The classic deficiency diseases of vitamin D are osteomalacia in adults and rickets in children. Until recently, vitamin D was considered an unimportant vitamin except for its role in preventing these diseases. However, within the last few decades it has become apparent that vitamin D, actually a hormone, is far more important physiologically than was once thought. Levels of plasma vitamin D that were considered sufficient for optimal health (>25 nmol/L or >10 ng/mL of hydroxyvitamin D) are now known to be insufficient, and low intakes of the vitamin are now associated with an increased risk of mortality. Increasing the vitamin D status of individuals with marginal plasma concentrations may improve health. In particular, insufficient vitamin D intake, defined by a plasma levels of hydroxyvitamin D of between 25 and 40 nmol/L (10 and 16 ng/mL), is associated with an increased risk of insulin resistance, metabolic syndrome and type 2 diabetes. This suggests that as vitamin D plasma levels fall blood sugar control diminishes.
The effects of vitamin D on insulin sensitivity have been tested in obese adolescent subjects with body mass indexes of 39.8 kg/m2. Subjects took 4000 IU of vitamin D3 per day or a placebo for 3 to 6 months and a number of physiological parameters was measured1. Supplementation of vitamin D to the subjects caused a significant increase (+48.8 nmol/L or 19.5 ng/mL) in the plasma levels of the vitamin D metabolite 25-hydroxyvitamin D, compared to the placebo (+7 nmol/L or 2.8 ng/mL). At baseline all of the subjects where either deficient (<50 nmol/L or <20 ng/mL) or insufficient (50-75 nmol or 20-30 ng/mL) in 25-hydroxyvitamin D. In addition, the vitamin D supplemented group also experienced a significant increase in fasting insulin levels. That the insulin system was improved by vitamin D supplementation was confirmed by improvements in insulin resistance as measured by the HOMA-IR and QUICKI scores. However, there was no significant difference in the anthropometric measurements between the groups.
These results confirm the findings from other studies that show a role for vitamin D in the maintenance of the insulin system. Further, during the study the authors reported no adverse effects from the vitamin D supplements. High calcium intakes are known to cause weight loss in humans and animals, and vitamin D can increase plasma levels of calcium. However, in this study, no changes were seen to the plasma levels of calcium following vitamin D supplementation, and so it is unlikely that this was the cause of the improvements in insulin function. The changes in HOMA-IR and QUICKI scores are interesting because they suggest that insulin resistance may in part be a vitamin D deficiency. Confirmation of the insulin sensitising effects of vitamin D were confirmed by improvements in the leptin to adiponectin ratio (a proposed marker of insulin resistance) in the vitamin D group. In fact, the best improvements in the leptin to adiponectin ratio in the subjects was seen in those with the greatest increase in 25-hydroxyvitamin D status.