Vitamin D deficiency and the risk of incident Type 2 diabetes

Cross-sectional studies have demonstrated that lower serum 25-hydroxyvitamin D(25[OH] D) levels are associated with obesity, the metabolic syndrome, impaired glucose tolerance and diabetes. However, as in all cross-sectional studies, the direction of causation is unclear. The article by Knekt et al. was the first prospective study to demonstrate that low 25(OH) D levels predict incident diabetes. This study utilized a nested case-control design of 412 incident diabetes cases and 986 age/sex matched controls drawn from two large Finnish cohorts totaling 7503 participants aged 40 years and over followed for up to 22 years. In men, although not in women, higher baseline 25(OH) D reduced the risk of incident diabetes by 72%. Recently, other prospective cohort studies have shown that baseline 25(OH) D deficiency is associated with incident cardiovascular disease events and mortality over follow-up, a relationship that may be mediated, in part, through incident diabetes. While animal studies and smaller interventional trials in humans suggest that vitamin D supplementation improves measures of insulin sensitivity and glucose tolerance, larger interventional trials are warranted to determine if vitamin D treatment at adequate doses can prevent diabetes.