Comparison of the effects of weight loss from a high-protein versus standard-protein energy-restricted diet on strength and aerobic capacity in overweight and obese men

Purpose: To compare the effects of two low-fat, hypoenergetic diets differing in carbohydrate-to-protein ratio, on strength and aerobic capacity measures in overweight and obese men. Methods: In a parallel design, 56 men (age, 45.5 ± 8.7 years; BMI, 33.6 ± 3.9 kg/m²) were randomly assigned to a low-fat, energy-restricted diet (7,000 kJ/day) with either high protein (HP: protein/carbohydrate/fat % energy, 35:40:25) or standard protein (SP, 17:58:25). Body weight, body composition, muscle strength and aerobic capacity were assessed at baseline and after 12 weeks. Results: Forty-two participants completed the study (HP, n = 21; SP, n = 21). Both groups experienced similar reductions in body weight (HP, -10.7 ± 5.3 kg [-9.8%]; SP, -8.7 ± 3.5 kg [-8.4%]) and fat-free mass (HP, -2.8 ± 3.6 kg; SP, -3.2 ± 2.7 kg; P < 0.001 time; P > 0.14 time × group interaction). There was a trend for a greater reduction in fat mass in the HP diet group, (-7.7 ± 4.3 kg [-21.2%] vs. -5.4 ± 3.3 kg [-15.1%]; P < 0.001 time; P = 0.06 time × group interaction). Absolute peak oxygen uptake did not change in either group (P = 0.39 time; P = 0.50 time × group interaction). Overall, in both groups, relative peak oxygen uptake increased (2.9 ± 2.8 ml kg^-1 min^-1 [8.9%]), peak isometric knee extensor strength increased (14.1 ± 35.7 Nm [7.1%]) and peak handgrip strength decreased (-1.6 ± 4.1 kg [-3%]) (P ≤ 0.02 time for all), with no diet effect (P ≤ 0.23 time × group interaction). Conclusion: In overweight and obese men, both a HP and SP diet reduced body weight and improved body composition with similar effects on strength and aerobic capacity.