Increased Protein Retains Nitrogen during Energy Restriction-Sports

A study by Walberg and colleagues examined the effects of varied dietary strategies on body composition, nitrogen retention, and muscular endurance in weight lifters during periods of reduced energy intake. After a I-week maintenance period to normalize diet and exercise programs, 19 experienced male weight lifters were assigned to one of three groups-

A control diet,
A moderate protein (0.8 g/kg/day),
A high protein (0.6 g/kg/day),
High carbohydrate hypo energy diet or
oderate carbohydrate hypoenergy diet.

Both hypo energy diets provided 18 kcal/kg/day. Following the I-week experimental period, the researchers discovered that, although body composition did not change, nitrogen balance significantly differed between the two hypoenergy groups. Specifically, the moderate protein group had a balance of -3.19 g/day, whereas the nitrogen balance in the high protein group was a positive 4.13 g/day. In both groups, isometric muscular endurance was unaffected in the biceps, but quadriceps endurance declined in the high protein group. In conclusion, a hypoenergy diet providing twice the recommended daily allowance (RDA) of protein was more effective in preserving body protein than was a higher carbohydrate diet providing the RDA for protein.

Commonly, athletes and exercise enthusiasts limit caloric intake in hopes of losing body weight, either for athletic pursuits or aesthetic intentions. Of course, the goal is to reduce body fat while maintaining or even increasing lean body mass. Because nitrogen balance measures the body's anabolic (or catabolic) condition, it is reasonable to conclude that a positive nitrogen balance is conducive to maximizing muscular mass.

The study by Walberg indicates that retaining body protein is possible even under conditions of intense energy restriction (a 70-kg male would receive only a meager 1260 kcal daily). Because weight lifters and those involved in anaerobic sports (gymnastics, wrestling) do not have the luxury of a high-energy expenditure, they must rely primarily on dietary restriction to decrease body fat levels. Therefore, it appears that elevating protein intake is an efficient method for avoiding the potentially detrimental effects of incurring lean tissue losses while dieting. Furthermore, it is likely that including resistance activity may lead to net muscle protein synthesis.

In contrast, the lack of sufficient carbohydrate (50% of calories) in the high-protein diet may diminish muscular endurance. This can be attributed to the corresponding glycogen depletion that presumably occurred in this group. Obviously a factor that cannot be over­looked, athletes must carefully consider this drawback when initiating a restrictive dietary regimen. Perhaps a gradual reduction in energy intake or a less drastic caloric decline can offset the potential performance decrements associated with this diet.

Finally, the authors also surmise that the combination of a higher protein intake combined with weight training may provide enough amino acids to allow net protein synthesis, while the moderate proteinlhigh carbohydrate diet led to the catabolization of more protein for energy. They concluded that this is consistent with the theory that the high-protein content of a low calorie diet maintains protein synthesis while energy needs are met primarily through the oxidation of fat. Additionally, it underscores the importance of increasing protein intake during energy restriction to conceivably promote losses in body fat.

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