Low physical activity (e.g. reduced daily steps) reduces the ability of dietary amino acids (AA) to support muscle protein synthesis, leading to eventual muscle loss. Interrupting prolonged sitting with short bouts of intermittent exercise can improve carbohydrate and lipid metabolism, however its ability to sensitize skeletal muscle to dietary AA has yet to be investigated. PURPOSE: To determine the ability of interrupting prolonged sitting with practical ‘activity snacks’ to enhance the postprandial incorporation of dietary AA into myofibrillar protein. METHODS: As a subset of a larger study, twelve participants (7 males and 5 females; ~23y; ~40.0mlO₂/kg/min; ~25.1kg/m²; ~4676 steps/d) completed three 7.5 hr trials in a randomized order consisting of prolonged sitting (SIT), sitting with intermittent walking (WLK; 2 min at 3.1mph every 30 min) or sitting with intermittent squatting (SQT; 15 ‘chair stands with calf raise’ every 30 min). Mixed-macronutrient meals (~55:30:15% carbohydrate:fat:protein) were provided at 20% (breakfast) and 30% (lunch) of daily energy requirements to be consistent with Western feeding patterns. Meals were enriched to 15% with ring-[²H₅]phenylalanine or ring-[¹³C₆]phenylalanine to model the metabolic fate of dietary AA. Muscle biopsies taken at the end of each trial as well as at the beginning of trial 2 were used to determine change in AA enrichment (LC/MS/MS) in the myofibrillar protein fraction (ΔMyo). RESULTS: ΔMyo was 0.032±0.004MPE in SIT and tended to be greater with SQT (0.038±0.003; P=0.10) and WLK (0.047±0.006; P=0.06) according to a prior comparisons (paired one-tail T-test). Relative to SIT, effect sizes were large for WLK (ES=0.88; 95% CI -0.30 - 2.07) and moderate for SQT (ES=0.55; 95% CI -0.60 - 1.71). CONCLUSION: Interrupting prolonged periods of sitting with intermittent bouts of body weight-dependent activity has the potential to improve the utilization of dietary AA for de novo muscle protein synthesis in young healthy adults. Our results add to the evidence that reducing sedentary time through ‘activity snacks’ may help maintain muscle mass and quality. Future research should determine whether at risk populations (e.g. aging, obese) may obtain a greater benefit from this simple lifestyle modification.
Supported in part by an ACSM Research Endowment Grant.