Obesity boost odds of breast cancer
Obesity is known to boost the risk of oestrogen-fuelled breast cancer in women past menopause. Now, recent research suggests a link between postmenopausal obesity and an especially aggressive type of breast cancer that doesn't depend on oestrogen to grow.
Breast cancer is not just one disease. It is a complex combination of many diseases. Epidemiologists have long noted a link between obesity and increased risk of postmenopausal breast cancer, as well as a decreased risk that comes with greater physical activity. A relationship between adipose tissue and estrogen is thought to contribute to this risk.
In a new study that looked at body mass index (BMI) - a measurement that takes into account height and weight - women with the highest BMIs had a 35 percent higher risk of developing the aggressive cancer known as triple-negative compared to those with the lowest BMIs. They also had a 39 percent higher risk of developing other breast cancers.
Triple-negative breast cancer is marked by a lack of oestrogen, progesterone and HER2 protein expression, hence the name. Only about 10 or 20 percent of breast cancers are triple-negative but the outlook is poor because of its aggressiveness and the lack of targeted treatments.
American researchers evaluated data from 155,723 women enrolled in the Women's Health Initiative, a large-scale study of postmenopausal women begun in 1993. The 15-year study looked at cancer, heart disease and osteoporosis. Participants, aged 50 to 79 years old, reported exercise habits, weight and height, and their BMIs were calculated.
During the study follow-up, which lasted a median 7.9 years, 2,610 women developed oestrogen receptor-positive breast cancer, which is fuelled by oestrogen, and 307 women developed triple-negative cancer. Women were divided into four groups, from lowest to highest BMIs. Those in the highest group had BMIs of 31 or higher; 30 and above is termed obese. The association between triple-negative breast cancer and obesity was what scientists call 'borderline' statistically significant. The researchers also found a modest link between greater physical activity and reduced risk of both kinds of breast cancer, suggesting - but not proving - that increased exercise might moderate a woman's risk of these cancers.
When evaluating the risks linked with obesity, researchers accounted for such factors as age, education, income, family history and ethnicity. The long-known link between obesity and oestrogen-positive breast cancers has been attributed to higher exposure to oestrogen in fat tissue. After menopause, most of a woman's oestrogen comes from fat tissue. But triple-negative cancers aren't responsive to hormones. The fact that we see such a similar association between the two subtypes and obesity tells us it may not just be the hormonal effect of the obesity driving the risk. The exact reasons are unknown but obesity increases markers of inflammation and levels of insulin and insulin-like growth factor.
The above study confirms that obesity and cancer risk is not just the problem of the oestrogen feeding the cancer cells. The fat cells may set up an environment conducive to the growth of breast cancer cells, beyond the effect of oestrogen. The findings call for further study on the possible role of growth factors or inflammation, and there is reason to repeat the message that keeping a healthy weight is crucial with age.
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