The Protein Paradox: Part I
by Greg Bradley-Popovich, MS, MS
It has long been debated among scientists as to whether active individuals or athletes require more dietary protein per unit body weight than that espoused by the various government agencies for normal sedentary subjects. This ongoing debate is between one school of thought, which believes that athletes and other active individuals do not require more protein than sedentary individuals, versus another that believes those engaged in physical activity have special needs. Increases in protein loss in athletes have been shown to occur through several means including, but not limited to, the following: source of energy, collagen repair, collagen hypertrophy, myofiber repair, myofiber hypertrophy, protein hidrosis (losses in sweat), and proteinuria.
When examining the protein needs of physically active individuals, one must further divide this diverse segment of the population into categories of subjects in search of specific training effects. As training techniques and goals vary, so does nutriture (Reimers, 1994). Athletes can generally be grouped into three broad categories: those individuals seeking aerobic conditioning, those seeking anaerobic conditioning, and those seeking a combination of these two forms of conditioning.
Of the aforementioned categories, the focus herein will be towards predominantly anaerobically-trained persons. Anaerobic athletes are athletes who participate in bursts of activity that last a short duration, generally less than two minutes.
Anaerobically-conditioned athletes usually develop characteristically hypertrophied skeletal muscles, usually resulting from the implementation of a resistance (weight lifting) regimen. This muscular hypertrophy is usually advantageous to the anaerobic athlete, for hypertrophy is positively correlated to muscular strength, an attribute which is desirable to most anaerobically conditioned athletes. Two outstanding and highly visible examples of athletes who seek strength and muscle gains are weight lifters and bodybuilders. Other examples of anaerobic athletes include sprinters, shot putters, and gymnasts.
Mike Mentzer Enjoys a Heavy Duty Meal
Of the various anaerobic athletes, most of the data regarding protein metabolism and protein requirements have been collected on weight lifters and bodybuilders. Because much of this problem report will focus primarily on these two groups, the two groups will be defined. Weight lifters desire to increase strength as much as possible and view muscle hypertrophy as a means of increasing strength. In contrast, bodybuilders are not preoccupied with strength gains per se, but with the accretion of muscle mass associated with strength development. In other words, for the bodybuilder, strength development is the means, not the end. Yet, both weight lifters and bodybuilders ultimately depend upon the hypertrophy of muscle. Additionally, bodybuilders strive to minimize body fat stores more so than weight lifters.
By no means is muscle hypertrophy desirable only in certain athletic populations. Rehabilitation patients, the active elderly, and general fitness enthusiasts may all qualify as athletes, though not in the traditional sense of the competitive context. Certainly non-competitive "athletes" comprise a far greater proportion of the population than competitive or elite athletes. Therefore, this problem report will include studies performed on a variety of participants from neophytes to professionals, all undergoing some form of resistance exercise, often to induce skeletal muscle hypertrophy.
The previously mentioned metabolic fates of protein as a result of exercise are general routes that apply to virtually all athletes, though to varying degrees depending upon the nature of the training. For example, under certain conditions, some protein will be oxidized for energy, as is commonly depicted in the glucose-alanine cycle, which uses the branched-chain amino acids, especially leucine, to provide an amine group for the formation of the amino acid alanine. The alanine, in turn, is carried from the muscle to the liver where it is used to form glucose to be consumed by the working muscles or the central nervous system. In addition to their role in the glucose-alanine cycle, the dietary essential branched-chain amino acids isoleucine, leucine, and valine are the primary amino acids oxidized under exercise stimuli, because the working skeletal muscles can oxidize them intramuscularly (Lemon, 1987). Other amino acids as well as the branched-chain amino acids can be degraded to Krebs Cycle intermediates in the liver to provide energy during exercise. Despite the detailed understanding of the biochemical pathways involved in transforming protein into energy-yielding substances, the overall loss of protein through such degradation for energy is agreed to contribute a very minor role in increasing the protein needs of anaerobic athletes (Lemon, 1995).
