Home / Articles / Supplementation / Protein Super Feature

Protein Super Feature
What You Need To Know About Protein and Growth

By Dr. John M Berardi, Ph.D.
First published in Pump Magazine, Oct 2000.

Printer friendly version

If you've been around the wonderful world of weight training you know that within the last few years there has been a tremendous "mainstream" surge in the popularity of strength training and nutritional supplementation. New advocates of training and nutrition are of course interested in the muscle and strength building that comes with intense weight training and proper supplementation. And this enthusiasm for building muscle has gone beyond the walls of local gyms and athletic field houses into medical centers and clinical populations.

Although athletes and bodybuilders have been shouting about the tremendous benefits associated with increased muscle mass and decreased fat mass for years, the scientific and clinical audiences have been slow to respond. But finally they're coming around. With a new acceptance of strength training and nutritional intervention to promote favorable changes in body composition, researchers and medical professionals are finally giving credence to what we've long known to be true. More importantly however, they are spending money on research looking at the effects of training and nutrition. And although this research is based on improving the clinical outcomes of patients with obesity and diseases of muscle wasting and weakness, bodybuilders can certainly benefit from these investigations because they can offer confirmation of what we have been doing all along.

Take growth hormone and testosterone research for instance. Long before researchers knew that they valuable in the building of muscle in patients with diseases of muscle wasting, bodybuilders had been using them for this very purpose. So although we are benefiting from their research, they have benefited from our trial and error. With this relationship, the science of muscle building and fat loss is spilling forward like a tidal wave, bringing with it new strategies for "gettin' huge".

Many Pump readers will remember that less than 10 years ago there was no creatine, andro, meal replacement powders, or the like. There were, however, nasty tasting protein powders and weight gainers. And there were also a few bottles of supplements that supposedly could promote muscle size and strength. Unfortunately, most of these supplements turned out to be crap in a bottle, not only because they had no muscle building effects, but also because they smelled and tasted like crap! But the times, they are a changin'. With of all of the research being done on supplements and all of the marketing hype based on this research, there are now hundreds of products to choose from. This is both an exciting and confusing time for bodybuilding. With all of the potential confusion, there is a need for information more than ever. According to John Cribbs, the owner of Pump magazine, his mission is to provide this information and to help readers find their way through the nutritional and muscle building supplement maze. And if he makes a little money along the way, no problem. We've all got to make a living.

In an attempt to help you through the supplement maze, this month's Super feature has one main focus: Protein. Although protein supplements have been around for quite some time, there is still a major lack of understanding in terms of how they should be used and what they should be used for. In addition, I don't think a lot of trainees really appreciate the need for a good protein supplement. If you want a quick summary of this feature, here it is: Eating lots of protein is the key to building muscle and losing fat. A high protein intake should be the foundation of any good diet and supplementation program.

Protein

Proteins are large biological molecules that are formed from amino acids. Although amino acids have specific biological properties in their own right, once joined by something called a peptide bond, they can take on chemical and biological properties. Joining two amino acids forms a dipeptide; three amino acids form a tripeptide, etc. This naming process continues until more than 50 amino acids are joined together. Once 50 amino acids get together by peptide bond, this is what scientists have called a protein.
In the body, proteins are everywhere. They play structural roles in the actin and myosin of muscle; the collagen in tendons and ligaments; the microtubules and microfilaments of the cell skeleton; and the keratin in the skin, nails, and hair. In addition, proteins such as enzymes, hormones, and neurotransmitters are responsible for metabolic regulation; hemoglobin and myoglobin are responsible for transport; and immunoglobulins are responsible for immune function. As you can see, proteins are involved in so many aspects of function that a deficiency would essentially render one physically useless. And you thought proteins were only important for their roles in muscle mass.

Skeletal Muscle Hypertrophy, Body Composition and Body Protein

When a trainee asks about how to get bigger, he is really asking about skeletal muscle hypertrophy. The next question to follow would be: "what exactly is skeletal muscle hypertrophy?" This phenomenon occurs when new protein is incorporated into the muscle cell, thus increasing the amount of contractile tissue in the muscle. The benefits of hypertrophy then, should be obvious. Increasing the contractile tissue (protein) in the muscle would lead to both increases in the diameter of the muscle as well as increases in the potential strength of the muscle. The more contractile tissue you have, the more forceful contractions you can produce.

