Q & A Library

Orginally Published 3/8/2005

Yes, I am raising my recommendation for vitamin D from 400 IU to 1,000 IU per day. That may sound like quite a leap, but evidence has been accumulating to suggest that the 400 IU now recommended for adults isn't enough for optimum health. We need vitamin D to facilitate calcium absorption and to promote bone mineralization as well as for protection from a number of serious diseases. I recently reviewed a series of studies on vitamin D and bone health with Walter Willett, MD, professor of epidemiology and medicine at Harvard Medical School. Dr. Willett has assembled a compelling review of clinical evidence suggesting that current vitamin D recommendations, including my own, are too low.

To sum up, most adults are simply not getting enough vitamin D for good bone health. We get vitamin D from such foods as fortified milk and cereals as well as from eggs, salmon, tuna, mackerel and sardines, and our bodies make vitamin D with exposure to sunlight. Unfortunately, many people don't spend enough time in the sun to get optimal exposure, particularly in northern latitudes during the gray winter months. In addition, sunscreen blocks vitamin D synthesis in the skin, and dermatologists have made people so paranoid about sun exposure that many people in lower latitudes and in summer don't get enough either.

And, clearly, many people are falling short of their vitamin D needs. A study published in the February 2001 issue of the American Journal of Clinical Nutrition showed that even while taking a 1,000 IU supplement, fewer than half of the participants in a Canadian study were getting enough to achieve optimal blood levels of vitamin D. And research published in the January 2003 issue of the American Journal of Clinical Nutrition showed that 1,700 IU was needed to bring blood levels of vitamin D to optimal levels during winter months in Nebraska. Dr. Willett told me that other studies have shown that an intake of 400 IU of vitamin D per day has no impact on the risk of fractures, but that 700 to 800 IU per day, with or without calcium supplements, does seem to reduce fracture incidence. He noted that some evidence further suggests that a higher intake of vitamin D may reduce the risk of prostate cancer, colon cancer, other malignancies, and multiple sclerosis.

Don't be concerned that 1,000 IU will give you too much vitamin D - exposure to sunlight in the summer can generate between 10,000 and 20,000 IU of vitamin D per day with no ill effects. What's more, no adverse effects have been seen with supplemental vitamin D intakes up to 10,000 IU daily.

Increasing my vitamin D recommendation to 1,000 IU daily may not be the last word on this subject. Depending on the amount of sunshine available where you live, you may need to take even more. Stay tuned.

Andrew Weil, M.D.