The tissue damage that occurs as a result of the high-intensity muscle contractions common to anaerobic athletes includes insults to the connective and muscle tissues. Connective tissue, primarily made of the protein collagen, has been shown to increase proportionally with muscle hypertrophy (MacDougall, 1986). Thus, the absolute amount of connective tissue in an anaerobic athlete increases with training. Muscle hypertrophy occurs by two means: increased size of myofibrils in a muscle fiber and possibly an increased number of myofibrils in a muscle fiber (MacDougall, 1986). These increases occur via the assimilation and incorporation of additional protein. Thus, the healing and hypertrophy resulting from strength training may be largely responsible for any increase in protein need.
Also, additional protein is lost in urine, a condition known as proteinuria. It appears to be positively related to exercise intensity. Proteinuria may be caused by a decreased reabsorption of amino acids in the kidney tubules during intense exercise. This avenue would be negligible, accounting for no more than 3 grams of protein per day (Poortmans, 1985).
Protein losses in sweat, protein hidrosis, also occur as a result of exercise. Both amino acids and proteins have been found in sweat. Approximately 1 gram of protein is lost in each liter of sweat. Again, these losses are relatively minor and training in a very warm environment would result in a loss of less than 3 g N/day (Celejowa and Homa, 1970; Consolazio, Johnson, Nelson, Dramise, and Skala, 1975; Lemon, Tarnoplosky, MacDougall and Atkinson, 1992; Walberg, Leedy, Sturgill, Hinkle, Ritchey, and Sebolt, 1988).
It appears that muscular growth accounts for the majority of additional protein needed by resistance-trained individuals. Additional dietary protein, then, mainly serves the needs of the growth mechanism.
Because the largest dry weight of muscle consists of protein, many athletes and researchers have suspected that increasing dietary protein intake above normal values may facilitate muscle hypertrophy. However, the wet weight of muscle is mostly water (Figure 1).
It is most appropriate that weight lifters and bodybuilders have primarily served as the anaerobic athletes on which many protein studies have been conducted. The lore and common "wisdom" promulgated in muscle magazines and gyms everywhere place protein high on a pedestal. Protein is generally regarded as an essential and almost mystical ingredient in achieving optimal progress in these sports.
At least two interesting arguments regarding protein requirements of resistance-trained individuals have been published in popular body building literature. These arguments arise not from physiology or nutritional biochemistry, but from mathematics. First, many bodybuilding journalists have emphasized that conventional nutrient allowances such as the Recommended Dietary Allowances establish a suggested protein intake that will be sufficient for approximately 98 percent of the population (National Academy of Sciences National Research Council, 1989). Therefore, they logically conclude that 2 percent of the population will not be covered by the recommended nutrient allowance. To quantify this conclusion, the bodybuilding authorities delineate that just 2 percent of the population of the United States (roughly 260 million people) is equal to more than 5 million people. They argue, therefore, that many bodybuilders and other athletes under strenuous training are likely to be part of this minority for whom the conventional allowance is inadequate.
On the other hand, some bodybuilding authorities point out that muscle growth is a relatively slow process and if muscle accretion alone is quantified on a daily basis, the need for copious amounts of protein above the conventional allowances are discredited. For example, if an individual were to increase his muscle mass by an impressive 20 pounds (9.09 kg) in one year, that would amount to about 2,000 grams of assimilated net muscle protein. (Each pound of muscle at 454 g is about 22 percent protein, and .22 X 454 = 99.8 g protein.) Averaged out over the course of a year, the 2,000 g would amount to only an additional 5.5 g of protein above daily maintenance requirements. This viewpoint is supported by Durnin who estimated this figure to be 7 g/day (1978). In contrast, Brotherhood (1984) cites research which indicates that the estimated value for lean body mass gains may exceed 50 g/day during aggressive muscle building programs.
It cannot be assumed that protein assimilation in muscle tissue occurs with 100 percent efficiency, as is the underlying assumption with the former argument which suggests a surplus of just 5.5 g of protein per day. There is no guarantee that all of the additional protein, once absorbed, will arrive at its intended destination (Di Pasquale, 1997, p. 73). This is because the amino acids in sufficient amounts must not only be made available during muscular growth, but must be available at the time a particular amino acid is being incorporated into a translated protein. Thus, because protein synthesis is continuous, it is a hit-or-miss situation in regards to the supplying of a particular amino acid at the correct time (Di Pasquale, 1997, p. 82). This imperfect efficiency accounts, at least partially, for the apparent mathematical discrepancies that have persisted for years: why it appears to require a lot of protein to build a comparatively small amount of muscle.
