Metabolic flexibility following resistance exercise and a high protein diet in older men: results from a 12-week randomized controlled trial

PURPOSE: Aging and sarcopenia are associated with metabolic inflexibility. This study investigated the effects of resistance exercise (RE) and a high protein diet (PRO) on metabolic flexibility (the ability to adjust rates of substrate oxidation to changes in fuel availability) in older men. METHODS: In a pooled groups analysis, 33 healthy older men [(mean±SE) age: 67±1 years; BMI: 25.4±0.4 kg/m²] were randomized to either RE (2×/week; n=17) or no exercise (NE; n=16), and either high protein diet [∼1.6 g/kg/day (∼25% of energy intake (EI))] via twice daily (25 g) whey protein supplementation (PRO; n=17) or control (CON, 2 × 23.75 g maltodextrin/day; n=16). An exploratory sub-analysis was also conducted between RE+CON (n=8) and RE+PRO (n=9). At baseline and 12 weeks, participants resided in whole-room indirect calorimeters for 24 h for measurement of metabolic flexibility via changes in relative substrate utilization [non-protein respiratory quotient (npRQ)] under different conditions (fasting sleep to awake, step exercise, and 2-h postprandial meal consumption, and peak step exercise to exercise end). RESULTS: Compared to NE, RE significantly increased (indicating medium-to-large effects on improved metabolic flexibility) ΔnpRQ (awake-sleep) (+0.02±0.004 vs. 0.00±0.05, p=0.01, f=0.48), and ΔnpRQ (steady state exercise-sleep) (p ≤0.045) and ΔnpRQ (peak exercise-exercise end) (p ≤0.04, f=0.39-0.64) for two step exercise bouts performed ∼2 h postprandially. Compared to CON, PRO increased ΔnpRQ (steady state-sleep) for one step exercise bout (+0.02±0.01 vs. -0.002±0.01, p=0.047, f=0.39). No significant differences occurred between the RE+CON and RE+PRO groups (p ≥0.06). CONCLUSION: In older men, RE improved metabolic flexibility. PRO had a limited benefit. No synergistic effects were observed.