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What is vitamin D?

Vitamin D is a mixture of hormone-like compounds (sterols) that are instrumental in bone metabolism. It is often called the "sunshine vitamin" because it is formed by the action of the sun's ultraviolet rays on sterols in either plants or humans. The plant form, vitamin D2, is produced when a plant sterol (ergosterol) is exposed to the sun. Vitamin D3, the human form, is manufactured in the skin from 7-dehydrocholesterol, a derivative of cholesterol.

As with other fat-soluble vitamins, fat or bile is needed in order for the body to absorb vitamin D. After vitamin D2 or D3 is absorbed into the bloodstream from the small intestine, it is converted, first by the liver and then by the kidneys, into its metabolically active form (1,25-dihydroxycholecalciferol) for the body's use.

Why is vitamin D important to nutrition?

Vitamin D enables the body to utilize and retain calcium and phosphorus, the minerals that are responsible for building and maintaining bones and teeth. Children who are deprived of vitamin D develop rickets, a disease characterized by soft, malformed bones. They may also have malformed or missing teeth. In adults, vitamin D deficiency can cause softening of the bones (osteomalacia) and also increase the risk of osteoporosis, a thinning of the bones due to excessive calcium loss.

How much vitamin D do I need?

The latest Recommended Dietary Allowances (RDAs) for vitamin D (in micrograms) are:

• 7.5 for infants up to 5 months of age.
• 10 for infants over 5 months and for children, adolescents, and adults up to age 24.
• 5 for adults over 24.
• 10 during pregnancy and breast feeding.

Vitamin supplements and nutrition labels often list vitamin

D in international units (IU). Ten micrograms of vitamin D (the D3 form) are equal to 400 IU.

What happens if I don't get enough vitamin D?
As noted earlier, bone disorders such as rickets, osteomalacia, and osteoporosis are the most common consequences of vitamin D deficiency. Without adequate vitamin D, calcium and phosphorus cannot be absorbed from the small intestine into the bloodstream. Instead, these minerals are excreted in the stools. As blood levels of calcium drop, the body must call upon the bones to release some of their stored minerals. Over time, the bones may become weak and damaged.

In children whose bones are still growing, the lack of calcium causes a softening of existing bone, resulting in skeletal deformities. The ribs bend, causing a deformed chest. Leg bones curve, resulting in bowlegs and knock knees. Curvature of the spine is also common. Teeth erupt late and may be small or malformed. They also decay more readily than normal teeth. Babies with rickets may develop a soft skull, and closing of the soft spot (fontanelle) will be delayed.

Vitamin D deficiency in older adults speeds osteoporosis. Studies of nursing-home residents and other older shut-ins have found an accelerated loss of bone tissue, which can be minimized or slowed by giving vitamin D supplements along with adequate calcium supplements.

Other possible manifestations of vitamin D deficiency include muscle cramps, spasms, twitching, or convulsions. These problems are also due to the abnormal calcium metabolism because muscles require calcium to function properly.

What happens if I get too much vitamin D?
As with other fat-soluble vitamins, excess vitamin D is stored in the liver rather than being excreted from the body. Prolonged use of vitamin D supplements can cause toxicity. Signs of vitamin D poisoning include loss of appetite, vomiting,

headache, drowsiness, diarrhea, and urinary abnormalities (either excessive thirst and urination or markedly reduced urinary output).

In an attempt to make use of the excessive vitamin D, extra calcium is absorbed. This can lead to serious metabolic complications, including calcium deposits in or calcification of the heart, the larger arteries, the kidney tubules, and other soft tissues.

Over time, relatively small, daily overdoses of vitamin D — as little as 25 to 50 micrograms (10,000 to 20,000 IU) in children and 250 micrograms (100,000 IU) in adults — can produce toxicity.

Should I take vitamin D supplements?
Vitamin D deficiency is rare in the United States. Just 10 to 15 minutes of exposure to the midday sun 2 or 3 times a week should provide all the vitamin D that a person needs. With age, however, the skin loses some of its ability to manufacture vitamin D, so up to 30 minutes of sun exposure may be needed. (Use moderation, however; remember that too much sun increases the risk of skin cancer.)

In some instances, doctors may recommend vitamin D supplements. For example, premature babies may require extra vitamin D. People with osteoporosis, kidney disease, or intestinal disorders that interfere with fat and vitamin D metabolism may require special formulations. In general, however, vitamin D supplements should not be given (especially to children) without medical supervision.

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