Obesity a cause of breast cancer?

Obesity is being reported as the greatest avoidable factor in breast cancer, but is this really what the study says?

The researchers at the University of Oxford performed a meta-analysis, a study that combines the results of several previously completed studies, on 13 published studies, combining data on 6,000 women.

However they did not analyse data on women who developed cancer, rather they investigated whether obesity and other lifestyle factors were related to levels of circulating sex hormones – which in turn are linked to cancer risk. Breast cancer in postmenopausal women has been shown to associate with levels of sex hormones (e.g. testosterone and oestrogen). But not all breast cancers are linked with these hormones; in premenopausal women there is a natural variation in the levels of sex hormones due to the woman’s menstrual cycle.

Study Results

    * The strongest correlation found was a directly positive one between sex hormone levels and women’s BMI scores that is higher sex hormone levels are found in those with higher BMIs.

    * However it was also shown that women who smoked 15 or more cigarettes a day also had higher levels of all hormones than non-smokers.

    * Similarly women who drank 20g (approximately 2.5 units) or more of alcohol a day also had higher levels than non-drinkers.

    * Other breast cancer risk factors, such as age of menopause, number of children, age at first pregnancy or family history, where not shown to be linked to the hormone levels.

The study authors claim their motivation is to understand the mechanism through which these lifestyles factors increase breast cancer risk. Whilst this study adds weight and new information in cannot conclude anything. In particular because it was a cross-sectional study. Cross sectional research observes a series of factors at only a single point in time, thus it cannot tell us how factors are related to each other, or how they vary over time. Thus, for example, the study cannot establish whether a woman’s weight gain causes her hormone levels to rise, or if the raise in hormone levels causes the woman’s weight to rise.

Although this study has been accurately reported the headlines implying that obesity is the largest cause of cancer is overstating the findings and confusing correlation with causation.

This research doesn’t prove that obesity, high levels of alcohol consumption and smoking directly cause cancer, however it fully endorses and adds weight to the current advice that avoiding the above lifestyle factors can reduce the risk of cancer, including breast cancer.