So what tools are available to the scientist in search of the truth? In general, there are four possible methods helpful in determining protein requirements in resistance-trained persons. These include the following: protein/amino acid turnover, muscle hypertrophy assessment, strength performance measurements, and nitrogen balance studies.
Of these various techniques, the focus herein will be on nitrogen balance. Nitrogen balance is considered an indirect index of lean tissue storage (Fern, Bielinski, & Schutz, 1991), like that which occurs as a result of resistance exercise and growth or the losses that occur during fasting. Nitrogen balance is simply nitrogen intake through diet minus nitrogen losses. Losses occur through feces, urine, and integumenta (skin, sweat, and hair). Mitchell (1924) first described nitrogen balance as the difference between intake and output. Nitrogen balance can be expressed as follows:
nitrogen balance = nitrogen intake
- [(urinary N - urinary endogenous N)
+ (fecal N - endogenous fecal N)
+ dermal N]
In this report, careful attention must be paid to semantics. It is important to keep in mind that the words "need" or "requirement" as they frequently appear in this paper do not simply refer to the minimum amount of dietary protein necessary to sustain health, but to the amount of protein necessary to achieve a highly positive nitrogen balance, which is necessary to increase the body's absolute lean tissue content.
Several studies have attempted to determine the protein requirement for maintaining nitrogen balance during resistance training. This project paper will review nine available studies. For the sake of comparison and uniformity, some original data have been expressed in different, but equivalent, units. For example, some studies originally expressed energy in joules or total energy per day, or expressed nitrogen balance in terms of mg/kg/day. However, for simplicity, this paper presents all of the energy data in terms of kcal/kg/day and the nitrogen balance data in terms of g/day.
Unless otherwise noted, all studies measured nitrogen losses in feces, urine, and sweat, but estimated or ignored losses in skin, nails, or hair. Additionally, when applicable, low- and high-protein diets in a given study are isoenergetic.
In the only study to observe older adults, Campbell, Crim, Young, Joseph, and Evans (1995) investigated nitrogen retention in 12 previously untrained senior citizens who averaged 65 years in age. It is also the only study which included both males and females, in the numbers of 8 and 4, respectively. Subjects were randomly assigned to two groups, a low-protein group consuming 0.8 g protein/kg/day (0.128 g N/kg/day), and a high protein group consuming 1.62 g protein/kg/day (0.259 g N/kg/day). Energy intake was designed to balance output including exercise, at about 30.0 kcal/kg/day for both groups. Subjects trained for 12 weeks in the following fashion: whole body with 3 sets per exercise for 4 exercises at 80% of each exercise's one repetition maximum for each individual. The workout routine was performed every-other-day and lasted about half an hour per session (Campbell, Crim, Young, and Evans, 1994). It was demonstrated during the two week period prior to initiation of the exercise program that a negative nitrogen balance was achieved at 0.8 g/kg/day, whereas a positive nitrogen balance resulted at 1.62 g/kg/day (-0.368 g N/day vs. 1.379 g N/day). During week 11, both groups had improved nitrogen retention to the same degree (0.631 g N/day vs. 2.06 g N/day, after correcting for posttraining body weight). In this study, total integumentary nitrogen losses were estimated. Campbell and colleagues concluded that the efficiency of nitrogen retention was higher in the low-protein group, but made no reference to the absolute differences between the groups (Campbell, Crim, Young, Joseph, and Evans, 1995).
In one of the earliest studies to address protein needs of weight lifters, Celejowa and Homa (1970) observed male competitive weight lifters, averaging 25 years of age, over 11 days during a training camp. The level of activity was considered "intense," and workout sessions lasted an average of 1.5 hours each. The researchers demonstrated a negative nitrogen balance in five out of ten competitive weight lifters who consumed protein in the amount of 2 g/kg/day during a training camp. The nitrogen balance mean value for the whole group was slightly positive (0.50 g N/day) while consuming an average of 50.0 kcal/kg/day, which resulted in an energy surplus of about 233 kcal/day/man. Celejowa and Homa concluded that a protein intake of 2 g/kg/day was too low for 50% of the weight lifters.
