Impact of timing of protein intake on nitrogen balance in exercising older individuals on a hypercaloric diet

Abstract

We have previously shown that in older adults, consumption of protein in the form of chocolate milk immediately after exercise enhances nitrogen balance (NBAL) when energy balance is maintained. Since it is known that hypercaloric diets increase nitrogen (N) retention, it is important to know if the timing of protein intake after aerobic exercise provides further increases in N retention compared to the consumption of carbohydrate only post exercise. PURPOSE: To investigate if consumption of protein and carbohydrate (PRO + CHO) immediately after exercise as opposed to earlier in the day can improve NBAL in older individuals consuming a hypercaloric diet. METHODS: In a randomized cross-over design, subjects completed two separate 3-day exercise and nutrition interventions. Exercise (60 minutes of stationary cycling at 55% of VO2max) was performed daily at 4:30 PM. Diets were hypercaloric (calculated at +15% daily intake), with a PRO+CHO or carbohydrate only (CHO) drink consumed at 10 am and the opposite drink consumed after exercise (5:30 PM). Both diets (1.2 g protein/kg bodyweight, 30% fat, and balance as carbohydrate) were isonitrogenous and isocaloric with only the timing of the drinks differing. A 24 hour stay in a metabolic chamber confirmed positive energy balance while 24-hour urine collections determined NBAL. RESULTS: The 3-day mean NBAL was not significantly different (p=.0881) (n=6) between the CHO trial (.970 ± .517 g N) and the PRO + CHO trial (1.659 ± .430 g N) although a trend toward increased NBAL with PRO+CHO was apparent. The mean energy balance was not significantly different (p=.2906) between the CHO trial (+13.09 ± 1.94%) and the PRO + CHO trial (+ 14.28 ± 1.75%). Further analyses comparing the positive energy balance cohort to previously completed negative, and even energy balance cohorts distinguished the role of energy balance and timing of nutrition effects. CONCLUSION: Older individuals in positive energy balance do not maintain a significantly more positive NBAL balance by consuming protein after aerobic exercise as opposed to earlier in the day although energy balance does change the effect of protein timing on NBAL.