©Journal of Sports Science and Medicine (2004) 3, 118-130
http://www.jssm.org
Review article
International Society of Sports Nutrition Symposium, June 18-19, 2005, Las Vegas NV,
USA - Symposium - Macronutrient Utilization During Exercise: Implications For
Performance And Supplementation
PROTEIN – WHICH IS BEST?
Jay R. Hoffman
and Michael J. Falvo
The Department of Health and Exercise Science, The College of New Jersey, Ewing, New Jersey, USA
Received: 26 May 2004 / Accepted: 28 June 2004 / Published (online): 01 September 2004
ABSTRACT
Protein intake that exceeds the recommended daily allowance is widely accepted for both endurance and
power athletes. However, considering the variety of proteins that are available much less is known
concerning the benefits of consuming one protein versus another. The purpose of this paper is to identify
and analyze key factors in order to make responsible recommendations to both the general and athletic
populations. Evaluation of a protein is fundamental in determining its appropriateness in the human diet.
Proteins that are of inferior content and digestibility are important to recognize and restrict or limit in the
diet. Similarly, such knowledge will provide an ability to identify proteins that provide the greatest
benefit and should be consumed. The various techniques utilized to rate protein will be discussed.
Traditionally, sources of dietary protein are seen as either being of animal or vegetable origin. Animal
sources provide a complete source of protein (i.e. containing all essential amino acids), whereas
vegetable sources generally lack one or more of the essential amino acids. Animal sources of dietary
protein, despite providing a complete protein and numerous vitamins and minerals, have some health
professionals concerned about the amount of saturated fat common in these foods compared to vegetable
sources. The advent of processing techniques has shifted some of this attention and ignited the sports
supplement marketplace with derivative products such as whey, casein and soy. Individually, these
products vary in quality and applicability to certain populations. The benefits that these particular
proteins possess are discussed. In addition, the impact that elevated protein consumption has on health
and safety issues (i.e. bone health, renal function) are also reviewed.
KEY WORDS: Sport supplementation, ergogenic aid, animal protein, vegetable protein.
INTRODUCTION
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
Protein ratings
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,
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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
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Hoffman and Falvo
Table 1. Protein quality rankings.
Protein
Protein
Biological
Net Protein
Protein Digestibility
Value
Utilization
Corrected Amino Acid
Type
Efficiency
Ratio
Score
2.9
80
73
0.92
Beef
0
0
0.75
Black Beans
2.5
77
76
1.00
Casein
3.9
100
94
1.00
Egg
2.5
91
82
1.00
Milk
1.8
0.52
Peanuts
2.2
74
61
1.00
Soy protein
0.8
64
67
0.25
Wheat gluten
3.2
104
92
1.00
Whey protein
Adapted from: U.S Dairy Export Council, Reference Manual for U.S. Whey Products 2nd Edition,
1999 and Sarwar, 1997.
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,
Protein ratings
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.
121
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.
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Hoffman and Falvo
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 lactoseintolerant 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.
Table 2. Composition (%) of whey protein forms.
Component
Whey
Whey
Whey
Powder Concentrate Isolate
11 – 14.5
25 – 89
90 +
Protein
63 – 75
10 – 55
0.5
Lactose
1 – 1.5
2 – 10
0.5
Milk Fat
Adapted from Geiser, 2003.
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-likegrowth 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 2weeks 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.
Protein ratings
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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.
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.
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.
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.
Table 3. Protein composition of soy protein forms.
Soy Protein Form
Protein Composition
Soy Flour
50%
Soy Concentrate
70%
Soy Isolate
90%
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
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 lipidlowering
effects,
increase
LDL-cholesterol
oxidation, and have beneficial effects on lowering
blood pressure.
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).
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Hoffman and Falvo
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 lowcarbohydrate, 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 multisite 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⋅kg1
⋅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
Protein ratings
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
125
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.
Table 4. Potential acid as sulfate from sulfurcontaining amino acids.
Food
mEq per g of protein
Oatmeal
.82
Egg
.80
Walnuts
.74
Pork
.73
Wheat (whole)
.69
White Rice
.68
Barley
.68
Tuna
.65
Chicken
.65
Corn
.61
Beef
.59
Milk
.55
Cheddar
.46
Soy
.40
Peanuts
.40
Millet
.31
Almonds
.23
Potato
.23
Adapted from Massey, 2003.
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
126
Hoffman and Falvo
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
Sources on Human Performance
Protein
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,
2004; 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
Protein ratings
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.
CONCLUSIONS
It does appear that protein from animal sources is an
important source of protein for humans from infancy
until mature adulthood. However, the potential
health concerns associated with a diet of protein
consumed primarily from animal sources should be
acknowledged. With a proper combination of
sources, vegetable proteins may provide similar
benefits as protein from animal sources.
Maintenance of lean body mass though may become
a concern. However, interesting data does exist
concerning health benefits associated with soy
protein consumption.
In athletes supplementing their diets with
additional protein, casein has been shown to provide
the greatest benefit for increases in protein synthesis
for a prolonged duration. However, whey protein has
a greater initial benefit for protein synthesis. These
differences are related to their rates of absorption. It
is likely a combination of the two could be
beneficial, or smaller but more frequent ingestion of
whey protein could prove to be of more value.
Considering the paucity of research examining
various sources of protein in sport supplementation
studies, further research appears warranted on
examining the benefits of these various protein
sources.
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Hoffman and Falvo
130
Journal of Clinical Nutrition 68 (suppl),1437S1443S.
AUTHORS BIOGRAPHY
Jay R. HOFFMAN
Employment
Department of Health and Exercise Science. The
College of New Jersey
Degree
PhD
Research interests
Endocrinology of sports performance and nutritional
supplementation.
Email:
[email protected]
Michael J. FALVO
Employment
Department of Health and Exercise Science. The
College of New Jersey
Degrees
B.S.
KEY POINTS
• Higher protein needs are seen in athletic
populations.
• Animal proteins is an important source of
protein, however potential health concerns do
exist from a diet of protein consumed from
primarily animal sources.
• With a proper combination of sources,
vegetable proteins may provide similar benefits
as protein from animal sources.
• Casein protein supplementation may provide
the greatest benefit for increases in protein
synthesis for a prolonged duration.
Jay R. Hoffman, Ph.D.
Department of Health and Exercise Science, The College
of New Jersey, PO Box 7718,Ewing, NJ 08628-0718,
USA