New Protein Requirements for Bodybuilders

A new study sought to assess protein requirements at the whole-body level using the IAAO technique in trained subjects.


The RDA for protein is established at 0.8g/kg as the minimum protein intake to offset protein deficiency (net nitrogen losses) for 97.5% of the healthy population above 19 years (1). Believe it or not, the RDA was even deemed appropriate for healthy adults undertaking resistance or endurance exercise (2).

The RDA represents the estimated average requirement plus 2 standard deviations (3), determined from selected nitrogen-balance studies of which very few were performed in older individuals (4,5).

Nitrogen balance studies suffer from several methodologic concerns (6-10). In addition to the impractical need for repeated 7- to 10-d adaptation periods necessary to produce accurate NB data for each of the several protein intakes needed to determine the requirement (6), the use of a linear regression line with a greater residual error for analyzing nonlinear data is not a good fit for either NB or oxidation data (6,8,11).

There is also an over- or underestimation of nitrogen intake and excretion, respectively (6,12). Furthermore, NB may be achievable at low protein intakes for the brief study durations often used because of more efficient amino acid (AA) utilization, reduced turnover rates, and/or accommodation (6,13,14,15).

Better methods such as the indicator amino acid oxidation (IAAO) technique show that the RDA underestimated protein needs by 30-50% (16,17,18). The IAAO technique identifies the plateau in AA oxidation that corresponds to the maximum rate of whole-body protein synthesis.

When dietary protein is inadequate the oxidation of all AAs, including the indicator AA, will be substantial (6). With increasing dietary protein oxidation of the indicator AA will decrease because more AAs are being incorporated into body protein, and once the dietary requirement is met there is no further change in the oxidation of the indicator AA and the resulting ‘‘breakpoint’’ is thought to be the requirement (6,10).

Isotope tracer methodology is considered to be a far more accurate technique, and the IAAO technique was considered an acceptable method to assess protein requirements back in 2005 (6,19).

A protein intake of 0.93-1.2 g/kg/day, exceeding the current requirement by as much as 50%, was unveiled in men using the using the indicator amino acid oxidation technique (18). The results were comparable with those estimated by the application of a biphase linear regression model to the data from nitrogen balance studies (0.91 and 1.0 g/kg/d) (18). 

Nevertheless, the few available NB studies on bodybuilders are quite variable, and even very positive NB (3.8–20 g/d) with protein intakes of 1.8–2.7 g/kg/d observed in men engaged in a rigorous strength-training (ST) program do not result in the expected fat-free mass (FFM) accrual (3,6,19,20). For example, a positive NB (12–20 g N/d) at a protein intake of 2.8g/kg should produce 300–500 g lean mass gain/d, however this was not observed (20).

Stable isotope identified 4 different states of protein metabolism studies (6,14,15,21,22):
1) ‘‘protein deficiency,’’ defined as the maximal reduction in protein synthesis to all but the essential organs;
2) ‘‘accommodation,’’ in which balance is achieved via a decrease in physiologic relevant processes;
3) ‘‘adaptation,’’ in which optimal growth, interorgan AA exchange, and immune function are present; and
4) ‘‘excess,’’ which is characterized by AA oxidization for energy and nitrogen excretion via urea, resulting in no further stimulation of protein synthesis (15).

Regardless of age, when protein intakes near the current RDA are combined with Strength training, accommodation results through increased nitrogen utilization efficiency and lower whole-body protein synthesis rates rather than adaptation (6,19,21,23).

Current dietary protein recommendation for bodybuilders varies widely, from the RDA of 0.85g/kg/d (16) to as much as 2.0 g/kg/d (24).

During exercise there is an increase in aminoacid oxidation (1-5% of the total energetic cost of exercise), increased catabolism and increased muscle protein synthesis (25,26,27). Under-recovery is also another concern (25,28).

Protein requirements for bodybuilders in nontraining days

A new study sought to assess protein requirements at the whole-body level using the IAAO technique, in 8 individuals who had undergone regular bodybuilding training for more than 3 years (6). Subjects had a mean 84kg of body mass, 72.4 kg of LBM, and a fat free mass index of 24.

They measured resting oxidation of L-[1-13C]phenylalanine on a nontraining day for a range of protein intakes above and below the current protein requirement as a reverse proxy for whole-body protein synthesis. The rationale is that as dietary protein intake increases, the rate of L-[1-13C]phenylalanine oxidation decreases, eventually reaching a plateau (6).

A plateau was observed at 1.7 g/kg (protein requirement). By using these data, researchers estimated that the RDA for the bodybuilders would be 2.2g/ kg, which is 2.6-fold greater than the current RDA for protein (6) which is 23% greater than the results reported previously for bodybuilders with the use of the NB method (3,19).

A meta-analysis of 22 RCTs that included 680 subjects involved in resistance type exercise also indicates a benefit from dietary protein in excess of the current RDA in both young and older individuals (29). Specifically, a supplement of 50 ± 32 g protein/d in addition to a regular dietary intake of 1.2 g/kg (daily protein intake of 1.8g/kg) increased fat free mass (FFM) and strength gains (29).

Another study also found that a daily protein intake of 1.9 compared with 1.3 g/kg resulted in increased skeletal muscle mass, strength, and muscle fiber size in young men (aged 22 ±1 y) after 12 weeks (30).

Training status

This study used trained subjects and therefore results should not be necessarily applied to novice subjects because it is know that the rate of muscle mass gain decreases with training experience (6).

For example, using the old NB method, novice subjects with less than 1 years of training experience had a requirement of 1.4g/kg of protein (3,20) and trained subjects with more than 3 years of experience had  a requirement of 0.82g/kg (20). This 70% greater protein need in novice compared with elite bodybuilders was attributed, at least in part, to a greater rate of muscle mass accrual in novices (6).

Interestingly, the difference in FFM between sedentary and trained subjects such as used in this study can be of 16kg in favor of trained subjects (6). 

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