Leucine supplementation does not attenuate the decline in daily muscle protein synthesis rates or preserve leg muscle mass during leg immobilization in young or older adults: a double-blind randomized trial

Muscle disuse leads to muscle atrophy that has been attributed to declines in basal and postprandial muscle protein synthesis (MPS) rates. Leucine regulates MPS and may attenuate disuse-induced declines in MPS rates and muscle mass. The purpose of this study was to evaluate the capacity of leucine supplementation to attenuate disuse-induced declines in MPS rates and muscle mass in young and older adults. In a randomized, double-blind, parallel-group design, 24 young (23±4y) and 24 older (69±4y) recreationally active adults (equal sex distribution) underwent 3 days of unilateral knee immobilization (leg casting) and received a leucine (LEU) or energy-matched carbohydrate (PLA) supplement (5g, 3×daily). Pre- and post-immobilization, quadriceps muscle cross-sectional area (CSA) was assessed in the immobilized (IM) and non-immobilized (NO-IM) leg by computed tomography. MPS rates were assessed in both legs during immobilization via 2 H 2 O coupled with saliva, blood, and muscle biopsy sampling. In young and older adults, MPS rates were ∼15 and ∼23% lower in the IM versus NO-IM leg (1.28±0.29 versus 1.50±0.26 and 1.10±0.16 versus 1.46±0.28 %•day -1 , respectively; leg: both P