No independent or combined effects of vitamin D and conjugated linoleic acids on muscle protein synthesis in older adults: a randomized, double-blind, placebo-controlled clinical trial

Abstract

Background: Aging is associated with skeletal muscle anabolic resistance (i.e., reduced muscle protein synthesis during anabolic conditions such as hyperaminoacidemia). The results from studies conducted in cell culture systems and animals suggest that both vitamin D and conjugated linoleic acids (CLAs) stimulate muscle protein synthesis.

Objectives: To conduct a randomized, double-blind, placebo-controlled clinical trial to determine the independent and combined effects of dietary vitamin D and CLA supplementation on myofibrillar protein synthesis rates in sedentary older adults.

Methods: Thirty-two sedentary, older adults were randomized to receive either: 1) 2000 IU vitamin D-3 (Vit D) per day; 2) 4000 mg CLA per day; 3) both Vit D (2000 IU/d) and CLA (4000 mg/d); or 4) placebo for 8 wk. Myofibrillar protein synthesis rates were evaluated by using intravenous [ring-2H5]phenylalanine infusion in conjunction with muscle biopsies during basal, postabsorptive conditions and during combined amino acid and insulin infusion before and after the supplementation period.

Results: Before the intervention, basal myofibrillar protein synthesis rates were not different among groups (Placebo: 0.033 ± 0.003; Vit D: 0.034 ± 0.002; CLA: 0.029 ± 0.005; Vit D + CLA: 0.038 ± 0.005 %·h-1), and hyperinsulinemia-hyperaminoacidemia increased myofibrillar protein synthesis rates by ∼35%. Compared with placebo, neither Vit D nor CLA nor combined Vit D + CLA supplementation affected the basal myofibrillar protein synthesis rates (placebo: 0.040 ± 0.004%/h; Vit D: 0.044 ± 0.006%/h; CLA: 0.039 ± 0.006%/h; Vit D + CLA: 0.040 ± 0.007%/h) or the hyperinsulinemia-hyperaminoacidemia-induced increase in myofibrillar protein synthesis (percentage increase from basal before and after the interventions: placebo, 30 ± 11 and 36 ± 11; Vit D, 38 ± 8 and 34 ± 10; CLA, 50 ± 14 and 51 ± 16; Vit D + CLA, 29 ± 15 and 35 ± 8).

Conclusions: Vitamin D and/or CLA supplementation, at the doses provided in our study, does not have muscle anabolic effects in sedentary older adults.The study was registered at clinicaltrials.gov (NCT03115775).

Keywords: aging; conjugated linoleic acids; dietary supplements; protein synthesis; sarcopenia; skeletal muscle; vitamin D.

Copyright © The Author(s) on behalf of the American Society for Nutrition 2020.

Figures

FIGURE 1

Treatment-induced changes in serum vitamin D-3 concentration (top) and contribution of linoleic acid to total plasma triglyceride fatty acid content (bottom) in the placebo (n = 8), vitamin D (Vit D, n = 9), conjugated linoleic acid (CLA, n = 7), and combined Vit D + CLA (n = 8) supplementation groups. Values are means ± SEM.

FIGURE 2

Myofibrillar protein synthesis rates during basal conditions (white bars) and during the hyperinsulinemic–euglycemic clamp procedure with concomitant amino acid infusion (black bars) before (top) and after (middle) the interventions, and the anabolic response (bottom), assessed as the ratio of myofibrillar protein synthesis rate during the hyperinsulinemic–euglycemic clamp to the myofibrillar protein synthesis rate during basal conditions, before (horizontally striped bars) and after (diagonally striped bars) the interventions in the placebo (n = 8), vitamin D (Vit D, n = 9), conjugated linoleic acid (CLA, n = 7), and combined Vit D + CLA (n = 8) supplementation groups. Values are mean ± SEM. Repeated measures ANOVA with group and condition (basal and clamp) as factors was used to evaluate the effect of hyperaminoacidemia–hyperinsulinemia among the groups before and after the interventions. *Different from corresponding basal value, P < 0.05. ANCOVA with the pretreatment value as covariate was used to evaluate treatment effects. No differences in treatment responses were observed among groups.

FIGURE 3

Hand-grip strength before (white bars) and after (black bars) the interventions in the placebo (n = 8), vitamin D (Vit D, n = 9), conjugated linoleic acid (CLA, n = 7), and combined Vit D + CLA (n = 8) supplementation groups. Values are mean ± SEM. One-factor ANOVA was used to compare outcomes among groups at baseline. ANCOVA with the pretreatment value as covariate was used to evaluate treatment effects. No differences in treatment responses were observed among groups.