In another study, Consolazio, Johnson, Nelson, Dramise, and Skala (1975) monitored young men over a 40 day period while they engaged in a "vigorous" physical conditioning program, which consisted of a variety of physical activities including "treadmill walking, riding the bicycle ergometer, calesthenics [sic], isometric exercises, and other sporting activities." (Calisthenics are exercises that use the body as its resistance, such as push-ups and abdominal crunches. Isometrics are exercises in which a muscle tenses but does not shorten, such as when a muscle attempts to contract against an immovable object or an individual flexes his muscles and holds a pose. Thus, for the purpose of this report, calisthenics and isometrics constitute resistance training.) The exercise time duration per workout was not reported. The 8 subjects averaged 21.5 years of age and consumed diets providing about 48.7 kcal/kg to balance energy intake with expenditure. Consolazio and colleagues observed greater nitrogen retention (0.533 g/day vs. 1.60 g/day) in resistance-trained athletes over a forty day training regimen when protein intake was 2.8 g/kg/day versus 1.4 g/kg/day.
Also supportive of these findings is a study which followed for four weeks two groups of young men whose average age was 24.5 years old (Fern, Bielinski, and Schutz, 1991). Both groups began whole-body strength training 3 times per week, with each session lasting for one hour. One group consumed their normal protein dietary intake of 1.3 g/kg/day while the higher protein group consumed this amount plus a protein powder supplement of 2 g protein/kg/day, giving a total of 3.3 g/kg/day. "Crude" nitrogen balance was determined to be 0.01 g N/day and 3.4 g N/day, respectively. It is insinuated that only urinary nitrogen was actually monitored.
Another study of four champion weight lifters ranging in age from 21 to 34 years old suggested an average protein intake of 2.2 g/kg/day resulted in a positive nitrogen balance of 1.85 g N (Laritcheva, Yalovaya, Shubin, and Smirnov, 1978). One lifter who consumed the least protein at 1.85 g/kg had a negative nitrogen balance of -0.88 g N. The weight lifters exercised 90-150 minutes per workout, and energy balance was approximately neutral. Nitrogen losses through the integumenta were not taken into consideration. It is assumed that the nitrogen retention data are from a single observation day for each weight lifter.
In a cross-over study examining the first two months of training for 12 untrained males averaging, 22.4 years of age, Lemon, Tarnoplosky, MacDougall and Atkinson (1992) divided subjects into a relatively low-protein group receiving 1.35 g/kg/day and a high-protein group consuming 2.619 g/kg/day. The two dietary phases were separated by a one-week ad libitum washout period. The trainees performed an "intensive" workout routine 6 days/week. In this 3-day split routine, chest and back were worked one day followed by legs the next day, and then finished with shoulders and arms on the third day. The cycle then repeated itself. Each workout consisted of 5-8 exercises, each performed for 4 sets at 70-85% of the individual's one repetition maximum. Energy was consumed in the amount of roughly 39 kcal/kg. The investigators reported an enhanced nitrogen retention at 2.62 g/kg/day as opposed to 1.35 g/kg/day (8.9 plus or minus 4.2 and -3.4 plus or minus 1.9 g N/day), respectively. In fact, all 12 participants were in negative nitrogen balance when consuming 1.35 g protein/kg/day. Using linear regression analysis, Lemon et al. predicted zero balance would have occurred in the low-protein group at 1.43 g/kg/day and the high-protein group at 1.53 g/kg/day. After adding a safety margin of 2 standard deviations, their minimum recommended protein intake was 1.63-1.73 g/kg/day.
Tarnopolsky and others documented the effects of two diets in a cross-over study on 6 male body builders training 1.5-2 hours/day. Nitrogen balance was determined after a control diet and after consuming the experimental diets for 3 days. A high-protein diet of 1.05 g/kg/day resulted in a slightly positive nitrogen balance in the body builders, while an even higher protein diet of 2.77 g/kg/day resulted in a more positive nitrogen balance (.62 g/day vs. 10.9 g/day) (Tarnopolsky, MacDougall, Atkinson, Blimkie, and Sale, 1986). The researchers concluded that body builders are able to maintain nitrogen balance on a dietary protein intake of less than 1.05 g/kg/day but that higher protein ingestion leads to a more positive balance.
