Karen Collins, MS, RD, CDN
American Institute for Cancer Research
As evidence has built connecting exercise to lower breast cancer risk, researchers have suggested the link is related to weight loss. Previous research found a connection between obesity and breast cancer risk, and exercise generally leads to lower weight. Now, intriguing evidence is emerging that some of exercise?s effect on lower breast cancer risk may be independent of weight.
The link between weight and breast cancer risk relates to estrogen. Body fat increases estrogen production and a high level of estrogen is linked with some types of breast cancer. After menopause, when fat becomes a significant source of the body?s estrogen, excess weight is strongly linked with greater risk of breast cancer.
The latest major study on exercise comes from the Iowa Women?s Health Study, which followed more than 36,000 women aged 55 to 69. After 18 years of follow up, researchers analyzed the activity level, weight and breast cancer status of the women. When researchers adjusted for weight, they found high activity levels reduced the risk of breast cancer by 9 percent. Researchers also found that women who were most active were 14 percent less likely to develop breast cancer than those least active. The study reaffirmed research that exercise benefits lean as well as overweight women. In this study the greatest protection from exercise was seen in tumors that were estrogen-sensitive but not sensitive to the hormone progesterone. This relationship was unaffected by weight.
Another long-term major study, the Women?s Health Initiative (WHI), followed more than 74,000 women and showed similar results. After adjusting for weight, women who participated in about two hours per week of brisk walking or other moderate exercise showed an 18 percent lower breast cancer risk. At most activity levels, women at a healthy weight showed more benefit from exercise than overweight women.
In a 2006 study sponsored by the National Cancer Institute, vigorous exercise reduced breast cancer risk 12 percent independent of participants? maintaining a healthy weight. In these and other studies the benefit of exercise is seen after controlling for most of the traditional breast cancer risk factors, including age, family history, pregnancy and hormone replacement therapy.
Researchers are working on understanding how exercise can affect cancer risk when it is unrelated to weight. What is well established now is that exercise offers protection. This is true even when people start exercising later in life. (Breast cancer develops over many years and protection seems to build over time. In the WHI study, vigorous exercise at age 50 brought an 8 percent lower risk, but the same exercise at age 35 reduced risk in later years by 14 percent.)
Studies also show that intensity and frequency work together to shape the impact of exercise. In the Iowa Women?s Health Study, the 14 percent overall drop in breast cancer risk was achieved by either at least two times a week of vigorous exercise (jogging, racket sports, aerobics) or more than four times a week of moderate exercise (golf, gardening, bowling, recreational walks). For now, some researchers say that the strongest evidence suggests that moderate-to-vigorous exercise three to four hours a week can potentially reduce the risk of breast cancer 30 to 40 percent.
AICRâ€™s Nutrition Hotline is a free service that allows you to ask a registered dietitian questions about diet, nutrition and cancer. Access it online at www.aicr.org/hotline or by phone (1-800-843-8114) 9 a.m. to 5 p.m. ET Monday-Friday. AICR is the only major cancer charity focused exclusively on the link between diet, nutrition and cancer. It provides education programs that help Americans learn to make changes for lower cancer risk. AICR also supports innovative research in cancer prevention and treatment at universities, hospitals and research centers. It has provided more than $78 million for research in diet, nutrition and cancer. AICRâ€™s Web address is www.aicr.org.