Abstract
The non-skeletal roles of vitamin D are widely recognized, including effects on the immune system, cardiovascular system and metabolism. Obesity and insulin resistance have been associated with low concentrations of serum 25-hydroxyvitamin D (25OHD). The mechanism is unclear and theories include sequestration of the fat-soluble vitamin into the excess adipose tissue, or impaired mobility and hence reduced sunlight exposure.
This Greek study recruited 44 obese women over a wide age range with a body mass index (BMI) of 36.7 ± 4.9 kg/m2 along with 25 age-matched normal BMI controls (23.8 ± 1.5 kg/m2). None had a raised fasting glucose nor were taking any vitamin or mineral supplementation. A weight loss diet was instituted and the daily intake of dietary vitamin D was estimated at 200 IU. Participants were encouraged to maintain their usual level of exercise and an account was also made for the season in which vitamin D levels were sampled. Markers were examined at baseline, four and 20 weeks. Overnight fasting serum 25OHD concentration was measured by an electrochemiluminescence immunoassay.
Following 20 weeks of diet, the obese group reduced initial body weight by 10%, along with a reduction in total cholesterol and triglyceride concentrations. Insulin concentrations fell significantly and insulin sensitivity increased. Concentrations of 25OHD significantly increased by 34%, while PTH concentrations remained unchanged. The concentration of 25OHD reached 18.3 ± 5.1 μg/L which is still deficient; hence the authors suggested that greater weight loss and supplementation would be required to increase 25OHD concentrations above a therapeutic threshold.
By demonstrating an improvement in insulin resistance related to increased 25OHD concentrations along with a reduction in weight, another possible explanation to the observed trend between obesity and vitamin D is highlighted. Although participants undertook routine levels of exercise, limited details about ultraviolet B (UVB) exposure were stated. Future investigation to substantiate this evidence on larger numbers could consider quantification of UVB exposure and inclusion of men and differing ethnic classes.