More recently, in a second study headed by Tarnopolsky, strength training males with at least 2 months of weight lifting experience served as the trainees. The average age of the 7 strength training participants was 21.6 years. The workout routine was executed 4 times/week yielding a weekly physical activity duration of 9.7 hours/week. All subjects was involved in three different experiments, each of 13 days. Each experimental period was preceded by an ad libitum washout diet period of at least 8 days. A low-protein group consumed the Canadian RNI of 0.86 g protein/kg/day. A moderate-protein group received 1.4 g/kg/day while a high-protein group ingested 2.4 g/kg/day. Energy was consumed in the amount of approximately 43 kcal/kg/day. Sweat losses of nitrogen were estimated based upon other studies. The low-protein group was shown to be in a negative nitrogen balance of -2.4 g N/day/day. The moderate-protein group had a modest positive balance of 0.7 g N/kg/day. Finally, the high-protein group revealed the highest retention of 3.8 g N/kg/day. Using linear regression analysis, the research team calculated an appropriate protein intake ("requirement" plus 1SD) to be 1.76 g protein/kg/day. They noted a discrepancy between the nitrogen balance method and other techniques they employed, and they recommended that future studies utilize nitrogen balance in conjunction with additional techniques (Tarnopolsky, Atkinson, MacDougall, Chesley, Phillips, and Schwarcz, 1992).
Similar to the findings of the most recent study by Tarnopolsky and others, Walberg and others have shown in heavily training body builders consuming a hypoenergetic diet that negative nitrogen balance occurred at 0.8 g/kg/day, but a positive balance was achieved at 1.6 g/kg/day (-3.19 g N/day vs. 4.13 g N/day). Each of the experimental groups consisted of 7 individuals about 21 years of age who had resistance-trained for at least 2 years. The experimental two-day split exercise routine was divided into biceps, back, and legs one day followed by triceps, shoulders and chest the next day. Participants exercised 6 days/week, performing 3 sets of 8-10 exercises for 8-10 repetitions, ranging in intensity of 60-80% of the one-repetition maximum. Each exercise session lasted 1.5-2 hours/day. Nitrogen balance data were assessed daily for 7 days. Both groups consumed a hypocaloric diet of 18 kcal/kg, which resulted in an average daily deficit of approximately -1,400. The authors concluded that a higher protein diet (1.6 g/kg/day) is superior to a lower protein diet (0.8 g/kg/day) at retaining nitrogen in weight lifters during short-term weight reduction. They suggested such a diet may be useful to minimize lean body mass loss immediately prior to a competition (Walberg, Leedy, Sturgill, Hinkle, Ritchey, and Sebolt, 1988).
My own statistical evaluation of the of the research findings in the literature calculated the average net protein utilization (NPU), a term introduced in 1961 by Platt et al (Munro, 1963). NPU is a biological measure of protein quality that included an evaluation of protein digestibility as well as of content of indispensable amino acids. NPU, in living humans, can be calculated from the following equation:
NPU = [N intake - (fecal N - endogenous fecal N) - (urinary N - urinary endogenous N)] / N intake
In my analysis, where the slope of the graph was equal to NPU, the slope of the line from the model of nitrogen balance per kilogram versus nitrogen intake per kilogram was 0.270. Hence, the NPU is equal to 27%. In practical terms, the body shows a combined efficiency of 27% at digesting and using dietary protein. Normally, a typical mixed U.S. diet has an NPU of 96% (National Academy of Sciences National Research Council, 1989). This apparent decreased efficiency could be due to several reasons. First, increased protein intake over long periods of time may upregulate gluconeogenesis (Di Pasquale, 1997, p. 83) and decrease the body's efficiency of protein use (Walberg et al.). Second, it could be speculated that muscular hypertrophy in an adult has very specific amino acid requirements at particular times as in the previously described "hit-or-miss" scenario (Di Pasquale, 1997, p. 73 and p. 82).
