WASHINGTON — Women who get plenty of vitamin D, especially early in life, are less likely to develop breast cancer than women who get smaller amounts of the vitamin, according to data from two studies presented at the annual meeting of the American Association for Cancer Research.
Julia Knight, Ph.D., of Mount Sinai Hospital in Toronto, and her colleagues conducted a case-control, population-based study of women aged 20–59 years from a Canadian cancer registry and the surrounding community. They identified 971 cases of breast cancer and 1,135 controls and asked the women about lifetime sun exposure and dietary vitamin D.
Logistic regression analyses adjusted for variables including age, ethnicity, age at menarche, age at first pregnancy, duration of breast-feeding, and family history of breast cancer.
Overall, participating in any outdoor work at any point in life resulted in about a 40% reduction in breast cancer risk (odds ratio 0.60). Participation in at least six types of outdoor activities at ages 10–19 years and at least five types of outdoor activities at ages 20–29 years reduced the risk by about 35%. Outdoor activities did not have to involve physical activity, Dr. Knight noted. However, the risk reduction was not as robust in women who reported that they had kept their arms and legs covered outside.
In addition, drinking at least nine glasses of milk per week at ages 20–29 years was significantly associated with reductions in breast cancer risk, as was the regular consumption of cod liver oil for at least 10 years.
“In general, most women drank more milk when they were younger,” Dr. Knight said. Vitamin D exposure during breast development or prior to pregnancy may be particularly important in preventing breast cancer later on, she added.
The evidence for the protective role of vitamin D appeared to weaken with age. By ages 45–54 years, there was some evidence of reduced risk associated with outdoor activity, but it was no longer significant, although the risk was no longer affected by whether the arms and legs were covered.
When asked about the role of vitamin D supplements, Dr. Knight admitted that supplements of any sort complicate the picture. “The problem with supplements is that there tends to be a reduced risk in anyone who takes any kind of vitamin supplements, whether they include vitamin D or not,” she said. People who take supplements tend to have other characteristics of a healthy lifestyle, she said.
“We did ask about sunscreen. We are still analyzing that because it is complicated, since sunblock use has only become common in the last 20 years,” she said.
Increasing the levels of serum vitamin D in the body, regardless of age, may reduce the risk of breast cancer, according to a second study presented at the meeting by Cedric F. Garland, Dr.P.H., of the University of California, San Diego.
Dr. Garland and his associates reviewed data on a total of 1,760 women in two studies. In the first, a serum 25-hydroxyvitamin D (25[OH]D) level of 35 ng/mL was associated with a 20% drop in breast cancer risk (Cancer Epidemiol. Biomarkers Prev. 2005;14:1991–7).
A second study of 179 women with breast cancer and 179 controls showed that as vitamin D levels in the blood increased, the risk of breast cancer decreased. Women with 25-(OH)D levels less than 50 ng/mL were six times more likely to develop breast cancer than those with levels greater than 50 ng/mL (Eur. J. Cancer 2005;41:1164–9).
Dr. Garland chose these two studies because 25 (OH) D has been shown to vary with geography. Death rates from breast cancer could be as much as three times higher in the Northeast, compared with the Southwest, because of reduced vitamin D intake from sunlight, he noted.
Based on these studies, “if we can get women up to 20–22 ng/mL of serum vitamin D, we could expect a 20% reduction in breast cancer risk. We think this is a good aim,” Dr. Garland said. Levels closer to 52 ng/mL—which are common among some women in sunny areas such as California—would be even better, he noted. Some women in the Northeast have vitamin D levels of about 15 ng/mL, and some women have levels so low that they are barely detectable, he added.
A vitamin D intake of 2,000 IU/day has been associated with vitamin D levels of 32 ng/mL. That intake is well above the average consumption of most American women, but below the upper limit of 2,400 IU that is currently recommended by the National Academy of Sciences. But even 1,000 IU/day may produce a vitamin D level of about 22 ng/mL.