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.

BACKGROUND

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.

OBJECTIVES

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.

METHODS

In a randomized, double-blind, parallel-group design, 24 young (23 ± 4 y) and 24 older (69 ± 4 y) recreationally active adults (equal sex distribution) underwent 3 d of unilateral knee immobilization (leg casting) and received a leucine [group of adult study participants who supplemented with 5 g of leucine 3 × daily with each main meal during 3 d of unilateral knee immobilization by means of a full leg cast (LEU)] or energy-matched carbohydrate [group of adult study participants who supplemented with 5 g of carbohydrate 3 × daily with each main meal during 3 d of unilateral knee immobilization by means of a full leg cast (PLA)] supplement. Preimmobilization and postimmobilization, quadriceps muscle cross-sectional area (CSA) was assessed in the immobilized (IM) and nonimmobilized (NO-IM) leg by computed tomography. MPS rates were assessed in both legs during immobilization via 2H2O coupled with saliva, blood, and muscle biopsy sampling.

RESULTS

In young and older adults, MPS rates were ∼15% and ∼23% lower in the IM compared with NO-IM leg (1.28 ± 0.29 compared with 1.50 ± 0.26 and 1.10 ± 0.16 compared with 1.46 ± 0.28%/d, respectively; leg: both P < 0.001), with no differences between LEU compared with 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 mm2 and from 5813 ± 1092 to 5750 ± 1096 mm2, respectively; leg × time interaction: both P < 0.001), with no differences between LEU compared with PLA (treatment: P = 0.374 and P = 0.998). IM leg MPS rates were lower in older compared with young adults [difference: -0.18 (95% confidence interval: -0.31, -0.04) %/d; P = 0.013]. No differences were observed in the absolute (mm2) or relative (%) decline in CSA between young and older adults (both P > 0.05).

CONCLUSIONS

Leucine supplementation does not attenuate the decline in daily MPS rates or muscle mass during short-term limb immobilization in young or older adults. Clinical Trial Register No. (Netherlands Trial Register): NL-OMON45771.