Over 100 studies have examined the associations between weight or BMI at different ages, central fat distribution, or adult weight gain and breast cancer incidence [1, 56].
Taken together, those studies found that women who were overweight or obese had a 30%-50% greater risk for postmenopausal breast cancer development than leaner women.
In contrast, overweight and obesity are associated with a lower risk of breast cancer developing during the premenopausal years.
The Women’s Health Initiative (WHI) Observational Study is a multiethnic, multisite cohort study of women aged 50–79 at study entry . Women underwent several clinic measures of adiposity when entering the cohort, including height, weight, and waist and hip circumference measurements, and reported their lifetime weight history.
Analysis of these data showed that anthropometric factors were associated with breast cancer risk, but only among those women who had never used hormone replacement therapy . Among these latter women, women with BMIs 31.1 had a statistically significant 2.5 times greater risk of developing breast cancer than women whose BMIs were 22.6.
The Nurses’ Health Study also found that the 60% greater risk for postmenopausal breast cancer associated with overweight and obesity was limited to women who had never used hormone replacement therapy . In another large cohort, heavier women with a family history of breast cancer had a greater risk of developing breast cancer than heavier women without a family history .
Results from case-control studies mirror those of cohort studies .
Adult Weight Gain
Adult weight gain has been quite consistently associated with a greater risk for postmenopausal breast cancer [58, 59, 61–68].
Findings from two of the largest cohort studies suggest that the doubling of risk associated with a gain in BMI from age 18 of >9.7 (WHI) or a weight gain >20 kg (Nurses’ Health Study) was limited to women who had never used postmenopausal hormone replacement therapy [58, 59].
In those studies, a 20% greater risk was observed for BMI gains of 3.5–6.2 (WHI) or weight gains of 2–20 kg (Nurses Health Study), although these gains were not statistically significant. Weight gain has also been found to be a consistent predictor of greater risk in case-control studies .
Greater central adiposity has been associated with an approximate doubling of breast cancer risk among postmenopausal women in cohort studies [58, 60, 61, 64, 69], independent of BMI.
In the WHI study, a statistically significant trend of increasing breast cancer risk with increasing waist and hip circumferences, but not waist/hip ratio, was observed, although this finding was limited to women who had never used hormone replacement therapy .
Women in the highest quintile of either circumference measure had approximately double the risk of women in the lowest quintile.
In the Nurses’ Health Study, RR for women in the highest versus lowest quintile of waist circumference was 1.2 among women overall and 1.9 among women who had never used postmenopausal hormone replacement therapy .
Data are less consistent for case-control studies, probably because of the difficulty with obtaining reliable measures of prediagnostic body circumferences after women are diagnosed with cancer.
Evidence on Effect of Intended Weight Control or Weight Loss
Data on the association between weight loss and breast cancer risk are limited. In three studies, weight loss occurring over a prolonged interval was associated with a nonsignificant slightly lower risk [61, 62, 70].
In another, weight loss in the decade before diagnosis was associated with a nonsignificant lower risk .
One study in premenopausal women found a statistically significant 36% lower risk with weight loss from age 20 to interview (age 20–44) that was present only among cases with low-grade tumors . One study in postmenopausal women found a statistically significant 24% lower risk with weight loss from age 18 to interview (age 50–74) .
These data suggest that weight loss may be beneficial but are difficult to interpret as it is not possible to determine the cause of weight loss in existing studies.
The majority of the studies on weight and breast cancer risk have been done in European and North American populations. Nevertheless, the available data suggest that greater adiposity increases the risk for breast cancer across race and ethnic groups .
There are several likely mechanisms linking adiposity to risk for breast cancer. After menopause, adipose tissue is the main site of estrogen production through aromatization of androgens to estrogens .
Overweight and obese postmenopausal women have higher concentrations of estrone, estradiol, and testosterone and lower concentrations of sex-hormone-binding globulin than leaner women .
Testosterone concentrations are greater in both premenopausal and postmenopausal overweight/obese women than in their leaner counterparts, perhaps due to greater conversion of androstenedione to testosterone in adipose tissue .
Insulin promotes cancer cell growth and, therefore, could explain part of the link between adiposity and breast cancer risk . Exercise reduces insulin resistance and, therefore, the circulating concentration of insulin both acutely and chronically.
Insulin-like growth factors (IGFs) stimulate cell turnover in most body tissues and have been associated with a greater risk of breast cancer .
IGF is downregulated by increased production of its binding protein (IGFBP-1), which can result from a greater level of exercise, lower caloric intake, and lower body weight [75, 76]. Decreased IGF activity may increase the hepatic synthesis of sex-hormone-binding globulin, resulting in diminished availability of free sex hormone.
Thus, greater levels of exercise could result in lower levels of biologically available endogenous sex hormones via a cascade of metabolic events, and thus, a lower risk of breast cancer.
Ann's NOTE: One thought is that there is no mention of the chemicals we know are stored in the fatty cells. For more on this, see Environmental Issues section (left side of page)
Secondly, this article seems to suggest that weight loss is not very useful in reducing risk. This could be the result of many factors but for those who wish to make a change, it is not just going on a 'diet'. Most dieters fail since their bodies are 'set' at a particular weight. Unfortunately it is much easier to gain than lose.
But a lifestyle change to healthier eating patterns and combining exercise would be of value to anyone. We offer many pages here that have suggestions for changes. See Food/Water/product section (left side of page).
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