Vitamin D deficiency is prevalent worldwide among both genders and various age groups. Spending more time indoors, avoiding direct sun exposure to reduce the risk of skin cancer, in addition to skin pigmentation and ageing are some of the contributing factors. There is currently no consensus about the optimal level of vitamin D but most experts define vitamin D deficiency as 25-hydroxyvitamin D (25OHD) levels below 50nmol/L. By this definition, about one third of Australian adults aged ≥25 years are vitamin D deficient.

The role of vitamin D in calcium homeostasis and bone health is well recognised. However, discovery of the vitamin D receptor in many tissues and the presence of the enzyme 1α hydroxylase which converts 25OHD to 1,25 dihydroxyvitamin D (1,25OHD) in various organs outside the kidneys have created new insights into the extra-skeletal roles of vitamin D.

Accumulating evidence has suggested a role for vitamin D in insulin secretion and insulin resistance. Compelling evidence comes from large observational studies indicating an association between low vitamin D levels and β cell dysfunction, impaired glucose tolerance and incidence of diabetes. Moreover, vitamin D deficiency has been inversely associated with cardiovascular risk factors including hypertension, dyslipidaemia, endothelial dysfunction, and cardiovascular morbidity and mortality.

To date, randomised clinical trials in humans have yielded inconsistent results on the effect of vitamin D supplementation on insulin secretion/sensitivity, incidence of diabetes, and other cardiometabolic risk factors. However, these trials often had small sample sizes, short durations of follow up, insufficient doses of vitamin D supplementation, and significant heterogeneity in sample characteristics and study designs. Larger and better quality trials are currently underway to determine the role of vitamin D in improving cardiometabolic outcomes.