Muscle hypertrophy is controlled by many factors including the nutritional state of the blood and muscle, the hormonal environment of the body, muscle fiber type, and muscle and nervous system stimulation. The idea that simply changing one's hormonal environment, simply adding a nutritional supplement into your program, or simply adding some resistance training to your daily activities will lead to significant long-tem increases in muscle mass is not logical. Instead, in order to promote real growth, a multi-faceted approach is needed. This approach would utilize strength training, hormonal manipulation, dietary intervention, and adequate rest and recuperative time.

One problem I often see when people discuss muscle growth is that of oversimplification. Many people don't really understand how much muscle there is in the body, how much that muscle can be expected to change, and what other "compartments" of the body can be altered in an attempt to change body fat and lean mass. Lets say that we have a 200 lb male that is about 15% body fat and 85% lean mass. While the total body fat is self-explanatory (15% of 200 is 30lbs), how much of his weight is actually muscle mass? Not 170lbs. 170lbs represents lean mass and lean mass is made up of bone, muscle, water, and other components including minerals. Still using our 200 lb male with 170lbs of lean mass and 30lbs of fat mass, it is then estimated that about 119lbs (70% of 170lbs) is water weight and 51 lbs (30% of 170lbs) is protein, bone, mineral, and other substances.

Now, although we have a better breakdown of body compartments, we still aren't much closer to a number for actual muscle mass. However, since about 50% of the lean body mass is muscle mass (muscle is composed mostly of water and protein), about 85lbs (or 42.5%) of our reference man's total body weight is muscle. Remember, this muscle is composed of mostly water and protein. Since this article is all about protein, lets figure out how much of the muscle is actually protein. Since it is estimated that about 20% of muscle is protein, our guy must have about 17lbs of muscle protein (20% of 85lbs). And although some of this consists of hormones, cell membrane components, cytoskeletal componenets, and enzymes; the bulk of the mass is contractile protein. The rest (68lbs) is water, mineral, etc. With all of this said, the following table is an approximate summary of the breakdown of body composition:

Total Mass: 200lbs
Lean Body Mass (muscle, water, bone, etc.): 170lbs
Fat Mass: 30lbs
Total Body Water: 119lbs
Total Non-Water Lean Mass (protein, mineral, etc): 51lbs
Total Muscle Mass: 85lbs
Muscle Water, Mineral, etc: 68lbs
Muscle Protein (mostly contactile): 17lbs

Although, these numbers are all approximations, they give a clear picture of how much of the muscle is actually protein. And many of you may be surprised to know that much less of your total body weight may be actually contractile muscle protein. I'm sorry to disappoint you. Hopefully though, rather than leaving sad and dejected, you will gain a renewed interest in increasing that mass.

Protein Turnover

One of the interesting things about proteins is that proteins, much like the skin, are in a constant state of synthesis or building and degradation or breakdown. Since proteins are continually subject to destruction and renewal, it is obviously important to consume sufficient protein to provide the raw materials for the rebuilding process of body proteins, especially muscle mass. In weight-trained individuals, the rates of protein synthesis, and protein breakdown (this is called protein turnover) are increased dramatically. Therefore increased dietary protein intake may be necessary to maintain optimal rebuilding and function. In addition, if increased muscle mass is the goal, you must consume more protein than you break down. This is known as positive protein balance. The extra protein that is retained in the body when in positive protein balance can be used for gains in muscle protein.

So how can we make a more positive protein balance? When looking at the research, there are quite a few factors that affect protein turnover. Hormonal factors such as testosterone, growth hormone, IGF-1, and insulin play large roles in protein turnover. In addition, dietary factors such as protein intake and total caloric intake can influence turnover. Exercise also plays a large role in protein balance.

