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Vitamin D, calciferol, is a fat-soluble vitamin. It is found in food, but also can be made in your body after exposure to ultraviolet rays from the sun. Vitamin D exists in several forms, each with a different activity. Some forms are relatively inactive in the body, and have limited ability to function as a vitamin. The liver and kidney help convert vitamin D to its active hormone form.
What does Vitamin D do?
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 -- skeletal diseases that result in defects that weaken bones.
What are the 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 US. 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. The list below includes selected food sources of vitamin D.
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.
How much vitamin D do we need?
The Recommended Dietary Allowance (RDA) is the average daily dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97-98%) healthy individuals in each life-stage and gender group. Unlike many other nutrients, there is insufficient evidence to establish an RDA for vitamin D. Instead, an Adequate Intake (AI) -- a level of intake sufficient to maintain healthy blood levels of an active form of vitamin D -- has been established. The 1998 AIs for vitamin D for adults are the same in males and females, but increase with age.
Ages 19-50: 200 International Units (IU)
Ages 51-69: 400 IU
Age 70 and older: 600 IU
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.
Who may need extra vitamin D to prevent a deficiency?
Older Americans (greater than age 50) are thought to have a higher risk of developing vitamin D deficiency. The ability of skin to convert vitamin D to its active form decreases as we age. The kidneys, which help convert vitamin D to its active form, sometimes do not work as well when people age. Therefore, some older Americans may need vitamin D from a supplement.
It is important for individuals with limited sun exposure to include good sources of vitamin D in their diets. Homebound individuals, people living in northern latitudes such as in New England and Alaska, women who cover their body for religious reasons, and individuals working in occupations that prevent exposure to sunlight are at risk of a vitamin D deficiency. If these individuals are unable to meet their daily dietary need for vitamin D, they may need a supplement of vitamin D.
Individuals who have reduced ability to absorb dietary fat (fat malabsorption) may need extra vitamin D because it is a fat soluble vitamin. Some causes of fat malabsorption are pancreatic enzyme deficiency, Crohn's disease, cystic fibrosis, sprue, liver disease, surgical removal of part or all of the stomach, and small bowel disease. Symptoms of fat malabsorption include diarrhea and greasy stools.
Vitamin D supplements are often recommended for exclusively breast-fed infants because human milk may not contain adequate vitamin D. The Institute of Medicine states that "With habitual small doses of sunshine, breast-fed or formula-fed infants do not require supplemental vitamin D." Mothers of infants who are exclusively breastfed and have a limited sun exposure should consult with a pediatrician on this issue. Since infant formulas are routinely fortified with vitamin D, formula fed infants usually have adequate dietary intake of vitamin D.
What is the health risk of too much vitamin D?
There is a high health risk associated with consuming too much vitamin D. Vitamin D toxicity can cause nausea, vomiting, poor appetite, constipation, weakness, and weight loss. It can also raise blood levels of calcium, causing mental status changes such as confusion. High blood levels of calcium also can cause heart rhythm abnormalities. Calcinosis, the deposition of calcium and phosphate in soft tissues like the kidney can be caused by vitamin D toxicity.
Consuming too much vitamin D through diet alone is not likely unless you routinely consume large amounts of cod liver oil. It is much more likely to occur from high intakes of vitamin D in supplements. The Food and Nutrition Board of the Institute of Medicine considers an intake of 1,000 IU for infants up to 12 months of age and 2,000 IU for children, adults, pregnant, and lactating women to be the tolerable upper intake level. Daily intake above this level increases the risk of adverse health effects and is not advised.
Selected food sources of vitamin D
The following lists suggests dietary sources of vitamin D. Fortified foods are a major source of vitamin D. Breakfast cereals, pastries, breads, crackers, cereal grain bars and other foods may be fortified with 10% to 15% of the recommended daily value for vitamin D. It is important to read the nutrition facts panel of the food label to determine whether a food provides vitamin D.
Cod Liver Oil, 1 Tbs: 1,360 IU
Salmon, cooked, 3 1/2 oz: 360 IU
Mackerel, cooked, 3 1/2 oz: 345 IU
Sardines, canned in oil, drained, 3 1/2 oz: 270 IU
Milk, nonfat, reduced fat, and whole, vitamin D fortified, 1 c: 98 IU
Margarine, fortified, 1 Tbs: 60 IU
Pudding, 1/2 c prepared from mix and made with vitamin D fortified milk: 50 IU
Dry cereal, Vitamin D fortified w/10% of the recommended daily value, 3/4 c: 40-50 IU (other cereals may be fortified with more or less vitamin D)
Liver, beef, cooked, 3 1/2 oz: 30 IU
Egg, 1 whole (vitamin D is present in the yolk): 25 IU
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*These statements about Vitamin D have not been evaluated by the Food and Drug Administration. Vitamin D is not intended to diagnose, treat, cure, or prevent any disease.
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