Older women who lose weight may have a lower risk of developing invasive breast cancer than those who maintain or gain weight, a large U.S. study suggests.
While obesity has long been linked to an increased risk of breast cancer, previous research has offered a mixed picture of the potential for weight loss to help reduce that risk. For the current study, researchers assessed weight and height to calculate body mass index (BMI) for more than 61,000 women twice, three years apart.
Then, researchers followed women for an average of 11.4 more years. During this time 3,061 women developed invasive breast cancer.
Compared with women who had stable weight during the initial three years of the study, women who lost at least 5 per cent of their body weight during those first three years were 12 per cent less likely to develop breast cancer over the next decade or so.
"Our results are consistent with a woman being able to lower their cancer risk, even if they remain overweight or obese after losing some weight, since almost none of the women in our current cohort analysis lost sufficient weight to achieve normal weight," said lead study author Dr. Rowan Chlebowski of the City of Hope National Medical Center in Duarte, California.
"That should be an encouraging result for women since modest sustained weight loss can be achievable by many, while weight loss sufficient to return to a non-obese or overweight category is quite difficult," Chlebowski said by email.
All of the women in the study had gone through menopause, when menstruation stops and production of the hormone estrogen drops. After menopause, women's main source of estrogen is fat tissue; being overweight or obese can increase the risk of cancer because estrogen can help tumors grow.
"Women who are overweight or obese likely have an increased risk of postmenopausal breast cancer due to increased hormone levels associated with fat cells," said Dr. Daniel Schauer of the University of Cincinnati College of Medicine, who wasn't involved in the study.
"These hormones, especially estrogen, can promote the development of postmenopausal breast cancer," Schauer told Reuters Health by email. "Losing weight decreases the levels of circulating hormones."
Among the roughly 41,000 women in the study who had a stable weight during the initial three years, participants had an average BMI of 26.7, which is considered overweight.
The 12,000 women who gained weight during the study also started out with an average BMI of 26.7.
Women who lost weight started out heavier.
The roughly 3,300 women who lost weight unintentionally started out with a BMI of 27.9 and half of them lost more than 17 pounds. Women who lost weight intentionally began with an average BMI of 29.9, just shy of the cutoff BMI of 30 to be considered obese, and half of them lost more than 20 pounds.
Weight gain of 5 per cent or more was not associated with an increased risk of breast cancer overall, the researchers report in the journal Cancer. But this amount of weight gain was associated with a 54 per cent higher risk of developing "triple negative" breast cancer, an aggressive and difficult to treat type of cancer.
The study wasn't a controlled experiment designed to prove whether or how weight changes over time might directly impact women's risk of developing or dying from breast cancer.
Researchers only measured women's weight twice, at the start of the study and again three years later, and any changes in weight women reported after that were not verified by medical exams.
For most people, weight creeps up over time, said Dr. Graham Colditz of the Washington University School of Medicine in St. Louis, who wasn't involved in the study.
"So, the first realistic goals is to work to stop gaining. There are health benefits to that, even if you're overweight," Colditz said by email.
"After that, sensibly and slowly losing weight is a good goal," Colditz added. "Five to 10 pounds is a great start that's more easily maintained over time."