Principle Study: Indicator Amino Acid–Derived Estimate of Dietary Protein Requirement for Male Bodybuilders on a Nontraining Day Is Several-Fold Greater than the Current Recommended Dietary Allowance

How much dietary protein does a bodybuilder require? Several organizations, such as the American College of Sports Medicine and the International Society of Sports Nutrition (shown in Figure 1), have recommended that physically active adults consume between 1.2-1.4 and 2.0 grams of protein per kilogram of bodyweight per day to allow for recovery from training and to promote the growth and maintenance of lean body mass. However, these recommendations are based primarily on studies that involve recreationally active or formerly untrained adults with normal amounts of lean body mass. Extending these recommendations to a resistance-trained bodybuilder looking to maximize lean body mass may not be appropriate.

Several studies have attempted to identify the minimal protein requirements of elite bodybuilders, novice bodybuilders, and strength-trained athletes. Using the nitrogen balance method, which attempts to determine protein requirements by measuring nitrogen intake and excretion, these studies suggested protein requirements upwards of 1.4 grams per kilogram of bodyweight. However, the nitrogen balance method has several limitations, such as the way in which the data is analyzed (linear fits used for nonlinear data), inaccurate estimations of nitrogen intake and excretion, and the one to two week adaptation period required before protein intake measurements can be taken.

The last point is especially important, considering that athletes’ bodies can adapt to lower protein intakes over that adaption period. Isotope tracer studies have suggested that there are four stages of protein metabolism: deficiency, accommodation, adaptation, and excess. Nitrogen balance studies may show that people are in nitrogen balance at lower intakes of dietary protein because the body adapts to this lower amount by downregulating physiologically relevant pathways, like muscle protein synthesis and immune function. However, for a bodybuilder interested in maximizing muscle growth, this accommodation is not beneficial. Rather, the focus should be on conditions when both optimal growth and immune function are present.

The Indicator Amino Acid Oxidation (IAAO) technique is a method for determining protein requirements that overcomes many of the shortcomings of nitrogen balance studies. For instance, only a minimal adaptation period is required before testing of protein requirements. The IAAO method is based on the concept that when one essential amino acid is deficient for protein synthesis, then all other essential amino acids, including the “indicator” amino acid, will be oxidized for energy because protein cannot be readily stored like carbohydrate or fat (see figure 1 of NERD #19, Volume 1, How much protein does grandpa really need?). Dietary amino acids must be incorporated into bodily tissues via protein synthesis or oxidized for energy and excreted from the body.

The study under review used the IAAO technique to determine the protein requirements of young male bodybuilders. This is the first study to use the IAAO technique in this population, and will provide important information for helping to establish protein recommendations for bodybuilders while avoiding the shortcomings of nitrogen balance assessments.

Protein requirements of physically active adults remains a controversial area of research due to the widespread differences in people who are regularly active. Bodybuilders are one such population and have limited data available. Several studies have attempted to determine the protein requirements of bodybuilders using the nitrogen balance method, but nitrogen balance measurements have notable limitations. The purpose of the study at hand was to determine the protein requirements of bodybuilders using a method that overcomes many of the shortcomings of nitrogen balance research, called the Indicator Amino Acid Oxidation (IAAO) technique.

Who and what was studied?

Eight healthy young men (average age of 22.5 years) were recruited from a university campus. All the participants had at least three years of resistance training experience and were currently strength training four or more days per week with minimal aerobic exercise (less than 20 minutes per week). Additionally, the participants had to be relatively weight stable, with less than four kilograms (10 pounds) of weight gain or loss in the past six months, and never having used anabolic steroids.

To ensure that the participants were near their theoretical maximum muscular potential, each had their fat-free mass index (FFMI; same as BMI but uses fat-free mass instead of body weight) calculated and compared to published values of Mr. USA winners during the pre-steroid era of 1939-1959. Only those participants within 10% of the muscularity of past Mr. USA winners were selected. On average, the eight male bodybuilders participating in this study had a FFMI of 24 (96% that of past Mr. USA bodybuilders).

The participants underwent several three-day test periods separated by at least one week. Each occasion consisted of a two-day adaptation period followed by the IAAO study day. During the adaptation days, the participants were provided with a maintenance diet supplying 1.5 grams of protein per kilogram of bodyweight, in order to be consistent with past nitrogen balance research in bodybuilders. During the IAAO test day, the participants were randomly assigned to receive a test protein intake ranging from 0.1 to 3.5 grams per kilogram, consumed as eight hourly meals after a 12-hour overnight fast. Importantly, testing days occurred on non-training days and at least 48 hours after a training session, since resistance training is known to increase muscle protein synthesis rates for up to two days after training.

