Effect of an Acute Prolonged Overnight Fast on Skeletal Muscle Insulin and Anabolic Sensitivity in Healthy Men

Background: Repeated prolonged overnight fasting enhances anabolic sensitivity and preserves myofibrillar protein synthesis. Whether a single fasting episode can enhance dietary protein-derived amino acid incorporation into skeletal muscle protein remains unclear. Objective: To investigate the effect of a single 16-h compared with 10-h overnight fast on postprandial insulin and anabolic sensitivity in humans. Methods: Randomized, crossover study. On 2 occasions, healthy participants (n = 9; age 22.6 ± 3.5 y; body mass index: 24.0 ± 2.5 kg/m²; homeostasis model assessment of insulin resistance 1.00 ± 0.54) ingested a liquid meal, which consisted of specifically produced, intrinsically [1-¹³C]phenylalanine–labeled milk protein (0.35 g/kg body mass, bm) and dextrose (0.8 g/kg bm), dissolved in 6 mL/kg bm water, after either a 10-h (Control) or 16-h (prolonged overnight fast trial, Long OF) overnight fast. Skeletal muscle and blood samples were obtained before, during, and 3 h after meal ingestion. Results: Postprandial incorporation of dietary protein-derived [1-¹³C]phenylalanine into myofibrillar protein did not differ between Control and Long OF (0.018 ± 0.005 compared with 0.022 ± 0.005 mole percent excess, d<inf>s</inf> = 0.22, P = 0.56). Forearm branched amino acid uptake incremental area under the curve [D (95% confidence interval, CI) 103 (-1618, 1823) µmol/min^.180 min, d<inf>s</inf> = 0.05, P = 0.892] and glucose uptake [D (95% CI) 24.7 (-25.3, 74.8) µmol/min, ?²p = 0.16, P = 0.280] also did not differ between treatments, but the postprandial serum insulin response [D (95% CI) -7.4 (-14.4, -0.4) mIU/L, ?²p = 0.42, P = 0.042] was lower in Long OF. Conclusions: A single prolonged overnight fast does not enhance postprandial skeletal muscle insulin sensitivity or dietary protein-derived amino acid incorporation into skeletal muscle protein in healthy, young adults. Whether susceptible populations, such as older, insulin-resistant individuals, may benefit from a more prolonged overnight fast warrants investigation.This study was registered at clinicaltrials.gov as NCT05420181.