared to whey (Demling
and DeSanti, 2000).
Protein supplementation provided a relative protein consumption
of 1.5 g·kg·day-1.
Subjects supplemented twice per day approximately 8-10 hours apart.
Only one study known has compared colostrum, whey and casein supplementation
(Fry et al., 2003).
Following 12-weeks of supplementation the authors reported no significant
differences in lean body mass, strength or power performances between
the groups. However, the results of this study should be examined
with care. The subjects were comprised of both males and females
who were resistance training for recreational purposes. In addition,
the subject number for each group ranged from 4-6 subjects per group.
With a heterogeneous subject population and a low subject number,
the statistical power of this study was quite low. However, the
authors did analyze effect sizes to account for the low statistical
power. This analysis though did not change any of the observations.
Clearly, further research is needed in comparisons of various types
of protein on performance improvements. However, it is likely that
a combination of different proteins from various sources may provide
optimal benefits for performance.
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