Lower extremity muscle size and strength and aerobic capacity decrease with caloric restriction but not with exercise-induced weight loss

Caloric restriction (CR) results in fat loss; however, it may also result in loss of muscle and thereby reduce strength and aerobic capacity (V̇O _{2 max}). These effects may not occur with exercise-induced weight loss (EX) because of the anabolic effects of exercise on heart and skeletal muscle. We tested the hypothesis that CR reduces muscle size and strength and V̇O_{2 max}, whereas EX preserves or improves these parameters. Healthy 50- to 60-yr-old men and women (body mass index of 23.5-29.9 kg/m ²) were studied before and after 12 mo of weight loss by CR (n = 18) or EX (n = 16). Lean mass was assessed by dual-energy X-ray absorptiometry, thigh muscle volume by MRI, isometric and isokinetic knee flexor strength by dynamometry, and treadmill V̇O_{2 max} by indirect calorimetry. Both interventions caused significant decreases in body weight (CR: -10.7 ± 1.4%, EX: -9.5 ± 1.5%) and lean mass (CR: -3.5 ± 0.7%, EX: -2.2 ± 0.8%), with no significant differences between groups. Significant decreases in thigh muscle volume (-6.9 ± 0.8%) and composite knee flexion strength (-7.2 ± 3%) occurred in the CR group only. Absolute V̇O_{2 max} decreased significantly in the CR group (-6.8 ± 2.3%), whereas the EX group had significant increases in both absolute (+15.5 ± 2.4%) and relative (+28.3 ± 3.0%) V̇O_{2 max}. These data provide evidence that muscle mass and absolute physical work capacity decrease in response to 12 mo of CR but not in response to a similar weight loss induced by exercise. These findings suggest that, during EX, the body adapts to maintain or even enhance physical performance capacity.