The Relationship between Weight and Breast Cancer
By Lauren Teras, PhD
Breast cancer is the most common cancer among women today. More than 1 million women world-wide are diagnosed with this cancer each year, mostly in the 50 and older age group. Breast Cancer Awareness Month highlights this international public health problem, and it is a good time to consider ways in which we can reduce our risk of this cancer. While many factors beyond our control contribute to risk, like age and family history, we do know of a few ways we can lower the risk of breast cancer.
Worldwide obesity has more than doubled since 1980. Once considered a problem only in high income countries, being overweight and obese is now dramatically on the rise all over the world, particularly in urban areas. As of 2008, the World Health Organization estimated that 1.4 billion adults were overweight, including 300 million obese individuals. In the year 2000, for the first time in human history, the number of adults worldwide who were overweight was greater than the number of adults who were underweight. In fact, approximately 65% of the world's population lives in countries where being overweight and obese kills more people than being underweight. The U.S. is near the front of the pack as the country with the 4th highest rate of obesity; about 2/3 of people in America are overweight, including approximately 1/3 are obese.
Reports from both the International Agency for Research on Cancer and the World Cancer Research Fund conclude that there is convincing evidence that being obese or overweight causes breast cancer after menopause. And a 2006 study from the American Cancer Society Cancer Prevention Study-II found that gaining weight as an adult was an even more important risk factor than current weight for post-menopausal breast cancer. The study found that women who gained 60 or more pounds after age 18 had double the risk of being diagnosed with post-menopausal breast cancer compared to women who maintained their weight over the same time period. Other studies have reported similar findings.
The higher risk of breast cancer for women who gain weight is likely due to higher levels of estrogen, since fat tissue is the largest source of estrogen among women who are post-menopausal.
Interestingly, current evidence suggests that heavier body weight does not increase breast cancer risk before menopause and may even slightly lower risk. Scientists do not fully understand why this is, but since breast cancer is much more commonly diagnosed after menopause, the risk of post-menopausal breast cancer far outweighs any potential benefit on pre-menopausal breast cancer risk. So we know that being overweight increases your risk of post-menopausal breast cancer, and we know that many people around the world are overweight or obese. The natural next question is, can this higher risk be reduced through weight loss? This turns out to be a difficult question for scientists to answer for many reasons. First and foremost, very few people actually lose weight during adulthood, so it is difficult to find a large enough group of people to study. Secondly, those who do lose weight don't usually keep it off. Maintained weight loss over a long period of time is quite rare.
One place to look for people who have successfully lost large amounts of weight is in groups of bariatric surgery patients, and the evidence from these studies is encouraging. Women who have undergone bariatric surgery were less likely to develop or die from cancer than women who were still obese. One study found that bariatric surgery was associated with an 83% lower risk of breast cancer. However, it is not clear that results of these surgery studies can be applied to the general public. Bariatric surgery studies tell us about large, rapid amounts of weight loss all at once, but they do not tell us about smaller or more gradual weight loss from exercise and changing one's diet. In addition, bariatric surgery patients, unlike the people they are being compared to, are screened for cancer and other conditions before the surgery; therefore, they are less likely to have an undetected disease at study entry. Another possibility is that the reduced risk of cancer after bariatric surgery may be linked to physical and biochemical changes from the surgery, rather than the fact that there's less fatty tissue. There is also, however, some evidence from general populations that weight loss that's kept off is associated with lower breast cancer risk. Research from the well-respected Nurses' Health Study found weight loss maintained for 4 or more years after menopause was associated with a 40% lower risk of breast cancer.
There is still much to learn about weight loss and breast cancer. How much weight loss is necessary to reduce risk? Does it depend on how much you weigh to start? Does the age at which you lose the weight make a difference?
While we don't yet fully understand the relationship between weight loss and breast cancer risk, we do know that avoiding further weight gain is helpful. We also know that losing even a little bit of weight (5%-10% of starting weight) improves your overall health by lowering your risk of cardiovascular disease and diabetes. What's more, exercise can also lower breast cancer risk above and beyond the impact it can have on losing weight. The good news is that body weight is one of the few breast cancer risk factors women have control over. So this month let's focus on the positive and do what we can to maintain a healthy body weight.
Dr. Teras is senior epidemiologist for the American Cancer Society.