Since the ultimate goal of this article is to discuss protein intake, I will only discuss the research that is relevant to protein intake and subsequent turnover. I will save the hormonal discussion for another article.
When eating a high protein diet, the protein is broken down into smaller peptides and individual amino acids in the GI tract. From here, these smaller molecules are sent to the liver and then into the blood. This would obviously lead to an increase in blood amino acid levels. As mentioned earlier, amino acids have important biological properties alone and as part of peptides and proteins. Apparently, when high blood levels of amino acids are present, these amino acids are shuttled into the cells in order to trigger an anabolic response or increased protein synthesis (1). This phenomenon, although not completely understood, may be governed by an interaction between insulin and these amino acids in order to promote more anabolism. Let me explain.

Insulin alone has been shown to both increase the activity of the protein synthesis machinery as well as decrease the protein breakdown process (2,3,4). This leads to a positive protein balance. Since insulin is also known to increase the uptake of amino acids into the cell, it appears that not only can insulin increase the process by which new muscle is built, but it can also help deliver the building blocks of that new muscle (2). Go insulin!
So how do we get our good friend insulin to come out to play? The answer is to eat protein and carbohydrates. Essentially, eating protein and carbs can increase insulin release. This insulin release then triggers anabolism and delivers the very amino acids that triggered its own release to the muscle for protein synthesis (2). It's a nice system.

So what about exercise? Since we all train, how does that effect protein turnover? A single bout of intense resistance exercise can result in a simultaneous increase in both synthesis and degradation (5,6,7). Unfortunately though, degradation seems to increase more than synthesis and therefore, there is a net negative protein balance after training. Although this seems like a negative thing, actually we can use exercise induced protein metabolism changes to our advantage. There will be more on this later.

Dietary Protein Quality, Digestion and Utilization

So now that we know what proteins are and how they can interact with other factors to promote growth, we need to discuss dietary protein consumption. Remember that proteins are long strings of individual amino acids. Due to this, the unique ratio of amino acids that each dietary protein source has determines the protein quality. The best dietary sources of protein have an amino acid ratio that is most closely associated with human protein make-up. To clarify further, quality protein has adequate levels of what are known as the indispensable amino acids. These indispensable amino acids must be present in certain amounts in the diet because without them, growth and protein synthesis is inadequate.

Scientific means have been developed to measure protein quality. The most discussed measure is that of the protein's biological value (BV). The BV of certain proteins is calculated by measuring the percentage of protein consumed and absorbed but not excreted. This gives an indication of how much of the protein that is consumed actually remains within the body to promote protein synthesis. This value is then compared with a "test protein" like egg protein and the value given is as a percentage BV relative to egg protein. Since egg has a high BV, proteins are represented relative to egg. A BV of 70% or above is considered good quality.

The second measure of protein quality is the protein-digestibility amino acid score (PCDAAS). This measure determines the indispensable amino acid content of the protein in question. These values are then compared to the known amino acid requirements in humans. Then, comparing the amino acid content of the protein with the human requirement for the indispensable amino acids, it is evident whether or not there are adequate amounts of each amino. The amino acid that is lowest in the protein is then termed the limiting amino acid because an amino acid is only as good as its weakest link. This value is then given to the protein of interest. I know this seems complex but here is an example. If only the amino acid lysine is very low in wheat protein relative to the known human needs for lysine, then lysine is the limiting amino acid. Since the adult requirement for lysine is 19mg lysine per 1g of protein, and wheat may only contain 8mg lysine per 1g of protein, the PCDAA of wheat protein is 8 divided by 19 or 42%. A PCDAAS of 1.00 or 100% is good quality. Any lower, and the protein must be combined with another source that may contain sufficient quantity of the low amino acid.

BV and PCDAAS of Selected Proteins

** adapted from:
a) Colgan, Michael PhD, The Right Protein for Muscle and Strength (1998) Progressive Health Series Colgan Institute, pg. 20 BIOLOGICAL VALUE OF PROTEINS;
b) FAO/WHO (1990) Report of a Joint FAO/WHO Expert Consultation on Protein Quality Evaluation, Food and Agruculture Organization of the United Nations, Rome;
c) Schaafsma, G. The Protein Digestibility-Corrected Amino Acid Score. J Nutr, 130: 1865S-1867S, 2000.

