Vitamin D 3 , 25‐Hydroxyvitamin D 3 , and 1α‐Hydroxyvitamin D 3 Modulate Fiber Type Distribution in Bicep Femoris of Growing Rats

An experiment was conducted to examine the effects of different types of vitamin D 3 on skeletal muscle development using growing rats. Three‐week‐old rats were fed a vitamin D 3 ‐deficient diet for 4 weeks and vitamin D 3 ‐replete diets for the next 4 weeks as each of vitamin D 3 (vit D 3 , 1,000 IU/kg diet), 25‐hydroxyvitamin D 3 (25‐OH D 3 , 400 IU/kg diet), and 1α‐hydroxyvitamin D 3 (1α‐OH D 3 , 4 IU/kg diet) was supplemented. Negative control group was fed a vitamin D 3 ‐deficient diet, and positive control group was fed a vitamin D 3 ‐supplemented diet (1,000 IU/kg diet) for the whole experimental period, 8 weeks. In results, body weight, feed intake, and body composition (lean and fat) were not affected by vitamin D 3 dose and type. Quantitative gene expression of skeletal muscle differentiation markers ( MyoD, Myogenin, Mrf4, Myf5, and Myostatin ) also showed no difference. However, skeletal muscle fiber type distribution (myofibrillar ATPase staining) was changed in bicep femoris (BF) of the rats fed different types of vitamin D 3 supplements.In positive control, type I was increased by 2.4 fold (7.22 % vs. 2.95 %) compared to negative control. In the rats fed vit D 3 ‐replete diets followed by 4‐weeks of vitamin D 3 deficiency, both type I and IIa were increased only 1.3 fold (3.72% vs. 2.95%, and 20.79 % vs. 15.74 %) compared to negative control. Percentages of type I were 5.0‐ and 6.9‐fold increased in BF of the rats fed 25‐OH D 3 (14.63 % vs. 2.95%) and 1α‐OH D 3 (20.45 % vs. 2.95 %) respectively, in comparison to negative control. The present study demonstrates that vitamin D 3 does not affect lean growth, but it might mediate skeletal muscle fiber type distribution in growing rats depending on the vitamin D 3 types.