A small group of eight young, experienced male bodybuilders consumed 0.1 to 3.5 grams of protein per kilogram of bodyweight across several IAAO test days to determine their protein requirements. Testing was performed on non-training days, at least 48 hours after the last training session.

What were the findings?

Figure 2 summarizes the study findings. Protein oxidation declined with increasing protein intake up to an average intake of 1.7 grams of protein per kilogram of bodyweight, at which point it plateaued. This suggests that the average dietary protein requirement for the study sample was 1.7 grams of protein per kilogram of body weight per day. The 95% confidence interval ranged from 1.2 to 2.2 grams per kilogram, indicating that there was a lot of variability in the bodybuilders’ specific protein needs. Using lean body mass rather than body weight, the average protein requirement and upper end of the confidence interval become 2.0 and 2.5 grams per kilogram.

What does the study really tell us?

The study under review suggests that the daily protein requirement that would cover the needs of young and experienced bodybuilders is likely to be between 1.2 and 2.2 grams of protein per kilogram of bodyweight per day, with the participants of this study averaging 1.7 grams per kilogram. From these statistics, the authors then infer that almost all experienced bodybuilders would have their protein requirements satisfied by eating 2.2 grams per kilogram. This could be in error, though, as this seems to be based on their reported 95% confidence interval, which may not be the correct way to get at this number when it comes to inferential statistics.

There are many important qualifiers of this study that cannot be overlooked when extending its findings to other populations, including strength-trained athletes. The participants training for a minimum of four days per week, for about an hour each day, but the specifics of their training routines were not provided. It is common for bodybuilders to use split routines with a focus on relatively higher repetition zones (6-12 repetitions per set is recommended to maximize muscle growth). It remains unknown how the training variables (frequency, intensity, and volume) influence protein requirements.

This is the first study to assess the protein requirements of bodybuilders using the IAAO technique, precluding direct comparisons to other research. Additionally, the participants were all young men with a significant amount of lean body mass (averaged a FFMI that was 96% that of past Mr. USA bodybuilders from the pre-steroid era) and at least three years of consistent strength training experience. So these findings may not apply to women or people with less muscle mass and training experience. Finally, important training variables may have influenced protein requirements at the time of study, such as the frequency, volume, and intensity of the participants’ training sessions.

Two previous nitrogen balance studies involving bodybuilders with less than one year and more than three years worth of experience suggested that protein requirements were 1.4 and 0.8 grams per kilogram, respectively. This difference could be attributed, at least in part, to a greater rate of muscle mass gain in novices compared to the more experienced bodybuilders. However, this would not explain the difference with the study under review, since both the previous nitrogen balance study and the study at hand involved bodybuilders with at least three years of training experience. Rather, this difference may relate to the stages of protein metabolism discussed in the introduction. Namely, the nitrogen balance study resulted in an accommodation to the lower protein intake while the current study did not.

Evidence for this latter possibility comes from randomized controlled trials showing superior gains in lean body mass with a protein intake around the requirement observed in the study under review. For instance, a meta-analysis found that supplementing a baseline diet containing 1.2 grams per kilogram with an additional 50 grams of protein per day (on average) led to significantly greater increases in lean body mass and skeletal muscle growth than consuming less protein. Moreover, when stratified for training status, resistance-trained groups were shown to have greater increases in their lean body mass (+0.98 kilograms) with a higher protein diet than their untrained counterparts (+0.75 kilograms).

Finally, it is important to note that this study tested protein requirements at rest, when muscle protein synthesis would presumably be uninfluenced by a previous resistance training session. Consequently, protein requirements on a training day could be different than requirements on non-training days because of an increased protein need to repair damaged muscle tissue, something that warrants further investigation. Consistent with this idea, research in rats showed that IAAO-determined protein requirements were greater following an endurance training session than at rest.

The study under review suggests that young, male, experienced, highly muscular bodybuilders require an average of 1.7 grams of protein per kilogram of bodyweight per day on non-training days, separated from workout sessions by at least 48 hours. The participant characteristics and timing of the testing procedures are important qualifiers that may influence protein requirements and therefore cannot be overlooked when attempting to extend these findings to other populations.

