Bone density. Calcium and magnesium, the predominant minerals in bone, need vitamin D to be incorporated into bone. Researchers found that large periodic doses of vitamin D (the equivalent of 800 IU daily) reduced the risk of fractures by 33 percent (British Medical Journal, 2003). Smaller amounts are insufficient for preventing fractures (British Medical Journal, 2005).

Muscle strength. Muscle synthesis requires vitamin D, and weakness and frailty (especially in the elderly) point to deficiency. Many doctors now believe that weak skeletal muscles, not bones, lead to falls and fractures. Studies show that 700 to 1,000 IU of vitamin D combined with 500 to 600 milligrams of calcium daily reduces the risk of falls by up to 65 percent. (Journal of the American Geriatric Society, 2005; Archives of Internal Medicine, 2006).

Diabetes. A three-year study at the Tufts-New England Medical Center found that a combination of 700 IU of vitamin D and 500 milligrams of calcium prevented increases in fasting blood sugar, while blood-sugar levels increased substantially among people taking placebos. Both vitamin D and calcium are needed for normal insulin function.

Immunity. Doctors have long known that vitamin D enhances immune function. Before the advent of antibiotics, treatment of tuberculosis consisted of sun exposure or cod liver oil, both vitamin D sources. In 2006, researchers discovered that vitamin D boosts levels of a powerful germ-killing peptide found in immune, skin and other cells (Science, 2006). Some researchers now believe that susceptibility to colds and flu is related to low vitamin D levels (Nature Medicine, 2006).

Cancer. More than 60 studies have found that high levels of vitamin D reduce the risk of several cancers. One clinical trial found that vitamin D and calcium supplements led to a 60 percent lower risk of leukemia, myelomas and breast, colon, lung and lymph cancers (American Journal of Clinical Nutrition, 2007).

By mid-winter, most people north of San Francisco, Wichita, Kan., and Richmond, Va., have used up their reserves of vitamin D and will need to supplement. During the winter at northern latitudes, the sun is too low to initiate vitamin D production.

Vitamin D is now regarded as exceptionally safe. A person would have to consume at least 40,000 IU daily for months to develop any signs of toxicity. Vitamin D3 (cholecalciferol) is the preferred form and is substantially more biologically active than vitamin D2 (ergocalciferol).

Vitamin D3 is produced from the lanolin of sheep's wool, so it is not suitable for vegans.