What Is It
Technically a hormone, vitamin D is produced within the body when the skin is exposed to the ultraviolet B (UVB) rays in sunlight. Theoretically, spending a few minutes in the sun each day supplies all the vitamin D your body needs, but many people don't get enough sun to generate adequate vitamin D, especially in the winter.
What's more, the body's ability to manufacture vitamin D declines with age, so vitamin D deficiencies are common in older people. But even young adults may not have sufficient vitamin D stores. One study of nearly 300 patients (of all ages) hospitalized for a variety of causes found that 57% of them did not have high enough levels of vitamin D. Of particular concern was the observation that a vitamin D deficiency was present in a third of the people who obtained the recommended amount of vitamin D through diet or supplements. This finding suggests that current recommendations for vitamin D may not be high enough.
What Does It Do
The basic function of vitamin D is to regulate blood levels of calcium and phosphorus, helping to build strong bones and healthy teeth.
• Aids in the body's absorption of calcium.
• Promotes healthy bones.
• Strengthens teeth.
• May protect against some types of cancer.
Studies have shown that vitamin D is important in the prevention of osteoporosis, a disease that causes porous bones and thus an increased risk of fractures. Without sufficient vitamin D, the body cannot absorb calcium from food or supplements - no matter how much calcium you consume. When blood calcium levels are low, the body will move calcium from the bones to the blood to supply the muscles - especially the heart - and the nerves with the amount they need. Over time, this reallocation of calcium leads to a loss of bone mass.
Scientists are continuing to discover more about the functions of vitamin D in the body. Some studies suggest that it is important for a healthy immune system. Others indicate that it may help prevent prostate, colon, or breast cancer .One study found that adequate vitamin D slowed the progression of osteoarthritis in the knees, although it did not prevent the disease from developing in the first place.
How Much You Need
The RDA for vitamin D is 200 IU a day for people under age 50; 400 IU for those ages 51 to 70; and 600 IU for those over age 70. Many experts, however, think the recommendations for people over age 50 are too low.
If You Get Too Little: A vitamin D deficiency can harm the bones, causing a bone-weakening disease in children (rickets) and increasing the risk of osteoporosis in adults. A deficiency can also cause diarrhea, in-somnia, nervousness, and muscle twitches. The likelihood of a child developing rickets today is remote, however, because vitamin D is added to milk. In addition, children typically spend enough time in the sun to generate ample vitamin D.
If You Get Too Much: Although your body effectively rids itself of any extra vitamin D it makes from sunlight, overloading on supplements may create problems. Daily doses of 1,000 to 2,000 IU over six months can cause constipation or diarrhea, headache, loss of appetite, nausea and vomiting, heartbeat irregularities, and extreme fatigue. Continued high doses weaken the bones and allow calcium to accumulate in soft tissues, such as the muscles.
How To Take It
Dosage: As little as 10 to 15 minutes of midday sunlight on your face, hands, and arms two or three times a week can supply all the vitamin D you need. But if you are over age 50; if you don't drink milk (which is fortified with vitamin D); if you don't get out doors much between the hours of 8 A.M. and 3 P.M.; or if you always wear sunscreen, you might want to consider vitamin D supplements. Many experts recommend 400 to 600 IU a day for people over age 50 and 800 IU for those over age 70; 200 to 400 IU a day is probably sufficient for younger adults.
Guidelines For Use: Supplements can be taken at any time of day, with or without food. Most daily multivitamins contain up to 400 IU of vitamin D. It is also often found in calcium supplements.
Vitamin D is added to milk; one cup contains 100 IU. Some breakfast cereals are fortified with 40 to 100 IU of vitamin D in each serving. Fatty fish, such as herring, salmon, and tuna, are naturally rich in the vitamin.
• Calcium and vitamin D supplements slowed bone loss and reduced the incidence of fractures in 176 men and 213 women over age 65 participating in a recent study. They took 500 mg of calcium and 700 IU of vitamin D a day for three years.
• Vitamin D may help prevent colon cancer. In a study of 438 men, researchers found that those with colon cancer had lower blood levels of vitamin D than those who did not have the disease. Across the board, men with the highest vitamin D intake had the best chance of avoiding colon cancer. More study is needed to confirm this finding and to see if the risk is the same for women.
Overuse of vitamin D supplements can result in elevated blood levels of calcium, leading to weight loss, nausea, and heart and kidney damage.