Effect of protein ingestion on energy expenditure and substrate utilization after exercise in middle-aged women

Research suggests that ingesting protein after resistance exercise (RE) increases muscle protein synthesis and results in greater muscle gains. The effect on energy expenditure and substrate utilization, however, is unclear. This study evaluated the effect of RE and postexercise protein on recovery energy expenditure and substrate utilization in 17 women (age 46.5 ± 1.2 y). A whey-protein supplement (120 kcal, 30 g protein) was ingested immediately after 1 bout of RE (PRO) and a noncaloric placebo after another (PLA). VO ₂ and respiratory-exchange ratio (RER) were measured before and for 120 min after each exercise session. RE resulted in a significant increase in VO₂ that persisted through 90 min of recovery (P < 0.01) and was not affected by protein supplementation. RE significantly lowered RER, resulting in an increase in fat oxidation for both PLA and PRO (P < 0.01). For PRO, however, RER returned to baseline values earlier than for PLA, resulting in a reduced fat-oxidation response (P = 0.02) and earlier return to preexercise baseline values than for PLA. Substrate utilization was significantly different between conditions (P = 0.02), with fat contributing 77.76% ± 2.19% for PLA and 72.12% ± 2.17% for PRO, while protein oxidation increased from 17.18% ± 1.33% for PLA to 20.82% ± 1.47% for PRO. Postexercise protein did not affect energy expenditure, but when protein was available as an alternate fuel fat oxidation was diminished. Based on these findings it might be beneficial for middle-aged women to delay protein intake after RE to maximize fat utilization.