Like vitamins A, E and K, vitamin D is a fat-soluble vitamin. In its pure form it is a white crystalline substance. As a supplement, however, it is usually found as a light yellow oil. It may be derived from cod-liver oil, which has 425 IU of vitamin D per teaspoon.
Vitamin D is unique among vitamins in two ways: first, it can be synthesized in the skin from sunlight,which is why it is sometimes called the sunshine vitamin. Given enough exposure to sunlight you need consume no vitamin D at all in the foods you eat. Second, vitamin D is the only vitamin whose biologically active form is a hormone - calcitriol.
Vitamin D is converted into this hormone in two stages, in the liver.
Q. What does vitamin D do?
A. Vitamin D's most important role is the regulation of two minerals, calcium and phosphorus. Both minerals are important for normal growth and development, especially the mineralization (hardening) of bone,
Vitamin D stimulate the absorption of calcium from the gut. Without it, calcium cannot be absorbed; It also helps to harden and strengthen bones and stimulates the kidneys to reabsorb some calcium, thus saving the body from excreting calcium.
Q. What happens to bones if you're not getting enough vitamin D?
A. If you become vitamin D-deficient, your body increases production of a hormone which actually removes calcium from your bones.
In children, the classic vitamin D deficiency disease is rickets, a condition characterized by bones so soft that they bend under the body weight. Children with rickets are bowlegged or knock-kneed. The adult equivalent of rickets, a condition called osteomalacia, is also characterized by soft bones. Symptoms include bone deforterized by soft bones. Symptoms include bone deformity, pain and tenderness, and muscle weakness,
Q. Does low vitamin D intake play a role in the development of osteoporosis?
A. Apparently. Although estrogen deficiency is generally considered to be the principal cause of osteoporosis, getting adequate vitamin D is important for preventing bone loss. In fact, a research study has shown that postmenopausal! women who took 400 1U of vitamin D (twice their RN1) daily reduced wintertime bone loss and improved bone density when compared with women who got approximately 100 IU of vitamin D a day. Both groups of women also took a supplement of calcium every day. It wasn't until the second winter, however, that the bone loss was seen to be greater in the placebo group than in the vitamin D-supplemented group.
Q. Is vitamin D beneficial in other ways in the body?
A. Preliminary research suggests that vitamin D, like vitamin A, may play a role in normal cell growth and maturation, which means it might help to prevent cancer. It also seems to be involved in the regulation of the immune system, which could be important in the prevention and treatment of infectious diseases.
Q. Does vitamin D have anything to do with cancer?
A. Researchers don't know for sure. Colon, rectal and breast cancer rates are highest, however, in areas where people are exposed to the least amount of natural sunlight. This correlation is found worldwide, except in Japan, where, researchers point out, people tend to eat lots of vitamin D-rich fatty fish, such as mackerel and salmon.
In laboratory experiments, the hormone form of vitamin D, calcitriol, does seem to have anti-cancer properties; It has recently been found to inhibit the growth of human colon-cancer and skin-cancer cells in test-tube experiments. Calcitriol may also play a role in the treatment of retinoblastoma, the most common eye tumor of childhood, It has been found to inhibit the growth of this kind of tumor in mice. Please note, however, that it would be the height of folly to rely on calcitriol to cure this potentially fatal condition. Retinoblastoma requires expert ophthalmic attention and usually surgery.
Q. And what about immune function?
A. So far there's no solid proof that vitamin D plays a role in building immunity. But some researchers speculate that one reason sunlight has long seemed to be helpful to people with conditions such as tuberculosis is that it stimulates the body's production of vitamin D. Two test-tube studies have recently shown that the hormone form of vitamin D, calcitriol, stimulates the production of macrophages, immune cells which gobble up bacteria. However, there's little real research evidence on vitamin D's possible role in immune function in humans
Q. How much vitamin D does most people get?
A. Studies show that most people don't reach the RN! of vitamin D from the foods they eat. On average, most men get about 80 IU and most women about 60 ID of vitamin D from foods. In one study, men and women aged 60 to 93 years averaged about 50 IU of a day of vitamin D from foods.
Q. That seems low. Does exposure to the sun make up for the low intake from foods?
A. Yes. Many authorities state that all the vitamin D you need can be obtained by synthesis in the skin under the influence of sunlight. Even minimal exposure - such as going in and out of doors to do daily activities - is more than adequate. During the winter months, however, blood levels of vitamin D tend to drop. And not everyone gets even the minimal amount of sunlight needed.
Q. Who's at risk of a vitamin D deficiency?
A. People in hospital or nursing homes or who simply don't go out much. Clothing is also relevant. Ethnic groups who cover up most of their skin can become vitamin D deficient older people also do not manufacture vitamin D in their bodies as well 35 younger people. And they may consume low amounts of vitamin D-containing foods and take medication which interferes with the absorption or metabolism of vitamin D. Such drugs include cholestyramine (used to control cholesterol and certain cases of diarrhoea), mineral oil, and certain anti-consultants.
Q. Anyone else at risk?
A. Other people who may be at risk include alcoholics, people who don't drink milk, people with absorption problems, and those who live in areas which receive little sunlight
QUICK-REFERENCE GUIDE
RNI
Men; young women aged 1 to 24: 400 IU Women aged 25 and older: 200 IU
Sources
Fatty fish (such as mackerel, tuna and salmon) liver, egg yolks, fortified milk and cereals.
Signs of Deficiency
In children, bones so soft they develop curves when subjected to the body's weight In adults, bone pain and tenderness and muscle weakness.
Possible Toxicity Problems
High doses of vitamin D may cause too high levels of calcium in the blood and calcium deposits in soft tissues. Symptoms include loss of appetite, nausea, vomiting, constipation and fatigue. Doses of less than 1,000 IU daily are unlikely to cause adverse affects.