In the next installment of this series, I will critically discuss these nitrogen balance studies and what implications they have for resistance-trained individuals.*** *** ***
Greg Bradley-Popovich holds a B.S. in Biology from Fairmont State College, an M.S. in Exercise Physiology from the School of Medicine at West Virginia University, and a second M.S. in Human Nutrition, also from WVU. He is currently a Doctor of Physical Therapy scholar at the highly acclaimed Creighton University in Omaha, Nebraska where he received the 1999 Developing Clinician Award. He is coauthor of the book Rational Strength Training, available at www.i-a-r-t.com. He is also a contributing author to the upcoming textbook Sports Supplements: A Complete Guide to Physique and Athletic Enhancement due out from Williams & Wilkins in the fall of 2000. Those interested in Greg's freelance scientific discussions on fitness and nutrition can contact him at firstname.lastname@example.org.
Reprinted from Exercise Protocol, spring 1999.
Allen, L. H., Oddoye, E. A., & Margen, S. (1979). Protein-induced hypercalciuria: a longer term study. American Journal of Clinical Nutrition, 32, 741-749.
Anand, C. R. & Linkswiler, H. M. (1974). Effect of protein intake on calcium balance of young men given 500 mg calcium daily. Journal of Nutrition, 104, 695-700.
Arnaud, C. D. & Sanchez, S. D. (1996). Calcium and phosphorus. In: E. E. Zigler & L. J. Filer, Jr. (Eds.), Present knowledge in nutrition, (7th ed.; pp. 245-255). Washington, D. C.: ILSI Press.
Brenner, B. M., Meyer, T. W., & Hostetter, T. H. (1982). Dietary protein intake and the progressive nature of kidney disease: The role of hemodynamically mediated glomerular injury in the pathogenesis of progressive glomerular sclerosis in aging, renal ablation, and intrinsic renal disease. The New England Journal of Medicine, 307, 652-659.
Brotherhood, J. R. (1984). Nutrition and sports performance. Sports Medicine, 1, 350-389.
Burr, D. B. (1997). Bone, exercise, and stress fractures. In: J. L. Holloszy (Ed.), Exercise and sport sciences reviews (pp. 171-194). Baltimore, MD: Williams and Wilkins.
Campbell, W. W., Crim, M. C., Young, V. R., & Evans, W. J. (1994). Increased energy requirements and body composition changes with resistance training in older adults. American Journal of Clinical Nutrition, 60, 167-175.
Campbell, W. W., Crim, M. C., Young, V. R., Joseph, L. J., & Evans, W. J. (1995). Effects of resistance training and dietary protein intake on protein metabolism in older adults. American Journal of Physiology, 268 (June), 1143E.
Catlin, D., Wright, J., Pope, H., Jr., & Liggett, M. (1993). Assessing the threat of anabolic steroids. The Physician and Sports Medicine, 21 (8), 36-41, 44.
Celejowa, I. & Homa, M. (1970). Food intake, nitrogen, and energy balance in Polish weight lifters during a training camp. Nutrition and Metabolism, 12, 259-74.
Conroy, B. P. & Earle, R. W. (1994). Bone, muscle, and connective tissue adaptations to physical activity. In: T.R. Baechle (Ed.), Essentials of strength and conditioning (pp. 51-66). Champaigne, IL: Human Kinetics.
Consolazio C. F., Johnson, H. L., Nelson, R. A., Dramise, J. G., & Skala, J. H. (1975). Protein metabolism during intensive physical training in the young adult. American Journal of Clinical Nutrition, 28, 29-35.
Di Pasquale, M. G. (1997). Amino acids and proteins for the athlete–the anabolic edge. Volume in I. Wolinsky (Series ed.), Nutrition in exercise and sport. New York, NY: CRC Press.
Durnin, J. V. (1978). Protein requirements and physical activity. In J. Parizkova & V. A. Rogozkin (Eds.) Nutrition, physical fitness and health (pp. 53-60) Baltimore: University Press.
Elliot, D. L., Goldberg, L., Kuehl, K. S., & Catlin, D. H. (1987). Chracteristics of anabolic-androgenic steroid-free competitive male and female body builders. The Physician and Sports Medicine, 15 (6), 169-179.
Fern, E. B., Bielinski, R. N., & Schutz, Y. (1991). Effects of exaggerated amino acid and protein supply in man. Experientia, 47, 168-72.
