Vitamin D
(Shakreez, Oil Power, flickr. CC: BY 2.0.)
Vitamin D deficiency has been linked to a number of conditions and has only recently been recognized as a major factor in chronic disease. In addition to its well-known impact on osteoporosis, vitamin D deficiency has been linked to a number of cancers, mood disorders, coronary artery disease and stroke. Those with low levels of vitamin D are also more likely to develop type 1 and type 2 diabetes. While deficiency has been linked to decreased insulin synthesis and secretion, supplementation to normal levels has been found to improve glucose control and insulin secretion (1). Studies of those with type 2 diabetes have shown that replacement to normal levels can improve endothelial function (2).
Repletion of vitamin D should be given a high priority given its recognized benefits and relative safety. This is particularly important for those living in northern climates due to limited vitamin D synthesis through sun exposure (i.e., most people living in Michigan require supplementation). The need for repletion of vitamin D may increase with age due to a variety of factors.
The appropriate lab test is 25-hydroxy-vitamin D and the target value should be around 40 ng/ml. A common dose is 2000 International Units (IU) Labor Day to Memorial Day and 1000 IU in the summer. Cholecalciferol (Vitamin D3) is recommended over ergocalciferol (D2) due to better absorption. It should be taken with food and is available widely over the counter. It is important to use serum levels to achieve adequate dosing. There are a number of polymorphisms is the metabolism pathway for vitamin D. This, along with other variables, makes dosing unpredictable. Toxicity is rare and is usually seen with daily intake over 10,000 IU.
1. Palomer X, et al. Role of vitamin D in the pathogenesis of type 2 diabetes mellitus, diabetes. Obes Metab, Mar 2008;10(3):185-97.
2. Sugden JA, et al. Vitamin D improves endothelial function in patients with type 2 diabetes mellitus and low vitamin D levels. Diabet Med, Mar 2008;25(3):320-5.