What you will notice from this chart is that the animal products seem to score better on both the BV and PCDAAS scales. This is why animal derived proteins like egg, milk, casein and whey are typically the proteins that bodybuilders will choose. Low quality proteins like beans, grains and peanuts can be eaten, but combining a few different low quality proteins like grains and milk proteins or rice and legumes (beans) would be necessary to get the required amino acids for growth.

At this point, you may be asking yourself, "So what does all this mean?" Well, the bottom line is that by choosing protein sources containing a BV of 70 or greater and a PCDAAS of 1.00, you will be providing your muscles with the best proteins and amino acids for muscle growth.

Daily Protein Requirements

Now let's get down to some specifics. Over the years the American Dietetics Association (ADA), despite evidence to the contrary, has continually asserted that the protein requirements of "normal" people are about 0.4g/lb. According to the ADA, this is estimated to meet the needs of 97.5% of the US population. This is all fine and dandy, but bodybuilders, strength athletes, and athletes in general, tend to have a larger protein turnover than 97.5% of the population and would therefore need more protein. Despite this fact, the ADA had previously asserted that athletes did not need more protein then the requirements for "normal" people. Recently, however this position has been modified based on new research.

Research by Dr Peter Lemon and Dr Mark Tarnopolski has shown that the protein requirements of athletes are higher than previously thought (8,9,10,11). Their recommendations of about 0.55g-0.65g/lb for endurance trainers and 0.65-0.80g/lb for weight trainers are based on studies of protein balance in humans. These levels of intake resulted in positive protein balance. Personally, although there are so many factors influencing protein needs, I think that people striving for perpetual muscle growth need a lot more protein, and that the protein needs are variable based on total caloric intake.

When consuming a high calorie diet, the excess of carbohydrates and fat tend to have a protein sparing effect. This leads to a lower requirement for protein intake of at least 1g/lb. There is some literature looking at high calorie diets with protein intakes of 1.6-1.8g/lb in Romanian weightlifters (12). This research showed that such high intakes might increase performance and muscle mass. So even on higher calorie diets, more protein may lead to more mass and strength. When total calories drop however, protein needs are increased due to the fact that the body may be oxidizing (burning) amino acids for energy in the absence of other carbs and fats (13). In this case, more dietary protein is required and intakes of 2.0g/lb or more may be required.

Although the research has mostly focused on the necessary protein to keep a neutral or slightly positive protein balance, a few investigations have taken a different approach. These studies have examined the metabolic repercussions of eating a high protein diet. The digestion and processing of food is well known to increase metabolic rate. This process is known as the thermic effect of food. Since the metabolic processing of protein is more complex than that of carbohydrates or fat, it has been shown that a high protein diet results in double the thermic effect vs. a high carbohydrate diet. Essentially, during the study period, the amount of calories expended to process the protein diet was about 10% of the total calories eaten. So if I am eating a high protein diet containing about 3000 calories, then about 300 of them will be burned from eating alone. The amount of calories expended to process the carbohydrate diet was only about 5% of the total calories eaten. This is only about 150 calories burned from eating alone. As you can see, at these levels, an extra couple hundred calories per day can be burned from eating high protein as opposed to high carbs (14).

When talking to bodybuilders, their testimonies have supported these recommendations, indicating that 1.5 g/lb or more might be beneficial for hard-training athletes, especially those who desire muscle hypertrophy. Although I think that these recommendations may lead to increased muscle mass beyond that of lower intakes, there are a number of considerations that must be taken into account when consuming a diet this high in protein.
When consuming a high protein diet, there are three main concerns: dehydration, calcium loss, and kidney abnormalities. In healthy individuals, none of the three are of major concern but they are worth mentioning. As far as dehydration, an adequate fluid intake of about ½ to 1 gallon of water per day will protect against this. Since calcium loss is accelerated with high protein intake, increasing calcium intake as part of the diet or as additional supplements will help with calcium balance. And as far as the kidney discussion, there is no scientific evidence that a high protein intake will sufficiently tax the kidneys of healthy individuals. If there are prior kidney problems, excess protein may present a problem, so before increasing protein intake considerably, a complete medical exam is in order.

