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

This is the authors' abstract. We don't add key points for this paper.

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 <0.001), with no differences between LEU versus PLA treatments (treatment: P =0.932 and P =0.742, respectively). CSA decreased by ∼1.2 and ∼1.1% in the IM leg in young and older adults (from 7162±1148 to 7076±1129 mm 2 and from 5813±1092 to 5750±1096 mm 2 , respectively; leg × time interaction: both P <0.001), with no differences between LEU versus PLA (treatment: P =0.374 and P =0.998). IM leg MPS rates were lower in older versus young adults (difference: -0.18 (95% CI: -0.31, -0.04) %•day -1 ; P =0.013). No differences were observed in the absolute (mm 2 ) or relative (%) decline in CSA between young and older adults (both P >0.05). Leucine supplementation does not attenuate the decline in daily MPS rates or muscle mass during short-term limb immobilization in young or older adults. This trial was prospectively registered at Netherlands Trial Register: https://onderzoekmetmensen.nl/en/trial/45771 as ID: NL-OMON45771