WASHINGTON—Vitamin D appears to lower the risk of breast cancer, but far greater amounts are needed than most women normally take in, according to two studies presented at the 97th Annual Meeting of the American Association for Cancer Research. Evidence has been mounting for several decades that sunlight or dietary intake of vitamin D can influence cancer risk. These two studies examined the sources of vitamin D and the levels of the vitamin associated with a reduced risk of breast cancer.

The first, an epidemiologic study, found that women who had been exposed to high amounts of vitamin D, particularly when they were young, had the greatest reductions in breast cancer incidence (abstract 4009). In this ongoing study, researchers from the Samuel Lunenfeld Research Institute at Mount Sinai Hospital in Toronto have conducted phone interviews with about 576 breast cancer patients and 1,135 age-matched controls from the same province, asking about their intake of vitamin D and levels of outdoor activity at various ages.

Results from these completed interviews show that working in an outdoor job between the ages of 10 and 19 reduced breast cancer risk by about 40%, and frequent outdoor activities between ages 10 and 29 reduced the risk by 35%. Outdoor activities included those that did not involve physical activity, said Mt. Sinai's Julia Knight, PhD, lead author of the study. Therefore, the reduction in risk is probably associated with early exposure to the sun rather than the level of exercise, she said.

Dietary intake of vitamin D also lowered risk. Taking cod liver oil between ages 10 and 29 reduced the risk by 25%, and consuming at least nine glasses of milk a week between ages 10 and 29 lowered it by 35%. Reductions in risk, for both diet and outdoor activities, remained significant even after adjusting for age, ethnicity, family history, age at menarche, and age at birth of a first child.

Meta-analysis

In the second study, researchers pooled data from earlier studies to determine the levels of vitamin D associated with a reduced risk of breast cancer (abstract 4008). Their meta-analysis found "a strong inverse dose-response relationship between the serum concentration of 25-hydroxyvitamin D and the risk of breast cancer," said Cedric Garland, DrPH, of the University of California, San Diego, School of Medicine.

Women with a vitamin D level of more than 52 ng/mL in blood serum had a 50% lower risk of breast cancer than women with vitamin D levels of less than 12 ng/mL. To achieve serum levels of 52 ng/mL, a typical individual would have to take in 1,000 IU of vitamin D a day, far more than the 320 IU that is the current median intake in the United States.

The authors of the study strongly urged that recommendations for vitamin D intake be revised. The National Academy of Sciences has set "adequate intake" at 200 IU a day up to age 50 years, 400 IU between ages 50 and 70, and 600 IU thereafter. There is some increased risk of kidney stones at higher levels, but the National Academy of Sciences has set an upper limit of 2,000 IU a day, and its lowest observed adverse effect level is 3,800 IU a day. "Vitamin D may be more important than people used to think," Dr. Knight said.