Clearly, increased protein intake is necessary to promote size and strength gains. Although much of a very high protein intake will be oxidized and lost in the urine, the ultimate protein balance still tends to remain higher than those on lower protein intakes. In addition, those who are consuming excess protein may have greater potential for fat loss due to an increased thermic effect of feeding. Based on this, it is evident that individuals who want to get bigger and leaner must consume high protein diets with intakes of at least 1g/lb and up to 2g/lb.

Protein Intake, Timing and Supplements

Now for a plan of attack. For a bodybuilder weighing 200 lbs, our current recommendations would mean a protein intake of at least 200g and up to 400g per day. Often times, people don't really realize just how difficult this is on a daily basis. If you try to eat 5 meals a day, this amounts to an average of about 40-80 grams of protein per meal, every meal. Since a can of tuna or a 6 oz chicken breast only contain about 30 g of protein each, getting all your protein is difficult indeed, especially from whole food sources. It's not this difficult to get these amounts of carbs and fats, but since there are very few protein snacks, protein intake must be planned and precise. This is where protein supplements come into play.

Protein powders, meal replacement powders, and protein bars allow quick and convenient ways of meeting your high protein needs. In addition, the protein sources contained in these supplements are often very high quality in terms of BV and PDCAAS. So, although protein supplements are not an absolute requirement for gaining mass, I have yet to know any person who had even half a life and was able to get 400grams of protein per day from cooking food. In addition, dollar for dollar, protein powders and meal replacement drinks tend to be more cost effective than whole food. Don't get me wrong, though. Protein powders are still supplements in my book. Supplement means an addition to the diet. I emphasize this because the focus of any diet should be food. Whole food is often preferable to powders because they can offer a whole spectrum of nutrients that powders cannot. But the bottom line is that both are necessary to achieve a complete nutritional balance as well as the desired level of protein intake, especially if you're not a big fan of cooking.

Lately, there has been much debate about which type of protein is better and some writers are speculating that casein may be better than whey. However, with the big surge in whey protein popularity, as well as the very high BV of whey, many are reluctant to accept that whey might not be clearly the best protein. Recent research is mixed on this subject. Two research papers have suggested that casein may be superior to whey in both a dieting situation as well as in a muscle-building situation (15,16). When given either whey or casein alone, it appears that casein may be more anti-catabolic while whey is more anabolic. The problem with whey is that it is very quickly digested and therefore quickly degraded. Also, the anabolic effects are short-lived. Casein on the other hand, is very slowly digested and may continually provide amino acids to the body over a much longer period of time. This may lead to better protein balance over time. Another study looking at muscular performance however showed that whey protein seemed to outperform casein when added to the diets of study subjects (17). Subjects on whey had better muscular performance than those on casein. Finally, another unpublished investigation showed no differences between whey, soy, and casein on muscle size and body composition.

Confused yet as to which is best? Me too. Although all this contradictory information is confusing, one thing is certain. Before you start throwing out your whey and buying pounds of casein, more research is needed to determine which, if either, is superior and what the effects of mixed protein meals are. In my opinion, mixed meals containing a variety of proteins like whey, casein, and egg; fats; and carbs, may be superior to single sources of protein in the long run. I think this is the case because high protein and calorie intakes spread out over the course of the day will supply a continuous stream of available amino acids and other nutrients for muscle growth. In addition, when consuming hundreds of grams of protein per day, it may not even matter one bit which source those proteins are coming from as long as they meet our BV and PDCAAS recommendations. There are certain times of the day that some more specific recommendations may be warranted, however. This manipulation of protein type and timing may prove an important variable in forcing muscle growth.

