Although calcium gets most of the publicity, vitamin D is equally important in preventing bone loss and fractures. Without it, our bodies can't properly absorb and utilize the calcium we take in. Vitamin D also helps maintain normal blood levels of phosphorus, another bone-building mineral. Vitamin D would be essential if it did nothing else, but researchers have discovered that it's active in many tissues besides bone and may play a role in warding off a range of diseases, including cancer, hypertension, and diabetes.
Physicians have known for years that vitamin D deficiencies often occur in people with liver and kidney disease, because these organs are involved in making the active form of vitamin D. Because our bodies make vitamin D when exposed to the sun, elderly and housebound people with poor diets are also known to be at risk. But recent studies suggest that the problem isn't confined to these groups, and that many adults have inadequate levels of vitamin D in their blood, especially in northern latitudes where long winters keep people indoors more. The shortfall may lead to fractures, osteoporosis, and other chronic conditions. Moreover, the incidence of nutritional rickets — a severe vitamin D-deficiency disease once thought to have been nearly eliminated — is on the rise in North America. And researchers at Boston 's Children's Hospital recently reported that 24% of Boston-area adolescents, especially African Americans, are vitamin D deficient.
As a result, some experts are beginning to question the adequacy of current recommendations for vitamin D. They're also speculating that the campaign to protect our skin from the sun has had a harmful effect on our ability to get enough of the sunshine vitamin.
How much more do we need?
The vitamin D recommendation was revised upward in 1997, to 400 IU/day for adults ages 51–70 and 600 IU/day for those over 70. These revised levels reflected a new understanding of vitamin D's role in preventing osteoporosis. But many experts believe the new recommendations are still inadequate for preventing osteoporosis and other conditions associated with low vitamin D.
Some experts believe that adults should take 800–1,000 IU daily of supplemental vitamin D to adequately prevent bone loss and possibly protect against some cancers and other chronic disorders. Another possibility might be to take a single weekly dose of 5,000 IU or a single 100,000-IU dose every few months. In a randomized trial in the United Kingdom , the latter strategy helped reduce fracture rates in elderly women and men. Subjects taking a single 100,000 IU vitamin D capsule every four months for five years had a rate of first fracture that was 22% lower than those taking a placebo, and they had a 33% lower fracture rate at the hip, wrist, and vertebrae.
What does this mean?Given the limitations of sun exposure and dietary vitamin D, the best strategy may be to simply take a vitamin D supplement. Until we know more about the optimal level for bone health, it may make sense to increase your daily intake to 800 IU, especially in northern latitudes. (Be sure you also get 1,000–1,200 mg/day of calcium.) The government has set the safe upper limit at 2,000 IU/day. You can't overdose on the vitamin D your skin makes, and it's unlikely you'll eat enough of it in food to get you into trouble.
February 2004 Update