The big picture

The United States and Canadian governments base their nutrition recommendations on the Dietary Reference Intake (DRI) values established by the Health and Medicine Division of the National Academy of Medicine (formerly known as the Institute of Medicine). The two most frequently relied upon DRIs are shown in Figure 3, the Estimated Average Requirement (EAR) and the Recommended Daily Allowance (RDA). The former represents the amount of a nutrient needed to meet the requirement of half of the population, while the latter represents an intake level that is sufficient to meet the nutrient requirement of 97-98% of the population. Both values are determined for a specific sex (male/female) and life-stage (categories of age plus pregnancy and lactation).

The current EAR and RDA for protein are 0.66 and 0.8 grams of protein per kilogram of bodyweight per day, respectively, for both men and women, and are based on nitrogen balance studies. Additionally, based exclusively on one nitrogen balance study in older adults beginning a resistance training routine and the previously discussed study in novice bodybuilders, the Health and Medicine Division concluded that “no additional dietary protein is suggested for healthy adults undertaking resistance or endurance exercise” (pg. 661).

An abundance of studies in resistance training populations have suggested that the recommendations put forth by the Health and Medicine Division are not evidence-based and greatly underestimate protein requirements of this population. Notwithstanding the limitations of the study at hand, it too suggests that resistance-trained people require substantially more protein than the 0.8 grams per kilogram RDA. Rather, the RDA should be at least 2.2 grams per kilogram (1.0 gram per pound or more).

Past research using the IAAO technique has also suggested that the protein RDA for healthy young men, older men, and older women should be greater than it is currently, around 1.2 grams per kilogram. NERD 19, Volume 1, How much protein does grandpa really need? discussed the study involving older men. Certainly, the protein RDA should be revisited for a variety of populations, especially considering research utilizing the IAAO technique. Even the Health and Medicine Division acknowledges that “on theoretical grounds, this method has advantages over other methods for estimating amino acid requirements, and is the chosen method for estimated amino acids requirements where data are available” (pg. 619).

The protein RDA for adults of all ages and activity levels is 0.8 grams of protein per kilogram of bodyweight. Several IAAO studies have suggested that this intake level for protein is not sufficient for everyone, instead supporting a requirement of 1.2 grams per kilogram for sedentary adults and at least 2.2 grams per kilogram for bodybuilders.

Frequently Asked Questions

Q. Do protein requirements change during a diet?

Even when dieting, eating 2.2 grams of protein per kilogram of bodyweight should be sufficient for most athletes unless they are working toward extreme leanness, like weight-category athletes and bodybuilders preparing for a show. Under these circumstances, there may be an advantage to eating more protein. One review of dieting athletes suggested that eating 2.3 to 3.1 grams of protein per kilogram of lean body mass was the most consistently protective intake range against losses of muscle mass, and this range was recommended for natural bodybuilding contest preparation.

Q. Will eating this much protein harm my kidneys or bones?

No to both. A meta-analysis in adults without established kidney disease suggests that eating more dietary protein causes an increase in GFR, serum urea, and urinary calcium excretion. However, it does not cause an increase in urinary albumin excretion, which is the most sensitive marker of kidney damage. These changes can be interpreted as normal physiological adaptive mechanisms induced by eating more protein. This conclusion is shared by the World Health Organization in their official report on protein when they state that "the most widely quoted potential problems of a high-protein diet relate to renal function and damage, but as discussed above the evidence for such claims in otherwise healthy individuals does not stand up to scrutiny" (pg 231).

As for bones, it is well documented that eating more protein results in an increase in urinary calcium excretion (see above meta-analysis). However, the excretion of calcium is only one piece of the puzzle, and randomized controlled trials have shown that eating more protein has no effect on whole-body calcium balance, probably because higher protein diets increase the absorption of dietary calcium to compensate for any losses. Furthermore, a meta-analysis of protein supplementation studies showed a small but significant benefit of increased protein intake on bone mineral density, possibly due to direct insulin-like growth factor-1 (IGF-1)-mediated anabolic influences on bone tissue.

What should I know?

Protein requirements for experienced bodybuilders, and possibly resistance-trained athletes in general, are likely to be greater than currently indicated by the RDA. The study at hand used the IAAO technique to determine protein requirements and found that, on average, the eight young, male bodybuilder participants required about 1.7 grams of protein per kilogram of bodyweight per day, with the corresponding “new” RDA being 2.2 grams per kilogram (1.0 gram per pound) or perhaps even more. Importantly, this finding was on a rest day separated by at least 48 hours from a previous training session, so protein requirements on training days may be different. The current study findings are supported by controlled trials showing greater muscle growth with protein intakes around the 1.7 to 2.2 gram per kilogram range, as compared to lower intakes.

You can also refer to our page on how much protein you need per day to figure out the exact number.