Long-term effects of vitamin D supplementation on myofibrillar protein synthesis rates in resting and exercised fed skeletal muscle of healthy adults

Background Vitamin D can promote skeletal muscle mass and strength, but the mechanisms remain unclear. We tested the hypothesis that long-term vitamin D enhances postprandial postexercise muscle protein synthesis (MPS) rates in healthy adults. Methods Seventeen healthy adults (age: 25±2 y, body mass index 24.9±1.1 Kg·m -2 ) underwent experimental visits pre and post 12 weeks of vitamin D (3000 IU·day -1 ; VITD; n =9) or placebo (PLA; n =8) supplementation in a randomised, parallel group, double-blind design. During each visit participants received a primed, continuous infusion of L-[ ring - 2 H 5 ]-phenylalanine and performed unilateral leg resistance exercise, before ingesting 22 g whey protein. Blood and m. vastus lateralis samples (t=-120, 0, 60 and 240 min) were collected throughout. Results Fasting vitamin D concentration remained unchanged in PLA pre- to post-intervention, but increased in VITD by 75±15%. Vitamin D augmented the rise in leucine and essential amino acid concentrations at 60 min of the postprandial postexercise period compared with placebo ( P <0.01). MPS increased over the 0 – 240 min period, from basal to postprandial postexercise (to 0.047±0.006 pre-intervention and 0.044±0.010 %·h -1 post-intervention in PLA), but was unaffected by vitamin D (0.072±0.008 pre-intervention and 0.066±0.009 %·h -1 post-intervention; interaction P >0.05). Intriguingly, MPS was 0.077±0.015 %·h -1 greater between 0-60 minutes of the postprandial postexercise period in VITD, compared with the same timepoint to pre-intervention, whereas PLA remained unchanged (interaction P <0.01). mTOR phosphorylation increased from basal to postprandial postexercise, but was unaffected by vitamin D. Conclusion In healthy adults, chronic, high-dose vitamin D supplementation does not augment MPS following exercise and nutrition. However, vitamin D may promote an earlier anabolic response, with potential applications to ageing, illness or injury.