Two weeks of reduced activity decreases leg lean mass and induces ‘anabolic resistance’ of myofibrillar protein synthesis in healthy elderly

Background: Alterations in muscle protein metabolism underlie age-related muscle atrophy. During periods of muscle disuse, muscle protein synthesis is blunted and muscle atrophy occurs in young and old. The impact of a short reduction in physical activity on muscle protein metabolism in older adults is unknown.

Purpose: To investigate the impact of 14 days of reduced daily steps on fasted and fed-state rates of myofibrillar protein synthesis (MPS) to provide insight into the mechanisms for changes in muscle mass and markers of metabolic health.

Methods: Prior to and following 14 d of reduced daily step-count, ten, healthy older adults (72±1 yr) underwent measures of insulin sensitivity, muscle strength, physical function and body composition. Using a primed constant infusion of L-[ring-13C6] phenylalanine with serial muscle biopsies, basal, postabsorptive and postprandial rates of MPS were determined before and after the 14 d intervention.

Results: Daily step-count was reduced by ∼76% to 1413±110 steps/d. Leg fat-free mass was reduced by ∼3.9% (P < 0.001). Postabsorptive insulin resistance was increased by ∼12% and postprandial insulin sensitivity reduced by ∼43% following step-reduction (P < 0.005). Concentrations of TNF-α and CRP were increased by ∼12 and 25%, respectively, following step-reduction (P < 0.05). Postprandial rates of MPS were reduced by ∼26% following the intervention (P = 0.028) with no difference in postabsorptive rates.

Conclusion: The present study demonstrates that 14 days of reduced steps in older adults induces small but measurable reductions in muscle mass that appear to be underpinned by reductions in post-prandial MPS and are accompanied by impairments in insulin sensitivity and systemic inflammatory markers and postprandial MPS.