Vitamin D

Vitamin D is unique for two reasons: it's synthesized by the body when the skin is exposed to sufficient sunlight; and because it's involved in regulating the function of specific organs, it's also a hormone.

Vitamin D's primary function is to enhance intestinal absorption of calcium and phosphorus in order to maintain adequate blood levels of the two minerals for calcification of bone and cartilage. As a hormone, vitamin D regulates the activity of an enzyme, 1-ahydroxylase, which is necessary to convert vitamin D to its active form in the body. Blood levels of vitamin D and calcium determine the amount of enzyme activity. When blood levels of calcium and vitamin D drop, enzyme activity increases in order to produce more of the metabolically active vitamin. In this way, we are protected against "low spots" in our intake of calcium and vitamin D. However, if enough of the metabolically active vitamin is produced-either because of a severe deficiency in calcium or a toxic overdose of vitamin D-calcium will be mobilized from the bones, too.

Vitamin D also plays a role in the proper mineralization of bone at the site of the bone itself. Little is known about exactly how this occurs, however.

Vitamin D also appears to be important to the structure and function of the thyroid and pituitary glands, since one of the effects of a deficiency is malfunction of those organs.

Low levels of extra vitamin D in the diet have also been found to lower the blood cholesterol. Even though the amount of vitamin D necessary to achieve this effect is far from toxic, not enough is known about this to recommend supplements of vitamin D for people with high cholesterol.

A deficiency in vitamin D results in severely diminished absorption of calcium and phosphorus from the intestine. Urinary and fecal levels of these minerals will rise and blood levels will drop. To maintain normal blood levels of calcium (necessary for functions other than bone mineralization), calcium will be mobilized from the bones. In adults, this softening of the bones is called osteomalacia. In children, whose bones are still growing, calcium and phosphorus are not deposited in the cartilage matrix, and the cartilage is not replaced as it should be; so the bones become swelled with cartilage at the ends and soft and malformed in the middle. This is called rickets. Vitamin D deficiency during development can also result in thin, irregular tooth enamel.

Low dietary intake and blood levels of vitamin D (and calcium) have also been associated with high blood levels of lead. The low blood and tissue levels of calcium are believed to mobilize lead from the bones and deposit it in the soft tissues of the body, where it can do more harm.

Vitamin D deficiency is more common than it should be. Many factors can contribute to a vitamin D deficiency, including diet, intestinal malabsorption, gastric surgery, and insufficient exposure to sunlight.

Several medications can interfere with the metabolism of vitamin D and cause a deficiency, including liquid paraffin taken as a laxative, anticonvulsant medications (primidone, phenytoin, phenobarbitone), the hypnotic glutethimide, and corticosteroids such as prednisone.

Vitamin D is, of course, the sunshine vitamin. When the skin is exposed to ultraviolet light at the correct wavelength, vitamin D is synthesized below the surface. There is some question among scientists as to which is more important as a source of vitamin D, sunlight or diet. A day in the sun in northern parts of America during the months of March through October can provide as much as 10,000 units of vitamin D. During the winter months, however, very little is provided.

Where is it found ?

Natural sources of vitamin D are scarce, unless you include sunshine. Fish, especially fish with heavy amounts of oil in the flesh (saltwater fish, salmon, sardines, herring) are good sources. Liver, egg yolk, and summer milk are also good sources. Most current dietary sources of vitamin D are actually foods which have been "fortified" with synthetic vitamin D.

Vitamin D supplements are available in doses ranging from a few units to several hundred units. Two forms of the vitamin are used, vitamin D2 and D3. D2 is the synthetic form, and is also called calciferol, or activated ergosterol. D3 is the "naturally occurring" form and is usually obtained from fish liver oils.

Who is likely to be deficient ?

Vitamin D deficiency, which causes abnormal bone formation, is more common after the winter due to restricted sunlight exposure in that season. Deficiencies are also more common in strict vegetarians (who avoid vitamin D-fortified dairy), dark-skinned individuals, people with malabsorption conditions, liver disease, or kidney disease, and alcoholics. People with liver and kidney disease can make vitamin D but cannot activate it.

One in seven adults has been reported to be vitamin D deficient. In hospitalized patients under age 65, 42% were reported to be vitamin D deficient. This same report found that 37% of the people studied were vitamin D deficient despite the fact that they were eating the currently recommended amount of this nutrient. Vitamin D deficiency is particularly common in the elderly.

How much to take

People who get plenty of sun exposure don't require supplemental vitamin D. (Sunlight increases vitamin D synthesis when it strikes bare skin.) Otherwise, 400 IU per day is a safe adult dose.

Side effects

Individuals with sarcoidosis who have elevated blood levels of calcium, and people with hyperparathyroidism should not take vitamin D without consulting a physician. Too much vitamin D taken for long periods of time can lead to headaches, weight loss, and kidney stones, and rarely to deafness, blindness, increased thirst, increased urination, diarrhea, irritability, failure to gain weight in children, and even death. Most people take more than 400 IU per day (a safe amount for adults), although one study showed that 800 IU per day prevented bone loss more effectively than 200 IU per day in postmenopausal women. Anyone wishing to take more than 1,000 IU per day for long periods of time should consult a physician. People should remember that the total daily intake of vitamin D includes vitamin D from fortified milk and other fortified foods, cod liver oil, and supplements that contain vitamin D. In addition, people who receive adequate sunlight exposure do not need as much vitamin D in their diet as do people who receive minimal sunlight exposure.

Vitamin D increases both calcium and phosphorus absorption. Vitamin D has also been reported to increase absorption of aluminum. Increased blood levels of calcium (which can be a marker for vitamin D status) have been linked to heart disease. Some, but not all, research suggests that vitamin D may slightly raise blood levels of cholesterol in humans.

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