Does protein right after exercise improve nitrogen balance during a low-calorie diet?

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The researchers had previously found that, in older adults maintaining energy balance, protein immediately after aerobic exercise improved nitrogen balance compared with protein earlier in the day. Because some older individuals consume lower-calorie diets, they wanted to know whether the benefit also occurs during calorie restriction. The study tested whether protein immediately after aerobic exercise, rather than earlier in the day, could improve nitrogen balance in older individuals on a hypocaloric diet.

In a randomized crossover design, healthy sedentary men (n=2; age 67.0±1.0 years; BMI 27.4±0.3 kg/m²) and women (n=8; age 63.0±1.8 years; BMI 22.3±0.6 kg/m²) completed two separate 3-day exercise and nutrition interventions. Each day they cycled on a stationary bicycle for 60 minutes at 55% of VO2max. The diets were hypocaloric, with daily intake reduced by 15%; participants had either a protein-plus-carbohydrate (PRO+CHO) or carbohydrate-only (CHO) drink in the morning and the opposite drink after exercise. Both diets contained 15% protein, 30% fat and 55% carbohydrate, with the same nitrogen and calorie content; only the timing of the drinks differed. A 24-hour stay in a metabolic chamber confirmed negative energy balance, and 24-hour urine collections determined nitrogen balance.

The 3-day mean nitrogen balance was not significantly greater in the PRO+CHO trial (0.097±0.526 g N) than in the CHO trial (-0.070±0.520 g N; p=0.280). The researchers concluded that, during negative energy balance, older individuals did not maintain a significantly more positive nitrogen balance by consuming protein after aerobic exercise instead of earlier in the day. The results differed from their previous work and indicated that energy balance is an important determinant of the anabolic effect of protein feeding.