Abstract and Introduction
Introduction: Recent studies have shown that vitamin-D intake can improve skeletal muscle function and strength in frail vitamin-D insufficient individuals. We investigated whether vitamin-D intake can improve the muscular response to resistance training in healthy young and elderly individuals, respectively.
Methods: Healthy untrained young (n = 20, age 20–30) and elderly (n = 20, age 60–75) men were randomized to 16 weeks of daily supplementary intake of either 48 μg of vitamin-D + 800 mg calcium (Vitamin-D-group) or 800 mg calcium (Placebo-group) during a period and at a latitude of low sunlight (December-April, 56°N). During the last 12 weeks of the supplementation the subjects underwent progressive resistance training of the quadriceps muscle. Muscle hypertrophy, measured as changes in cross sectional area (CSA), and isometric strength of the quadriceps were determined. Muscle biopsies were analyzed for fiber type morphology changes and mRNA expression of vitamin-D receptor (VDR), cytochrome p450 27B1 (CYP27B1) and Myostatin.
Results: In the vitamin-D groups, serum 25(OH)D concentration increased significantly and at week 12 was significantly different from placebo in both young men (71.6 vs. 50.4 nmol/L, respectively) and elderly men (111.2 vs. 66.7 nmol/L, respectively). After 12 weeks of resistance training, quadriceps CSA and isometric strength increased compared to baseline in young (CSA p < 0.0001, strength p = 0.005) and elderly (CSA p = 0.001, strength p < 0.0001) with no difference between vitamin-D and placebo groups. Vitamin-D intake and resistance training increased strength/CSA in elderly compared to young (p = 0.008). In the young vitamin-D group, the change in fiber type IIa percentage was greater after 12 weeks training (p = 0.030) and Myostatin mRNA expression lower compared to the placebo group (p = 0.006). Neither resistance training nor vitamin-D intake changed VDR mRNA expression.
Conclusion: No additive effect of vitamin-D intake during 12 weeks of resistance training could be detected on either whole muscle hypertrophy or muscle strength, but improved muscle quality in elderly and fiber type morphology in young were observed, indicating an effect of vitamin-D on skeletal muscle remodeling.
During recent years increased attention has been drawn to the influence of vitamin-D on the development of a variety of diseases. A relationship between vitamin-D insufficiency and osteoporosis as well as increased risk of bone fractures has been established.[1–4] Furthermore, low vitamin-D levels are connected to the development of diabetes, cardiovascular diseases,[3,6] cancer,[3,7] depression, osteoarthritis, multiple sclerosis and maintenance of a healthy immune system, although the specific mechanisms have not been fully elucidated.
It is well known that both muscle strength and vitamin-D levels decrease with age.[1,11] Low vitamin-D levels are associated with decreased muscle strength and poor physical function in elderly individuals. In addition, it has been proposed that vitamin-D plays an important role for obtaining optimal skeletal muscle function.[13,14]
Expression of the vitamin-D receptor (VDR) in skeletal muscle tissue has been questioned. However, recent data strongly indicate that VDR is expressed in C2C12 myoblasts and myotubes,[16,17] in murine skeletal muscle, and in situ detection of VDR in human skeletal muscle points towards a role of vitamin-D on muscle function.[19,20] In addition, VDR has been located in skeletal muscle cells that promote de novo protein synthesis. Addition of the active form of vitamin-D (1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3)) to C2C12 myotubes has been shown to increase skeletal muscle protein synthesis and Akt/mTOR-signaling, down-regulate the expression of Myostatin and increase the size of myosin heavy chain (MHC) type II positive myotubes. Moreover, vitamin-D signaling has been reported to alter gene expression and increase C2C12 myotube size. These findings support the hypothesis that vitamin-D has a direct positive effect on the contractile filaments and thus muscle strength. Furthermore, preliminary data from another exercise experiment in our lab suggest that resistance exercise increases mRNA expression of VDR and 25-hydroxy vitamin-D (25(OH)D) hydroxylase cytochrome p450 27B1 (CYP27B1), which is responsible for converting the inactive form of vitamin-D, 25(OH)D, to the active form, 1,25(OH) 2 D 3, further supporting a positive role for vitamin-D in skeletal muscle.
Studies in humans have shown positive effects of concomitant intake of vitamin-D on muscle strength[24–29] and intramyonuclear VDR level in vitamin-D insufficient individuals. Moreover, it has been shown that vitamin-D intake increases cross sectional area (CSA) of skeletal muscle fibers, the diameter and number of type II muscle fibers and specifically type IIa fibers. However, it appears that elderly vitamin-D insufficient people may benefit the most from vitamin-D intake whereas young people might not.
Based on these findings we wanted to examine the hypothesis that vitamin-D is important for the hypertrophic response to resistance exercise; specifically whether serum vitamin-D levels may influence the muscular response to resistance training. UVB radiation from the sun is the primary source for vitamin-D production and is very low at northern latitudes from September to April. The resulting seasonal fluctuations in serum vitamin-D levels could influence muscle strength and function. If this is so, it is possible that low serum vitamin-D levels during the winter months could partly blunt the positive effect of resistance training, and in such case, vitamin-D supplementation could be important for maximizing improvements in skeletal muscle mass and strength during resistance training.
The aim of the present study was to investigate whether vitamin-D intake during 12 weeks of resistance training has an additive effect on muscle hypertrophy and strength. We hypothesized that intake of vitamin-D plus calcium would improve the outcome of three months of resistance training in healthy untrained individuals resulting in greater muscle strength and hypertrophy compared to a training control-group supplemented with calcium alone (placebo). Moreover, we hypothesized that resistance exercise would increase the mRNA expression of VDR and CYP27B1. The study included a group of young and a group of elderly individuals to elucidate a possible blunted hypertrophic response in the aging muscle.
Nutr Metab. 2015;12(32) © 2015 BioMed Central, Ltd.