Vitamin D Status and Its Relation to Muscle Mass and Muscle Fat in Young Women

Abstract

Previous studies have demonstrated that increased muscle lipid content is associated with reduced muscle strength and physical performance, independent of muscle mass. An association between elevated muscle lipid content with skeletal muscle attenuation has been reported in a large number of studies. The relationship between vitamin D serum levels and muscle strength is well established. Patients with vitamin D deficiency have increased body fat and decreased muscle strength. A similar association between vitamin D levels and muscle strength has been reported both in the elderly and in healthy adolescent postmenarcheal girls. In several studies, skeletal muscle attenuation was demonstrated in vitamin D deficient patients with neuromuscular disorders. It is unclear whether vitamin D insufficiency is also related to adipose tissue infiltration in muscle. In the present study, the investigators hypothesized that there was an inverse relationship between serum vitamin D levels and adipose tissue infiltration in muscle, which is independent of muscle mass. This cross-sectional study tested this hypothesis by examining the relationship between serum 25-hydroxyvitamin D (25OHD) and skeletal muscle lipid content and muscle mass in a cohort of healthy young women. The study subjects were 90 postpubertal females, aged 16 to 22 years. Study outcome measures included anthropometric characteristics, serum 25OHD values determined with radioimmunoassay, and values of fat, muscle mass, and percent muscle fat determined using computed tomography (CT). The participants were divided into 2 groups based on baseline vitamin D levels: 41% (n = 37) were 25OHD sufficient (≥30 ng/mL) and 59% (n = 53) were 25OHD insufficient (≤29 ng/mL); 24% of the latter were 25OHD deficient (≤20 ng/mL). The data demonstrated a strong inverse relationship between serum 25OHD levels and CT measurements of percent muscle fat (r = −0.37; P < 0.0003), whereas no such relationship was found between the 2 vitamin D groups for fat infiltration in the thigh muscle area (r = 0.16; P = 0.14). Multiple regression analysis showed that the association between 25OHD levels and percent muscle fat was independent of body mass or CT measures of subcutaneous and visceral fat. Compared with the vitamin D sufficient group, percent muscle fat was significantly higher in the vitamin D insufficient group (3.15 ± 1.4 vs. 3.90 ± 1.9; P = 0.038). These findings indicate that vitamin D insufficiency has an inverse association with fat infiltration in muscle, which is independent of body mass.