As a result of a few recently published papers showing that meal timing and protein percentage may alter protein turnover, many have discussed the concept of "pulse feeding". In one study, older women consumed either one large protein meal consisting of about 80% of their daily protein requirements or 3 protein meals of equal intake. This study showed that the "pulse pattern" or the 1 big protein meal increased protein balance (18). In addition, another study conducted has shown that 3 protein meals led to greater protein balance when compared to hourly protein feedings (19). This data has led many to speculate that altering the classic recommendation to split protein intake up equally over all meals might end up leading to a better protein balance. Although there is some merit to these ideas, there is some limitation to applying the data to bodybuilders. First, the initial study was done in elderly women and they are not exactly analogous to bodybuilders. When this study was repeated in young women, there was no difference between the two diets in protein turnover (20). And although the study examining hourly vs. 3 protein meals might be more applicable, small hourly protein feedings are not representative of the normal bodybuilder's protein intake. In addition, the total daily protein intake in this study was only about 0.45g of protein per lb of body weight. As mentioned, this is far below what bodybuilders would actually be consuming.

Despite the fact that we can't easily generalize the findings to bodybuilders, the idea that protein intake could be staggered throughout the day to increase synthesis does make sense. For example, if your target protein intake is 300g/day, then eating a pulse pattern of 3 meals of 75g and 3 meals of 25 g might be beneficial. And not only are the levels of intake important, but the timing of the intake relative to sleep and training are also important. Consuming the large protein meals upon rising, after working out, and before bed, might prove to be the best strategy for timing protein intake. To take this pattern one step further, one could also speculate that the specific type of protein consumed at each feeding might be manipulated for maximum results.

Consuming a breakfast of mixed proteins, a post-workout drink of whey protein, and a pre-bed meal high in casein might maximize the needs of the body during such periods. At breakfast, both quick digesting and slow digesting proteins are required in order to quickly deliver amino acids to the body after the fasted sleep period. In addition, some slowly digesting aminos should be also eaten in order to slowly provide aminos for the morning hours after the meal. After the workout, the most effective way to enhance anabolism and decrease catabolism is to provide a very rapid infusion of amino acids. Since whey protein is the fastest protein in terms of digestion and absorption, then perhaps post-workout is the best time for such a supplement. Finally, before bed, one must anticipate a long 6-8 hour period of "fasting" during sleep. At this time, a large serving of the slow digesting casein might help provide a slow release of amino acids into the body in order to prevent the over-night catabolism associated with sleeping.

"Second and Third Generation" Protein Products

Although protein powders have been around for a while, the last few years have brought with them a host of what I call "second and third generation" protein products. Traditional thinking was that protein was protein and that there was no difference between whey, soy, egg, casein, beef, chicken, fish, etc for muscle growth. Today however, savvy supplement companies are using modern science to modify their protein formulations in order to increase the absorption and the biological effects of their products. This has led to "second generation" protein products. For example, whey proteins that contain amino acid combinations in smaller peptide forms may be more readily absorbed than regular whey protein (21). In addition, with special filtration processes, more of the bioactive immunoglobulins in whey protein can be preserved in order to provide added nutrient benefits.

The next step beyond the treatment of proteins in an attempt to enhance their bioavailability and functional properties was to add special ingredients such as methoxyflavones for anabolism, insulin potentiators such as inzitol and glucomannon to increase insulin sensitivity, l-arginine to potentially increase blood flow, and l-carnitine and lecithin for thermogenesis. I call these products "third generation" protein products as they have been developed to deliver both quality protein as well as other "adjunct" nutrients. These products are advancing the science of protein nutrition as they may be able to offer the synergistic effects of stimulating the body to begin protein synthesis while providing the amino acids necessary for this synthesis.

Caution should be used when examining these products however, because some supplements are more effective when taken on an empty stomach. When added to protein powders, their effectiveness may be diminished. If products are formulated correctly though, the combination of protein and other supplements may offer the advantage of delivering the very nutrients needed by the muscle at the same time that supplements are creating the desired metabolic changes. New research needs to be conducted on such products, however, as they may behave differently in terms of digestion, absorption, and metabolism than conventional protein powders. To this end, a whey protein containing insulin potentiators and blood flow enhancers, is designed to behave much differently than conventional whey protein. If it does, then the traditional whey protein research does not apply to such a product and it must be evaluated on its own merit.

