Effects of Vitamin D Supplementation on Muscle Protein Synthesis

Overview

Background: Healthy adults maintain muscle tissue by continuously building up and breaking down muscle proteins throughout the day. Studies have shown that vitamin D (VitD) is essential for maintaining muscle mass by activating cellular pathways involved in building muscle via muscle protein synthesis (MPS). Although, various candidate molecules have been identified in animal models, it is not known whether these pathways are activated in humans. Interestingly, animal studies indicate that 20% of VitD is stored in human muscle cells, which may help maintain VitD sufficiency during winter in Northern latitudes when there is not much sunlight Objectives: We will investigate whether VitD supplementation increases the MPS response to feeding and exercise, VitD storage in muscle cells, and cellular pathways that are involved in healthy sedentary or moderately active adults. Methods: Participants will consume either a placebo (sucrose) or the intervention (vitamin D3, 3000IU/day) for 12 weeks in a double-blinded randomised study. Before and after the intervention participants will have body composition measured. Blood and muscle samples will be taken before and after a bout of exercise and ingestion of 20 g protein in order to measure MPS.

Recent in vitro studies have demonstrated an anabolic role of vitamin D directly targeting skeletal muscle via vitamin D receptors (VDR) present in myotubes \[1,2,3\]. However, this has yet to be translated to in vivo human models. 25-hydroxyvitamin-D (25OHD) is the primary circulating metabolite and reference measurement for vitamin D status. This may then either be converted to 24,25-dihydroxyvitamin D3 (24,25OHD) to prevent intoxication \[4\] or be activated in the kidneys to 1,25-dihydroxyvitamin D (1,25OHD)\[5\]. Evidence support a biological role for 1,25OHD in skeletal muscle\[1-4,7\]. With focus on muscle hypertrophy, a study demonstrated that 25OHD can also be activated to 1,25OHD in myotubes\[8\] and promote cell proliferation, growth and differentiation of myocytes in in vitro skeletal muscle cells\[7,9-13\]. The mechanisms proposed include (i) gene expression of endocytic receptors for vitamin D binding protein (VDP) (megalin/cubulin) on the muscle cell surface membrane and (ii) high affinity for VDP to bind to actin inside the muscle cell. Furthermore, epidemiological studies support a positive role for vitamin D in human muscle function\[14-21\] and mechanistic studies implicate intracellular 25OHD in the regulation of protein metabolism. Cell culture and in vivo animal models demonstrate that 25OHD activates anabolic cell signalling proteins of the mTORC1 pathway in response to anabolic stimuli\[21,22\], which translates into an increased stimulation of muscle protein synthesis\[17\]. Despite these exciting results from cell culture and in vivo animal studies, no study has replicated these findings in in vivo human models. The length of the intervention in studies investigating the effects of vitamin D supplementation on muscle health outcomes and MPS varies between studies; however, evidence supports improvements in fast-twitch muscle fibres in elderly women\[18\], muscle strength in humans and animals and an increased in MPS in rats and mice following a minimum of 12 weeks intervention \[22\]. Thus, this study plans to have 12 weeks of intervention to ensure there is sufficient time for a physiologically effect to take place. Seasonal variations in blood 25OHD concentrations have been evaluated in Caucasians residing in Northern Ireland\[4\]. Thirty-four percent were deficient (\<25nmol/L) in winter months\[4\]; however, despite insufficient sunlight in winter to synthesise vitamin D in skin, a significant proportion of a population resident in the same latitude, in Scotland, maintained blood 25OHD concentrations \>50nmol/L\[6\]. These data and a recent review\[8\] suggest that humans have evolved a storage mechanism, which allows 25OHD, produced in the summer, to be conserved and used more efficiently in winter.