Vitamin D

Vitamin D is called the sunshine vitamin because people form it in their skin through a biochemical process when they are exposed to sunlight.  One of the main functions of vitamin D is to help maintain bone strength.  It does this by increasing the absorption of calcium and phosphorus from the intestinal tract.  It also acts as a hormone to directly promote bone mineralization.  If vitamin D is not present, bone development is impaired, even when there is adequate calcium.  A deficiency of vitamin D in children can result in the bone disease known as rickets.

People in northern latitudes such as the northeastern United States, may have inadequate vitamin D status in the winter because they are exposed to lower levels of sunlight and spend more time indoors. To address this problem a public health program was put into place to fortify milk with vitamin D. Now each cup of milk is supplemented to contain 100 International Units of vitamin D. 

• How much Vitamin D is required? The Food and Drug Administration lists a Daily Value of 400 International Units for vitamin D, which can be considered a minimum target for vitamin D intake.  However Walter Willet, who is head of nutrition department at the Harvard School of Public Health thinks that 400 iu is too low and recommends that adults get 800 to 1000 International Units of vitamin D per day.  The best food sources for vitamin D are cold water fish such as salmon and mackerel.  But it is difficult to obtain that much vitamin D from  a normal diet so a nutritional supplement is a good idea. 

• What to look for in a Vitamin D supplement  Many vitamin D supplements contain a form of vitamin D called ergocalciferol, or vitamin D2.  There is research to show that cholecalciferol, or vitamin D3, is more active than vitamin D2.  Cholecalciferol is the form that is used to fortify milk.  So it makes sense to check the label when buying a vitamin D supplement to make sure that cholecalciferol is being used.  Cholecalciferol supplements are not difficult to find.  The Walgreen brand offers a vitamin D supplement made of cholecalciferol.

Recent studies have also found that higher vitamin D levels are linked to a reduced risk of several types of cancer.  In support of this is the observation that the incidence of several types of cancer increases in a gradient from the southern United States to the northern United States.  Lower levels of sunlight in the winter months at northern latitudes result in lower vitamin D levels and this may be the link to the higher incidence of certain types of cancer at more northern latitudes.    Vitamin D has also been found to help maintain a healthy immune system, provide cardiovascular benefits, and offer protection against multiple sclerosis and rheumatoid arthritis [1].

There is also some evidence that low Vitamin D levels in the winter contribute to the higher incidence of influenza at that time of year.

In light of all this recent information, vitamin D may be one of the most important vitamins to include as a dietary supplement.

1. Harvard Health Letter, March 2006; 31(5):1-2.