Vitamin D absorption depends on several variables, including:

• Geographical latitude

• Season

• Time of day

• Age

• Skin pigmentation

• Health status of the GI tract

For example, researchers found that at a latitude equal to that of Boston, sunlight alone cannot produce vitamin D in the skin between the months of November and February. In Canada, that situation persists from October to March.

Age is also a big factor. One study found that less than 30 percent of people over 70 were able to produce adequate vitamin D with the same amount of sun exposure as younger people.

Other research has found that in nursing homes, as much as 67 percent of the patients had very low vitamin D.

There may also be a link between vitamin D deficiency and falling with advancing age. And falls in the elderly that cause a hip fracture lead to a 50 percent death rate.

Muscle biopsies in people with osteomalacia (softening of the bones), which is common in cases of vitamin D deficiency, demonstrated atrophy, fat accumulation, and fibrosis (scarring) in fast-acting muscle fibers — the kind of fibers that prevent hard falls.

Studies that have been conducted to see if giving elderly people vitamin D with calcium would reduce hip fractures demonstrated a positive, though small, effect. Unfortunately, most such studies used insufficient levels of vitamin D. In general, it takes at least 1,000 IU to as much as 2,000 IU of daily vitamin D to raise blood levels.

It has also been shown that vitamin D plays a role in high blood pressure and the risk of heart attack. This is not surprising due to the fact that vitamin D affects the muscles within arteries and the heart muscle itself.