Studies have shown people with lower levels of vitamin D (25-OH-D) in their blood have an increased risk of type 2 diabetes compared to those with higher levels. Hypertension and other cardiovascular diseases, some types of cancer, and bone fractures are also more common in people with low levels of 25-OH-D.
But before supplementation can be recommended, we need a double-blind controlled clinical trial to prove the association is causal. Recall that higher blood levels of vitamin E and beta-carotene levels were shown in many observational studies to be correlated with a reduced risk of CVD and cancer. However, when the data from all the previous placebo controlled clinical trials were pooled they indicated supplements of vitamin E and/or beta-carotene are likely harmful at least for some groups of people.
An earlier placebo controlled trial designed to examine the impact of a combined 700 IU Vitamin D plus 500mg calcium supplements on bone health did note that the 40% of subjects who took the supplement whose blood sugars were modestly elevated were less likely to progress to having type 2 diabetes over the next 3 years.1
A small study looked at a group of 81 insulin resistant women age 23-68 in New Zealand. Subjects were randomly assigned to receive either a 4000 IU vitamin D supplement or a placebo. After 6 months those taking the vitamin D supplement had significant improvements in insulin sensitivity as well as lower fasting insulin levels. The researchers noted that insulin resistance was most improved in those whose blood 25-OH-D levels rose into the 80 to 119 nmol/L range, which is well in excess of the 30 nmol/L range believed adequate for optimal bone health. Such a high level of 25-OH-D cannot generally be attained by diet alone in most people and would require either fairly high dose supplements of Vitamin D3 and/or a significant amount of sun exposure.2 Improving insulin sensitivity should improve blood sugar control in people with type 2 diabetes and also help prevent the development of type 2 diabetes as well.
Bottom Line: The current DRI (200 to 600 IUs depending on age) and UL (2000 IU) for vitamin D established nearly 10 years ago now appear inadequate, and too low, respectively for reducing morbidity and mortality from CVD and likely type 2 diabetes, some cancers and other ills.
By James J. Kenney, PhD, RD, FACN
1. Diabetes Care 2007;30:980-86
2. Br J Nutr doi:10.1017/
Stephanie Ronco has been editing for Food and Health Communications since 2011. She graduated from Colorado College magna cum laude with distinction in Comparative Literature. She was elected a member of Phi Beta Kappa in 2008.