Friedl, K. E. (1994). Performance-enhancing substances: Effects, risks, and appropriate alternatives. In: T. R. Baechle (Ed.), Essentials of strength training and conditioning (pp. 188-209). Champaign, IL: Human Kinetics.
Goldenburg, L., Elliot, D. L., Schutz, R. W., & Kloster, F. E. (1984). Changes in lipid and lipoprotein levels after weight training. JAMA: The Journal of the American Medical Association, 252 (4), 504-506.
Hatfield, F. C. (1993). Hardcore bodybuilding: A scientific approach. Chicago, IL: Contemporary Books.
Hegsted, M., Schuette, S. A., Zemel, M. B., & Linkswiler, H. M. (1981). Urinary calcium and calcium balance in young men as affected by level of protein and phosphorus intake. Journal of Nutrition, 111, 553-562.
Holloway, J. B. (1994). Individual differences and their implications for resistance training. In: T. R. Baechle (Ed.), Essentials of strength training and conditioning (pp. 151-162). Champaign, IL: Human Kinetics.
Hurley, B. F., Seals, D. R., Hagberg, J. M., Goldberg, A. C., Ostrove, S. M., Holloszy, J. O., Wiest, W. G., & Goldberg, A. P. (1984). High-density lipoprotein cholesterol in body builders v powerlifters: Negative effects of androgen use. JAMA: The Journal of the American Medical Association, 252 (4), 507-513.
Kersey, R. D. (1993). Anabolic-androgenic steroid use by private health club/gym athletes. Journal of Strength and Conditioning Research, 7 (2), 118-126.
Kleiner, S. M., Bazzarre, T. L., & Ainsworth, B. E. (1994). Nutritional status of nationally ranked elite bodybuilders. International Journal of Sport Nutrition, 4, 54-69.
Kraemer, W. J. (1992 a). Hormonal mechanisms related to the expression of muscular strength and power. In: P. V. Komi (Ed.), Strength and power in sport (pp. 64-76). Cambridge, MA: Blackwell Science.
Kraemer, W. J. (1992 b). Endocrine responses and adaptations to strength training. In: P. V. Komi (Ed.), Strength and power in sport (pp. 291-304). Cambridge, MA: Blackwell Science.
Laritcheva, K. A., Yalovaya, N. I., Shubin, V. I., & Smirnov, P. V. (1978). Study of energy expenditure and protein needs of top weight lifters. In Parazkova & Rogozkin (Eds) Nutrition, physical fitness and health (pp. 144-163). Baltimore: University Press.
Lemar, H. J., Jr. (1998 a). Growth hormone use and abuse. In: M. T. McDermott (Ed.), Endocrine secrets, 2nd edition (pp. 172-179). Philadelphia, PA: Hanley and Belfus Medical Publishers.
Lemar, H. J., Jr. (1998 b). Use and abuse of anabolic-androgenic steroids. In: M. T. McDermott (Ed.), Endocrine secrets (2nd ed.; pp. 291-295). Philadelphia, PA: Hanley and Belfus Medical Publishers.
Lemon, P. W. R. (1987). Protein and exercise: update 1987. Medicine and Science in Sport and Exercise, 19 (5), S179-S190.
Lemon, P. W. R. (1994). Protein requirements of soccer. Journal of Sports Science, 12, S17-22.
Lemon, P. W. R. (1995). Do athletes need more dietary protein and amino acids? International Journal of Sport Nutrition, 5, S39-S61.
Lemon, P. W. R. (1997, September). Protein needs of athletes. Symposium conducted at the International Conference on Sport Nutrition, Williamsburg, Virginia.
Lemon, P. W .R. & Proctor, D. (1991). Protein intake and athletic performance. Sports Medicine, 12 (5), 313-325.
Lemon, P. W. R., Tarnopolsky, M. A., MacDougall, J. D., & Atkinson, S. A. (1992). Protein requirements and muscle mass/strength changes during intensive training in novice bodybuilders. Journal of Applied Physiology, 73, 767-775.
Lombardo, J. (1993). The efficacy and mechanisms of action of anabolic steroids. In C. E. Yesalis (Ed.), Anabolic steroids in sport and exercise (pp. 89-106). Champaign, IL: Human Kinetics.
