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.

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 all 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.

When does vitamin D deficiency most often occur?
The most common vitamin D deficiency diseases are rickets and osteomalacia. Rickets usually occurs in children, causing skeletal deformities because the body’s ability to a utilize calcium is inadequate. Osteomalacia is an adult vitamin D deficiency disease resulting in muscular weakness and weak bones.

Vitamin D deficiency occurs, not only when the dietary intake of vitamin D is inadequate, but also when there is limited exposure to sunlight, when the kidneys cannot convert vitamin D to its active form, or when there is a gastrointestinal absorption problem.

In today’s age of vitamin D fortified milk and other products, this deficiency is more common in people over 50 years of age rather than children. As we age the skins’ ability to convert vitamin D to its active form decreases and kidneys which help to convert vitamin D to its active form tend to be less efficient.

Those most needing of vitamin D supplementation are individuals who have limited exposure to sunlight, people in northern areas, and anyone who keeps their bodies covered. Also at greater risk are those with fat malaborption disease since vitamin D is a fat-soluble vitamin. Chron’s disease, sprue, cystic fibrosis, liver disease, pancreatic enzyme deficiency, and bowel disease may be causes of fat malabsorption. Also one who has had part of their stomach removed or stapled for weight loss is also at risk of fat malaborption disease.

Supplementation of vitamin D may be necessary for exclusively breast-fed infants who have limited exposure to sunlight, since human milk often does not contain enough vitamin D.

Osteoporosis:
Osteoporosis, a disease of fragile bones, afflicts at least 25 million adults in the United States and is one of the primary causes of bone fractures in the elderly. Having normal levels of vitamin D in your body helps keep your bones strong and may help prevent osteoporosis in elderly, non-ambulatory individuals, in post-menopausal women, and in individuals on chronic steroid therapy.

Normal bone is constantly being broken down and rebuilt. During menopause, the balance between these two systems is upset, resulting in more bone being broken down than rebuilt. Estrogen replacement, which limits symptoms of menopause, can help slow down the development of osteoporosis by stimulating the activity of cells that rebuild bone, but supplementation is sometimes necessary.