MedWire News: Data from the Women's Health Initiative (WHI) study indicate a link between obesity and estrogen receptor positive (ER+) breast cancer, and also suggest that obese women have an increased risk for triple-negative breast cancer.
In addition, the study results show that physical activity appears to decrease the risk for both breast cancer subtypes
The researchers note that the associations were all modest in magnitude, and not statistically significant for the triple-negative subtype.
"However, given the potentially modifiable nature of these exposures, these results suggest that there may be ways postmenopausal women can impact their risks of both ER+ and triple-negative breast cancer," they write in the journal Cancer Epidemiology, Biomarkers and Prevention.
Amanda Phipps (Fred Hutchinson Cancer Research Center, Seattle, Washington, USA) and colleagues used data from 155,723 women enrolled in the WHI to assess the relationship between potentially modifiable breast cancer risk factors - body mass index (BMI), waist and hip circumference, and recreational physical activity - and the risk for triple-negative and ER+ breast cancer.
During a median follow-up period of 7.9 years, 307 triple-negative and 2610 ER+ breast cancers were recorded.
The researchers report that women in the highest baseline BMI quartile (≥31.05 kg/m2) had a significant 39% increased risk for ER+ breast cancer and a nonsignificant 35% increased risk for triple-negative breast cancer, compared with those in the lowest quartile (<23.75 kg/m2).
In addition, waist and hip circumference were positively associated with the risk for ER+ breast cancer, but were not associated with triple-negative breast cancer.
In terms of recreational physical activity, women in the highest activity tertile (≥16.50 metabolic equivalent hours per week) had a significant 15% lower risk for ER+ and a nonsignificant 23% lower risk for triple-negative breast cancer compared with women who reported no physical activity at baseline.
"It has been suggested that [the association between BMI and ER+ breast cancer] is largely attributable to the positive relationship between BMI and endogenous estrogen levels, because adipose tissue is the primary source of androgen conversion to estrogen in postmenopausal women not using HT," remark Phipps and co-authors.
"On the contrary, triple-negative breast cancers are hormone receptor-negative by definition, yet we observed a similar positive association between BMI and breast cancer risk for both subtypes," they add.
This, say the researchers, suggests that nonhormonal factors, such as increased inflammation and increased levels of insulin and insulin-like growth factors, may also play a role in mediating associations between obesity and breast cancer risk.
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