MacDougall, J. D. (1986). Morphological changes in human skeletal muscle following strength training and immobilization. In N. L. Jones, N. McCartney, & A .J. McComas (Eds.), Human muscle power (pp. 269-288). Champaign, IL: Human Kinetics.
McCarthy, P. (1989). How much protein do athletes really need? The Physician and Sports Medicine, 17 (5), (170-175).
Mitchell, H. H. (1924). A method of determining the biological value of protein. Journal of Biological Chemistry, 58, 873-903.
Munro, H. N. (1964). Introduction to part II. In: H.N. Munro & J.B. Allison (Eds.), Mammalian protein metabolism: Vol. 2. (pp. 1-39). New York, NY: Academic Press.
National Academy of Sciences National Research Council. (1989). Recommended dietary allowances ( 9th ed.) Washington, D.C.: National Academy Press.
Oser, B. L. (1965). Hawke's physiological chemistry. New York, NY: McGraw-Hill.
Paul, G. (1989). Dietary protein requirements of physically active individuals. Sports Medicine, 8, 154-176.
Poortmans, J. (1985). Postexercise proteinuria in humans: facts and mechanisms. Journal of the American Medical Association, 253, 236-240.
Reeds, P. J. & Beckett, P. R. (1996). Protein and amino acids. In: E. E. Zigler & L. J. Filer, Jr. (Eds.), Present knowledge in nutrition, (7th ed.; pp. 67-86). Washington, D.C.: ILSI Press.
Reimers, K. J. (1994). Evaluating a healthy, high performance diet. Strength and Conditioning, 16 (6), 28-30.
Roy, B. D., Tarnopolsky, M. A., MacDougall, J. D., Fowles, J., & Yarasheski, K. E. (1997). Effect of glucose supplement timing on protein metabolism after resistance training. Journal of Applied Physiology, 82 (6), 1882-1888.
Sargent, R. G. and Hohn, E. (1993). Protein needs for the athlete. National Strength and Conditioning Journal, 15 (1), 54-56.
Schutte, S. A. & Linkswiler, H. M. (1982). Effects of Ca and P metabolism in humans by adding meat, meat plus milk, or purified proteins plus Ca and P to a low protein diet. Journal of Nutrition, 112, 338-349.
Sherwood, L. (1993). Human physiology: From cells to systems, 2nd ed. St. Paul: West.
Sterck, J. G., Ritskes-Hoitinga J., & Beynen, A. C. (1992). Inhibitory effect of high protein intake on nephrocalcinogenesis in female rats. British Journal of Nutrition, 67 (2), 223-233.
Tarnopolsky, M. A., Atkinson, S. A., MacDougall, J. D., Chesley, A., Phillips, S., & Schwarcz, H. P. (1992). Evaluation of protein requirements for trained strength athletes. Journal of Applied Physiology, 73, 1986-1995.
Tarnopolsky, M. A., MacDougall, J. D., Atkinson, S. A., Blimkie, C., & Sale, D. (1986). Dietary protein requirements for body builders vs. sedentary controls [Abstract]. Medicine and Science in Sports and Exercise, 18, S64.
Tesch, P. A. (1992). Training for bodybuilding. In: P. V. Komi (Ed.), Strength and power in sport (pp. 370-380). Cambridge, MA: Blackwell Science.
Ullrich, I. H., Reid, C. M., & Yeater, R. A. (1987). Increased HDL-cholesterol levels with a weight-lifting program. Southern Medical Journal, 80 (3), 328-331.
Vega, F. & Jackson, R. T. (1995). Dietary habits of body builders and other regular exercisers. Nutrition Research, 16 (1), 3-10.
Walberg, J. L., Leedy, M. K., Sturgill, D. J., Hinkle, D. E., Ritchey, S. J., & Sebolt, D. R. (1988). Macronutrient content of a hypoenergy diet affects nitrogen retention and muscle function in weightlifters. International Journal of Sports Medicine, 9, 261-266.
Wallace, M. B., Moffatt, R. J., Haymes, E. M., & Green, N. R. (1991). Acute effects of resistance exercise on parameters of protein metabolism. Medicine and Science in Sport and Exercise, 23 (2), 199-204.