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