Conclusion

At this point, if you've hung in there with me through the entire article, you are sure to have absorbed some valuable information about what protein is, how it can impact the body, and how much you need to consume to increase body proteins. Now it's time to put that information into practice. In celebration of the completion of this article, lets mix up a 50 gram protein shake and have a big, frothy toast to contractile proteins and muscle mass.

References

1) Bennet, W.M. et al. Euglycemic hyperinsulinemia augments amino acid uptake by human leg tissues during hyperaminoacidemia. Am J Physiol. 259: E-185-E194, 1990.
2) Biolo, G. et al. Physiologic hyperinsulinemia stimulates protein synthesis and enhances transport of selected amino acids in human skeletal muscle. J Clin Invest 95:811-819, 1995.
3) Newman, E. et al. The effect of systemic hyperinsulinemia with concomitant amino acid infusion on skeletal muscle protein turnover in the human forearm. Metabolism 43: 70-78, 1994.
4) Roy, B.D. et al. Effects of glucose supplementation timing on protein metabolism after resistance exercise. J Appl Physiol 82: 1882-1888, 1997.
5) Biolo, G. et al. Increased rates of muscle protein turnover and amino acid transport after resistance exercise in humans. Am J Physiol. 268: E14-E520, 1995.
6) Phillips, S. et al. Mixed muscle protein synthesis and breakdown after resistance exercise in humans. Am J Physiol, 273: E99-E107, 1997.
7) Roy, B.D. et al. Macronutrient intake and whole body protein metabolism following resistance exercise. Med Sci Sports and Exerc, 32 (8): 1412-1418, 2000.
8) Lemon, P.W.R. Protein and Exercise: Update, 1997. Med Sci Sports Exerc. 19 (Suppl):S179-S190. 1987.
9) Lemon, P.W.R., et al. Effects of exercise on protein and amino acid metabolism. Med Sci Sports Exers. 13: 141-149, 1981.
10) Tarnopolsky, M.A. et al. Evaluation of protein requirements for trained strength athletes. J Appl Physiol. 73(5):1986-1995, 1992.
11) Tarnopolsky, M.A. et al. Influence of protein intake and training status on nitrogen balance and lean body mass. J Appl Physioil. 64(1): 187-193, 1988.
12) Dragon, G.I. et al. Effect of increased supply of protein on elite weight-lifters. In: Milk Proteins. T.E. Galesloot and B.J. Tinbergen, eds. Wageningen, The Netherlands: Poduc. 1985. pp. 99-103.
13) Lemon, P.W.R. et al. The effect of initial muscle glycogen levels on protein catabolism during exercise. J Appl Physiol. 48:624-629, 1980.
14) Robinson, S.M. et al. Protein turnover and thermogenesis in response to high-protein and high-carbohydrate feeding in men. Am J Clin Nutr. 52(1):72-80, 1990.
15) Boirie, Y. et al. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci, USA. 94: 14930-14935, 1997.
16) Demling, R.H. et al. Effect of a hypocaloric diet, increased protein intake and resistance training on lean mass gains and fat mass loss in overweight police officers. Ann Nutr Metab. 44(1):21-29, 2000.
17) Lands, L.C. et al. Effects of supplementation with a cysteine donor on muscular performance. J Appl Physiol. 87(4): 1381-1385, 1999.
18) Arnal, M. et al. Protein pulse feeding improves protein retention in elderly women. Am J Clin Nutr. 69: 1202-1208, 1999.
19) Arnal, M. et al. Protein feeding pattern does not affect protein retention in young women. J Nutr. 130L 1700-1704, 2000.
20) El-Khoury, A. et al. The 24-h kinetics of leucine oxidation in healthy adults receiving a generous leucine intake via three discrete meals. Am J Clin Nutr. 62: 579-590, 1995.
21) Grimble, G.K. et al. Effect of peptide chain length on amino acid and nitrogen absorption from two lactalbumin hydrolysates in the normal human jejunum. Clin Sci 71: 65-69, 1986.