Correlation between Plasma 25‐hydroxyvitamin D and Fiber Composition of Skeletal Muscle in Response to Dietary Vitamin D3 or 25‐hydroxyvitamin D3 Supplementation

Abstract

An experiment was conducted to examine the effects of dietary vitamin D3 (D3) level and 25‐hydroxyvitamin D3 (25OHD3) on plasma 25‐hydroxyvitamin D and skeletal muscle. Growing rats (3‐week‐old) were fed either D3‐sufficient diet (1000 IU/kg diet) or D3‐deficient diet for 4 weeks (Phase I). During Phase II for another 4 weeks, rats fed D3‐deficient diet were allotted to 7 treatments. Experimental diets included D3 at 0, 400, 700, 1000, 1500, and 2000 IU/kg diet or 400 IU 25OHD3/kg diet. Positive and negative control groups were fed either D3‐containing (1000 IU/kg diet) or D3‐deficient diet throughout the experiment. Body weight, plasma 25‐hydroxyvitamin D, bone mineral density (BMD), lean percentage, and bicep femoris (representative of skeletal muscle) mass were determined. Also, fiber type composition and relative gene expression levels of myosin heavy chain (MHC) isoforms, PGC‐1α and Mitofusin 1 (Mfn1) in bicep femoris were determined. Dietary D3 levels or 25OHD3 did not affect body weight, BMD, lean percentage, or skeletal muscle mass in growing rats. However, plasma 25‐hydroxyvitamin D level was increased in a dose‐dependent manner upon dietary D3 level, and 25OHD3 group resulted in the highest level among treatments. Compared to the same dose of D3 (400 IU/kg diet), dietary 25OHD3 showed 2.3‐fold increase in plasma 25‐hydroxyvitamin D level (23.5±5.1 ng/mL vs. 53.3±1.5 ng/mL). Skeletal muscle fiber type composition was also altered by dietary D3 levels and 25OHD3. Vitamin D deficiency decreased type I fiber percentage and this decrease was diminished by dietary D3 in a dose‐dependent manner. Dietary 25OHD3, on the other hand, showed much more increase in type I percentage and decrease in type II percentage compared to same dose of D3 (400 IU/kg diet) (15.1±1.6 % vs. 5.4±1.7 %, and 63.4±1.6 % vs. 79.1±6.7 %), and showed the highest percentage of type I fiber than all the other treatments. However, relative gene expression levels of MHC isoforms did not coincided with the phenotypic difference. MHC I, PGC‐1α and Mfn1 transcript levels were numerically increased in response to dietary D3 level with the highest value resulted from 1500 IU/kg D3 supplementation. It showed 3‐fold increase in MHC I (3.4±2.4 vs. 1.0±0.7) and PGC‐1α (3.6±0.8 vs. 1.0±0.5), and 2‐fold increase in Mfn1 (2.4±0.6 vs. 1.0±0.8) transcript levels compared to positive control. In summary, 4 weeks of dietary D3 or 25OHD3 supplementation did not affect growth, lean percentage, skeletal muscle mass, and BMD in growing rats. However, plasma 25‐hydroxyvitamin D level, percentage of type I fiber and its gene expression levels in skeletal muscle responded to dietary D3 levels and 25OHD3. Also, it is notable that correlation between plasma 25‐hydroxyvitamin D level and percentage of type I fiber in bicep femoris was observed (r2=0.86). The present study demonstrates responses of plasma 25‐hydroxyvitamin D level and skeletal muscle fiber type composition upon dietary D3 and 25OHD3 with greater efficacy.