Normal Weight Women Have Higher Breast Cancer Risk Than Obese Women Before Menopause

Contrary to the common belief that obesity increases the risk of breast cancer, a new study has found that women with normal weight should be more cautious about breast cancer than obese women before menopause. This study, conducted by a joint research team from Kangbuk Samsung Hospital’s Healthcare Data Center and Hanyang University, followed over 120,000 women aged 20 to 54 for an average of 6.7 years.

These women underwent health screenings at the Kangbuk Samsung Hospital’s Comprehensive Examination Center from 2011 to 2019. The research team analyzed the relationship between body composition metrics such as body fat percentage, body mass index (BMI), waist circumference, and muscle mass, and the incidence of breast cancer.

The results revealed that for premenopausal women, a 10% increase in body fat percentage was associated with a 12% decrease in the risk of breast cancer. Specifically, with every one-unit increase in BMI (kg/m2), waist circumference (cm), and body fat mass (kg), the risk of breast cancer decreased by 1-4%. In contrast, muscle mass showed no significant correlation with breast cancer risk.

It is generally known that excessive exposure to the female hormone estrogen increases the likelihood of developing breast cancer. Therefore, obesity has also been recognized as a major risk factor for breast cancer because fat cells, in addition to the ovaries, secrete estrogen.

However, this conventional wisdom is more applicable to postmenopausal women and may be exceptional for younger, premenopausal women, as highlighted by this study. The researchers pointed out that obese premenopausal women might have lower exposure to estrogen due to factors such as irregular menstrual cycles, which reduce ovarian estrogen secretion.

Professor Jang Yoo-Soo from Kangbuk Samsung Hospital advised, “Although the risk of breast cancer may decrease, it is crucial to maintain an appropriate weight for overall health, as obesity increases the risk of other chronic diseases such as diabetes, hyperlipidemia, and cardiovascular diseases.”

The findings of this study were published in «JAMA Network Open», a journal of the American Medical Association (AMA).