Number of Risky Lifestyle Behaviors and Breast Cancer Risk

Abstract

Background Lifestyle factors are associated with overall breast cancer risk, but less is known about their associations, alone or jointly, with risk of specific breast cancer subtypes. Methods We conducted a case–control subjects study nested within a cohort of women who participated in the Norwegian Breast Cancer Screening Program during 2006–2014 to examine associations between risky lifestyle factors and breast cancer risk. In all, 4402 breast cancer cases subjects with information on risk factors and hormone receptor status were identified. Conditional logistic regression was used to estimate odds ratios (ORs), with 95% confidence intervals (CIs), in relation to five risky lifestyle factors: body mass index (BMI) of 25 kg/m² or greater, three or more glasses of alcoholic beverages per week, ever smoking, fewer than four hours of physical activity per week, and ever use of menopausal hormone therapy. Analyses were adjusted for education, age at menarche, number of pregnancies, and menopausal status. All statistical tests were two-sided. Results Compared with women with no risky lifestyle behaviors, those with five had 85% (OR = 1.85, 95% CI = 1.42 to 2.42, Ptrend < .0001) increased risk of breast cancer overall. This association was limited to luminal A–like (OR = 2.20, 95% CI = 1.55 to 3.12, Ptrend < .0001) and luminal B–like human epidermal growth factor receptor 2 (HER2)–positive (OR = 1.66, 95% CI = 0.61 to 4.54, Ptrend < .004) subtypes. Number of risky lifestyle factors was not associated with increased risk of luminal B–like HER2-negative, HER2-positive, or triple-negative subtypes (Ptrend > .18 for all). Conclusions Number of risky lifestyle factors was positively associated with increased risk for luminal A–like and luminal B–like HER2-positive breast cancer.

Menopausal hormone therapy and breast cancer risk: effect modification by body mass through life

“…It is well established that HT use is more strongly associated with hormone receptor positive breast cancer [10], whereas overweight at a young age may confer a lower risk for all breast cancer subtypes [23]. Unfortunately, detailed information about breast cancer subtypes was not available to us, but we acknowledge that an effect modification of HT by life-course body mass patterns, may be more relevant for some than for other subtypes of breast cancer.…”

“…In most previous studies of HT use and breast cancer risk, information on BMI was restricted to adult or current BMI, and effect modification was assessed on a multiplicative scale [1,[5][6][7][8][9][10][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75]. Two studies, using the additive scale, have quantified effect modification of HT use by current BMI [45,76], but found no clear evidence for effect modification.…”

“…In addition to socio-demographic factors, the questionnaires covered age at menarche, use of hormonal contraception, age at first birth, number of pregnancies, information on smoking habits (never, former, current), and alcohol consumption. Information about current and previous health was included, as well as BMI at the time of participation (termed baseline BMI from here) [10]. The women were also asked to categorize their height and weight relative to their peers in childhood (age 7 years) and adolescence (age 15 years).…”

“…Other studies have evaluated effect modification of HT by BMI on a multiplicative scale [5,6,10]. To enable comparison with these studies, we also applied a multiplicative scale, and used likelihood-ratio tests to assess effect modification [38].…”

“…One challenge is that the effect of HT use may be modified by other factors that influence risk [5][6][7][8]. Previously, it has been suggested that the risk increasing effect of HT is weaker in overweight than in normal weight women [1,7,9,10], and that HT use may not contribute to additional risk in women who are overweight around menopause [5,6]. However, in most previous studies, relative risks were compared directly between subgroups with different baseline risk, an approach that may not be appropriate to assess the importance of effect modification [11].…”