The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. It promotes bone mineralization in concert with a number of other vitamins, minerals, and hormones. Without vitamin D, bones can become thin, brittle, soft, or misshapen. Vitamin D prevents rickets in children and osteomalacia in adults, which are skeletal diseases that result in defects that weaken bones.

What are the sources of vitamin D?

Food sources of Vitamin D

Fortified foods are the major dietary sources of vitamin D. prior to the fortification of milk products in the 1930s, rickets (a bone disease seen in children) was a major public health problem in the United States. Milk in the United States is fortified with 10 micrograms (400 IU) of vitamin D per quart, and rickets is now uncommon in the US.

One cup of vitamin D fortified milk supplies about one-fourth of the estimated daily need for this vitamin for adults. Although milk is fortified with vitamin D, dairy products made from milk such as cheese, yogurt, and ice cream are generally not fortified with vitamin D. Only a few foods naturally contain significant amounts of vitamin D, including fatty fish and fish oils (4). The table of selected food sources of vitamin D suggests dietary sources of vitamin D.

Vitamin D and exposure to sunlight

Exposure to sunlight is an important source of vitamin D. Ultraviolet (UV) rays from sunlight trigger vitamin D synthesis in the skin. Season, latitude, time of day, cloud cover, smog, and sunscreens affect UV ray exposure. For example, in Boston the average amount of sunlight is insufficient to produce significant vitamin D synthesis in the skin from November through February. Sunscreens with a sun protection factor of 8 or greater will block UV rays that produce vitamin D, but it is still important to routinely use sunscreen whenever sun exposure is longer than 10 to 15 minutes. It is especially important for individuals with limited sun exposure to include good sources of vitamin D in their diet.

Vitamin D: How much?

• 1 to 50 years of age: 200IU Daily

• 51 to 70 years of age: 400IU Daily

• Over 70 years of age: 600IU Daily

• Pregnancy or Lactation 200IU Daily

Numbers are based on the Tolerable Upper Intake Level (UL). Unless you have a special condition, you would want to go a bit under the IU. The numbers state the maximum amount, or the amount you should not exceed.

Food Sources of Vitamin D:

Cod liver oil (1-tablespoon) 1,360IU

Salmon, cooked (3.5 ounces) 360IU

Milk, vitamin D-fortified (1-cup) 100IU

Dry cereal, fortified at 10% DV (3/4-cup) 40 to 50IU

Beef liver, cooked (3.5 ounces) 30IU

Egg yolk, large (1) 25IU

Margarine (1-teaspoon) 20IU

When can vitamin D deficiency occur?

A deficiency of vitamin D can occur when dietary intake of vitamin D is inadequate, when there is limited exposure to sunlight, when the kidney cannot convert vitamin D to its active form, or when someone cannot adequately absorb vitamin D from the gastrointestinal tract.

The classic vitamin D deficiency diseases are rickets and osteomalacia. In children, vitamin D deficiency causes rickets, which results in skeletal deformities. In adults, vitamin D deficiency can lead to osteomalacia, which results in muscular weakness in addition to weak bones

Vitamin D Supplements:

If you take a supplement with vitamin D, do the following:

• Check the label's Supplement Facts to help you keep your total daily intake UNDER the UL.

• Check with your physician whenever you begin taking any supplement, especially if you have any known health conditions.