Walking or body weight squat "activity snacks" increase dietary amino acid utilization for myofibrillar protein synthesis during prolonged sitting

Interrupting prolonged sitting with intermittent exercise enhances postprandial glycemic control but has unknown effects on sensitizing skeletal muscle to dietary amino acids. We hypothesized that brief walking or body weight squats would enhance the utilization of dietary phenylalanine for myofibrillar protein synthesis (MyoPS) during prolonged sitting. Participants (7 males and 5 females; ∼23 yr; ∼25.1 kg/m²; ∼7,300 steps/day) completed three 7.5-h trials consisting of prolonged sitting (SIT) or sitting with intermittent (every 30 min) walking (WALK) or body weight squatting (SQUAT). Two mixed-macronutrient meals (∼55:30:15% carbohydrate:fat:protein), enriched with l-[ring-²H₅]phenylalanine or l-[ring-¹³C₆]phenylalanine, were provided to mimic breakfast and lunch. Tracer incorporation into myofibrillar protein was determined from the vastus lateralis with MyoPS estimated using plasma enrichment as precursor surrogate. Phosphorylation of candidate anabolic signaling proteins was determined by immunoblotting. There was no difference between conditions (P ≥ 0.78) in the time course or area under the curve for plasma phenylalanine enrichment. MyoPS was greater (P < 0.05, weighted planned comparison) in SQUAT (0.103 ± 0.030%/h) and WALK (0.118 ± 0.037%/h) compared with SIT (0.080 ± 0.032%/h). When compared with SIT, there were moderate-to-large effect sizes, respectively, for SQUAT [effect size (ES) = 0.75; 95% CI -0.10-1.55] and WALK (ES = 1.10; 95% CI 0.20-1.91). Fold change in rpS6^Ser240/244 phosphorylation was greater in SQUAT compared with SIT (7.6 ± 2.7 vs. 1.6 ± 0.45-fold, P < 0.05) with no difference (P ≥ 0.21) in any other targets measured (4E-BP1^Thr37/46, eEF2^Thr56, mTOR^Ser2448, ERK1/2^{Thr202/Tyr204}). Interrupting prolonged sitting with short "activity snacks" improves the utilization of dietary amino acids for MyoPS. The long-term impact of this practical lifestyle modification for muscle mass or quality should be investigated.NEW & NOTEWORTHY Prolonged sitting can impair postprandial glycemia, lipidemia, and insulin sensitivity regardless of previous health status. We demonstrate that interrupting prolonged sitting with brief periods of activity, such as body weight squats or short bouts of walking, improves the efficiency of dietary amino acid utilizations for muscle contractile protein synthesis. This further emphasizes the importance of minimizing sedentary time to improve the postprandial metabolism of all macronutrients.