Vitamin D is an essential nutrient that our bodies can make on any reasonably sunny day. In fact, it’s called the sunshine vitamin because we can make all we need if we have enough sunlight hitting our skin. Ultraviolet rays from the sun convert one type of chemical compound in our skin to a substance called previtamin D3, which is then converted to vitamin D3. This in turn is converted twice more, in the liver and kidneys, into active vitamin D. While either liver or kidney disease may cause a shortage, people who have liver disease can usually make enough to get by. People with kidney disease, on the other hand, often need to take the active form of vitamin D, which is available only by prescription, to prevent deficiency.
In the body, vitamin D acts more like a hormone than a vitamin. In other words, it has a direct effect on organs such as the intestines, kidneys, and bones. The cells of these organs have receptor sites, similar to docking platforms, just for vitamin D. All of these organs respond to the vitamin by making calcium available for bone growth.
Harden Up and Grow Right
Open just about any nutrition textbook to the pages on vitamin D and you’ll see haunting photographs of children with rickets, a deficiency condition. Their heads are large because their skull bones haven’t fused properly, and their legs are bowed because their bones are too soft to support their weight.
Rickets is no longer common in the United States, but "vitamin D deficiency may play a contributing role in two common conditions that affect older adults," says Hector DeLuca, Ph.D., chairman of biochemistry at the University of Wisconsin in Madison and a leading vitamin D researcher. One of these conditions is osteomalacia, or softening of the bones, and the other is osteoporosis, or bone loss. "Low blood levels of vitamin D make both of these conditions worse," he says. In fact, just a few weeks’ worth of the Daily Value, which is 400 international units (IU), or 10 micrograms, will improve some symptoms of these conditions if vitamin D levels are low. "People improve dramatically once they start getting enough vitamin D," Dr. DeLuca says.
Vitamin D helps to make calcium and phosphorus available in the blood that bathes the bones. As these essential minerals are deposited, the bones mineralize, or harden.
It makes calcium and phosphorus available in three ways. In the intestines, it sends cells the message, "Absorb more calcium and phosphorus." In the kidneys, the instruction is "Don’t pee out that calcium and phosphorus—recirculate it." And when blood levels of calcium begin to drop, vitamin D sends bone cells the message, "Start breaking down bone and get that calcium into the bloodstream."
That final message is important because calcium is needed throughout our bodies to maintain normal muscle and nerve function. Low levels can lead to fatal convulsions. And if you have only calcium in circulation—without the vitamin D that’s necessary to help incorporate it—you could end up with a calcium deficiency. You need that calcium, and you also need vitamin D to help it do its job.
SUPPLEMENTSNAPSHOT
| Vitamin D May help: Osteomalacia, osteoporosis, and multiple sclerosis. Daily Value: 400 international units (10 micrograms). Who’s at risk for deficiency: People who have chronic intestinal absorption problems or chronic liver or kidney disease, people who don’t eat many dairy products and aren’t exposed to sunlight, and those who use sunscreen at all times. Good food sources: Fatty fish such as mackerel and salmon and fortified milk and milk products. (The best nonfood source of vitamin D is the sun.) Cautions and possible side effects: Large doses (more than 2,000 IU daily) over several months can cause high blood levels of calcium, kidney stones, and calcium deposits in heart and kidney tissues, which can be fatal. |
Why D Is a Big Deal
Bones aren’t the only parts of our bodies that vitamin D befriends. Researchers are finding more and more places where it’s active.
Forms of vitamin D are being studied in the laboratory for the treatment of breast, prostate, and colon cancer, plus a deadly skin cancer, melanoma. One form of vitamin D, as a topical cream, can cure up to 70 percent of people who have psoriasis.
Researchers don’t know exactly how vitamin D works in these cases, but like vitamin A, it promotes a process called cell differentiation. Like a traffic cop, it directs developing cells in the right direction, toward their proper, mature form. In the skin, for example, it normalizes the growth rate of cells called keratinocytes. In psoriasis, these cells proliferate like crazy without going through their normal growth process. That’s what causes the snowstorm of flaky skin.
Also, a deficiency of vitamin D "almost certainly plays a role in the development of type 1 (insulin-dependent) diabetes," Dr. DeLuca adds. The vitamin is needed for cells in the pancreas, called islet cells, to produce insulin, the hormone that allows cells to take up blood sugar. Without adequate vitamin D, islet cells don’t produce insulin.
In animals with an inherited tendency to develop type 1 diabetes, the active vitamin D hormone helps prevent the disease, Dr. DeLuca says. While studies have not yet been done in humans, researchers speculate whether it might prove to have a similar effect.
Help for T-Helpers
Vitamin D also helps to regulate some of the white blood cells that make up the immune system. In animals, it helps one kind of "supervisor" (T-helper) cell release biochemicals that stop another kind of T-helper cell from attacking the body’s own tissues. Thus, in animals at least, this vitamin helps block the destructive assault that occurs in autoimmune diseases such as rheumatoid arthritis, lupus, and multiple sclerosis, Dr. DeLuca says.
Researchers at the University of Wisconsin may soon learn whether fairly small doses of the active form of vitamin D can help stop nerve-damaging lesions in people who are newly diagnosed with multiple sclerosis.
Milk May Let You Down
Vitamin D deficiency is a real problem in the United States, and it becomes increasingly likely in people age 50 and older, says Michael Holick, M.D., Ph.D., chief of the section on endocrinology, nutrition, and diabetes at Boston University Medical Center. People just don’t get enough of the few foods that contain vitamin D—milk and fatty fish such as mackerel and salmon.
"In fact, milk may not be as reliable a source as people think it is," Dr. Holick says. In the United States, milk is generally fortified with 400 IU of vitamin D per quart. When Dr. Holick’s laboratory analyzed samples of milk in all regions of the United States and western Canada, however, researchers found that many didn’t contain as much vitamin D as they were supposed to. Fat-free milk was worst: One out of every six samples contained no vitamin D at all.
"Most older people, especially those who use sunscreen, probably don’t get enough sun to meet their vitamin D requirements," Dr. Holick says. Nevertheless it is possible to get an adequate amount of vitamin D from sunlight, he says. Dr. Holick recommends that you expose your hands, face, and arms to sunlight in the midmorning or afternoon. If you live in the north—about the latitude of Boston—you’ll have enough exposure if you get 5 to 15 minutes of sunlight three times a week in spring, summer, and fall. Winter sun isn’t strong enough to meet your needs for vitamin D unless you live in Florida or a similar clime.
Supplements are another reliable way to get what you need if you’re not getting enough vitamin D from dairy sources or sunlight. You can take a multivitamin/mineral supplement that offers the Daily Value of 400 IU.
The National Research Council, which sets standard, official guidelines for U.S. health agencies, has recommended a daily dose of 600 IU for people over 70. Some studies show that people over 65 benefit from getting up to 800 IU a day, especially during the winter months.
