medwireNews: Increased adiposity is associated with a decreased risk for developing breast cancer before menopause, confirms a pooled analysis published in JAMA Oncology.
“The inverse association was strongest for hormone receptor [HR]–positive breast cancer, was evident across the entire distribution of body mass index [BMI], and did not materially vary by attained age or other characteristics of women,” say the study authors.
They note that these findings not only confirm previous case–control studies, but also show that the magnitude of the association is greater than that observed in prior analyses.
But in light of the “many adverse effects” of obesity on general health, the researchers stress: “[W]e do not advocate weight gain as a preventative measure against premenopausal breast cancer.
“However, understanding the mechanistic action underlying the inverse association of premenopausal adiposity with breast cancer risk could potentially identify modifiable pathways.”
For the study, the team pooled individual-level data from 758,592 premenopausal women (median age 40.6 years) who were participating in 19 prospective cohort studies. Among these women, 13,082 cases of in situ or invasive breast cancer were recorded over a median 9.3 years of follow-up.
The risk for breast cancer decreased with increasing BMI, with the results more marked in the younger age groups. Specifically, the hazard ratio (HR) for breast cancer per 5-unit difference – after adjustment for factors such as attained age, age at menarche, and family history of breast cancer – was 0.77 for women in the 18–24 year old age bracket, rising to 0.85, 0.87, and 0.88 for the 25–34, 35–44, and 45–54 year old age groups, respectively.
Even among women classed as a healthy weight, higher BMI was associated with a reduced breast cancer risk. For instance, in the youngest age group (18–24 years), women with a BMI of 23.0–24.9 kg/m2 had a significant 20% lower risk for breast cancer than their counterparts with a BMI of 18.5–22.9 kg/m2.
The inverse relationship between adiposity and breast cancer risk was observed in all age groups for HR-positive disease, but the association was significant for HR-negative disease only in the youngest age group. And the findings were inconsistent across age brackets for triple-negative breast cancer.
“Our estimated 12% to 23% reduction in premenopausal breast cancer risk per 5.0-U difference in BMI depending on age is substantially stronger than that from meta-analyses, which have reported 5% to 9% reductions among women overall without analysis by age at BMI, and a study reporting a 10% reduction using measured BMI at ages 16 to 19 years,” remark Minouk Schoemaker (The Institute of Cancer Research, London, UK) and fellow members of the Premenopausal Breast Cancer Collaborative Group.
They add: “The associations of BMI with risk also did not appear to be appreciably modified by risk factors for breast cancer later in life, with the possible exception of nulliparity and oral contraceptive use.”
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