Background: Vitamin D deficiency is now being recognized as an emerging problem worldwide. Obesity has been found to be associated with lower serum 25-hydroxyvitamin D (25(OH)D) concentrations due to various mechanisms. There is increasing evidence showing extra-skeletal health benefit of vitamin D. Previous studies demonstrated the relationship between vitamin D and adiposity. However, the association between vitamin D status and skeletal muscle mass has not been established in healthy obese individuals in tropical countries.
Objectives: This cross-sectional study was designed to assess the vitamin D status and its relationship between serum 25(OH)D concentrations and body composition, including skeletal muscle mass and adiposity in healthy obese non-diabetes subjects who live in Thailand, which is located near the equator.
Results: We enrolled163 Thai obese subjects (59.5% were female). The prevalence of vitamin D deficiency (25(OH)D <20 ng/ml) and vitamin D inadequacy (25(OH)D <30 ng/ml) were 49/163 (30.1%) and 148/163 (90.8%) respectively. In all, 98% of obese subjects with BMI>35 kg/m2 had vitamin D inadequacy. Serum 25(OH)D concentrations were negatively associated with percent body fat (%BF) (r = - 0.23, P =0.003). Moreover, vitamin D status was positively associated with skeletal muscle mass (SMM) (r = 0.18, P =0.03) and the association still exist after controlling for body fat mass and age (P= 0.003). Interestingly, in the subjects with lowest tertile of %BF, multiple linear regression analysis revealed that the significant positive predictors of %SMM were the vitamin D status and male gender, the negative predictor was the body mass index after adjusting with age and exercise duration.
Conclusions: Our study demonstrated the high prevalence of vitamin D deficiency in obese, non-diabetes Thai populations. Vitamin D status was an independent predictor of %SMM of patients with lowest tertile of %BF. We speculated that adiposity might play a role in the relationship of vitamin D and skeletal muscle mass.
Alex’s Notes: Vitamin D deficiency is a worldwide problem, and there are plenty of epidemiological studies showing associations between various parameters of body composition and vitamin D status. The current study sought to continue the association band-wagon in 163 obese Thai adults without diabetes, a history of disease that might affect vitamin D status (e.g. liver; renal), medications that involve vitamin D metabolism, and who were not supplementing vitamin D over 1200 IU daily.
The participants were all interviewed at a hospital to obtain information about factors associated with vitamin D status such as diet, exercise duration, sunlight exposure, sunscreen usage, and so forth. Body composition was assessed after an eight hour fast with bioelectrical impedance (BIA), which despite its bad rap has been shown to be accurate in obese persons. Blood draws were performed last to assess 25(OH)D levels, and fasting glucose & insulin.
Overall, 60% of the participants were women and the average BMI was 33.9kg/m2 with an average age of 41 years. The prevalence of vitamin D inadequacy (<30 ng/dL) and deficiency (<20 ng/dL) was 90.8% and 30.1%, respectively. I want to emphasize this point, as Thailand is located in a tropical zone near the equator where average sunlight exposure is 5-9 hours per day, and exposure is expected year round. Many persons advocate daily sunbathing to synthesize vitamin D, but this method is unreliable. The difference in vitamin D status cannot be explained by diet or lifestyle either, as there were no significant differences in daily sunscreen usage, time spent in the sun (over an hour), exercise duration, or consumption of vitamin D-rich foods between vitamin D adequate and inadequate participants.
The subjects with adequate vitamin D status were significantly older (~7 years) and had significantly more skeletal muscle mass. They also had significantly less fat mass, and although the BMIs were not different, all but one subject with BMI > 35kg/m2 had vitamin D inadequacy. There was a significant, albeit modest, positive association between 25(OH)D levels and skeletal muscle mass and inverse association for fat mass. However, when subjects were grouped into tertiles of body fat percentage, the independent predictors of skeletal muscle mass were gender, higher 25(OH)D status, and lower BMI only in the subjects in the lowest tertile. These three characteristics explained roughly 63% of the variance in muscle mass.
The average body fat percentage of the tertiles was 31.6, 42, and 49.6%. It does thus appear that there is a ceiling whereby increasing fat mass overrides vitamin D’s effect on skeletal muscle. Being fat soluble, it could be that the adipose tissue sequesters the vitamin D for storage. Similarly, the “marbling” effect of obesity on increasing intramuscular triglycerides could also potentially inhibit binding affinity.