Serum levels of 25-hydroxyvitamin D [25(OH)D] are the established marker for vitamin D status. Generally, an association exists between serum 25(OH)D and obesity but the exact interactions are not fully understood. Poor vitamin D status may play a role in the development of insulin resistance which can lead to obesity, and supplementation of vitamin D has been shown to increase insulin sensitivity. In addition, the vitamin D metabolite 1,25-dihydroxyvitmain D [1,25(OH)2D] may improve the synthesis and secretion of insulin from the β-cells of the pancreas (here). The poor vitamin D status of individuals with obesity may explain the increased incidence of cancer and cardiovascular disease in this sub-group of the population. Because being over-weight or obese is associated with poor vitamin D status, researchers1 have investigated the effects of weight loss on the serum levels of 25(OH)D over the course of 12 months.
Overweight and obese postmenopausal women were randomly assigned to receive dietary modifications, an exercise regimen, both dietary modification and an exercise regimen or placed in a control group with no intervention. The dietary modification involved a 30% reduction in energy intake and consultations with a dietician. The exercise intervention involved 45min of moderate intensity aerobic exercise 5 days per week, 3 of which were supervised to assure compliance. At baseline, serum 25(OH)D varied depending on the season (December to February: 18.7ng/mL; March to May: 19.6ng/mL; June to August: 22.5ng/mL; September to November: 21.1ng/mL). Dietary intake of vitamin D was 6.1µg/d from dietary sources and 49.5% used a vitamin D supplement supplying 13.3 µg/d. After 12 months, the mean weight loss was 2.4% in the exercise group, 8.5% in the diet group, 10.8% in the diet plus exercise group and 0.8% in the control group.
The baseline vitamin D status was not associated with the degree of weight loss. However, changes to the BMI of the women were inversely associated with the increase in 25(OH)D and the magnitude of the weight loss seen was associated with the observed increase in 25(OH)D when all women randomly assigned to interventions were grouped together. Women who lost <5%, 5-9.9%, 10-14.9% and ≥15% bodyweight had increases in 25(OH)D of 2.1, 2.7, 3.3 and 7.7ng/mL. In addition the magnitude of fat loss was associated with the rise in 25(OH)D. Taken as a whole, theses results suggest that weight loss is associated with rises in 25(OH)D. This supports evidence to show that during the autumn and winter, overweight subjects are able to release vitamin D into their circulation from adipose tissue, which increases circulating levels of 25(OH)D above those of lean subjects (here).
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