Vitamin D is a fat soluble vitamin that is synthesized by the body after exposure to sunlight, specifically the UV rays from the sun. It also occurs naturally in some foods, and is fortified in other foods.

There are various forms of vitamin D, but calciferol is the most active form.

Once vitamin D is in the body, the liver and kidney convert it into its active form. This active form acts like a hormone, sending messages to the intestines to promote the absorption of calcium and phosphorus and helps regulate cell growth. Vitamin D is key to healthy bones because it increases the absorption of calcium to form and maintain strong bones.

What Are the Best Sources of Vitamin D?

The best source is sunlight. It only takes 10 minutes of unprotected sun exposure for the body to produce vitamin D. Any body parts exposed to the sun for more than 15 minutes should be protected with sunscreen. In northern latitudes, the UV rays between November and February are not strong enough for the body to produce vitamin D, according to the National Institute of Health. During these months, or for people who do not get 10 minutes of unprotected sunlight every day, the best way to get the recommended daily intake (RDI) of vitamin D is through diet.

How Much?

The Institute of Medicine recommends 200 international units (IU) for children and adults younger than 50. For men and women over 50, the RDI is 400 IU.

Which Foods Have Vitamin D?

Many foods are fortified with vitamin D, most notably milk. One cup of milk provides 98 IU, almost half the recommended amount for adults under 50. Many breakfast cereals are also fortified with vitamin D, but many only provide 10 percent of the RDI. Cold water fish such as salmon, tuna and sardines, are excellent sources of vitamin D. Just 3 1/2 ounces of salmon, half of an average restaurant portion, provides 360 IU of vitamin D.

There are also a variety of vitamin D vitamin supplements. The NIH estimates 30 to 40 percent of older adults are vitamin D deficient and could benefit from a supplement. Also, people living in northern latitudes and people with a high amount of melanin in the skin (darker skin) are prone to vitamin D deficiency.

Is Vitamin D Deficiency Dangerous?

Rickets and osteomalacia are the most common diseases associated with vitamin D. Rickets, a bone disease in children, has declined considerably in the U.S. since milk became fortified, but it is beginning to reemerge. It is important for children to get enough vitamin D from breastfeeding through the teen years, when bones are still forming.

Osteomalacia leads to muscular weakness and weak bones in adults.

While osteomalacia is an extreme condition due to vitamin D deficiency, osteoporosis is all too common. The NIH estimates more than 25 million adults in the U.S. are at risk or have osteoporosis. Calcium is key to preventing this disease, characterized by fragile bones, but vitamin D is key to the absorption of calcium in the body.

How Much is Too Much?

While getting enough vitamin D is key to healthy bones and a strong immune system, overdosing on vitamin D is equally dangerous. The Institute of Medicine has set the highest tolerable intake levels at 2,000 IU for children over 3-years-old and adults.

The most common cause of vitamin D toxicity is an overdose through a supplement.

It is very rare to overdose through diet unless an excess of cod liver oil is consumed.

Vitamin D toxicity can cause nausea, vomiting, loss of appetite, weakness, weight loss, as well as confusion and heart rhythm abnormalities, according to the NIH.

A varied diet of nutrition-rich foods coupled with brief, unprotected exposure to sunlight is the best recipe to ensure the RDI of this important vitamin. Any supplement should be discussed with a doctor before it is added to a daily regimen.

This article was reviewed by Dr. Manny Alvarez.