Obesity increases the risk of breast cancer in postmenopausal women, a large-scale study co-led by a University of North Carolina Lineberger Comprehensive Cancer Center researcher found the opposite is true for premenopausal women: higher body fat was linked to lower breast cancer risk. Heavier women have a lower overall risk of breast cancer before menopause. Breast cancer is most common in older women.

Obesity has been linked to higher risk for breast cancer in women after menopause which occurs on average at age 51. Since breast cancer is less common in younger women, researchers pooled data from 19 different studies to investigate breast cancer risk for a group of 758,592 women who were younger than 55 years. Studies often have smaller numbers of premenopausal breast cancers since breast cancer is less common at younger ages, and the evidence was not as strong as for postmenopausal breast cancer.

Their analysis linked a higher body mass index, or BMI, to lower breast cancer risk for younger women across this age group, even for women within a normal weight range. There was no threshold at which having a higher BMI was linked to lower cancer risk. The largest reductions in risk were for BMI between the ages of 18 and 24, with a 23 percent lower breast cancer risk linked to each five-unit increase in BMI during this time period.

At ages 25 to 34, each five-unit increase in BMI was linked to 15 percent lower risk. There was a 13 percent lower risk for BMI at ages 35 to 44, and a 12 percent lower risk for BMI at ages 45 to 54 years. Researchers saw the risk decreases linked to higher body mass index for estrogen- or progesterone-receptor positive breast cancer, but they did not see a consistent relationship for BMI and triple negative breast cancer, or hormone-receptor negative breast cancer.

Multiple factors could be contributing to the link between higher BMI and lower breast cancer risk in younger women, such as differences in hormones, including estrogen, growth factors, or breast density. Estrogen can be a driver of breast cancer, but there are different levels and sources of estrogen before and after menopause. Before menopause onset, the primary source of estrogen comes from the ovaries, and estrogen produced by fatty tissue may help to downregulate the amount of estrogen produced by the ovaries.

The amount of estrogen produced by ovaries is driven by feedback loops in the body, the small amount of estrogen produced by fat tissue before menopause may help tell the ovaries that they can produce less estrogen and also regulate other hormones or growth factors. After menopause, the ovaries are no longer the primary source of estrogen. Most estrogens come from adipose, or fat, tissue, women who have more adipose tissue after menopause usually have higher levels of estrogen which can contribute to higher breast cancer risk.
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