Obesity plays a much bigger role in causing cancer than researchers had previously believed, accounting for 14 per cent of cancers in men and 20 per cent in women, according to a massive new study by the American Cancer Society.
An estimated 90,000 Americans die each year of cancer caused primarily by obesity and excess weight, according to the study, published in the New England Journal of Medicine. That makes weighing too much second only to smoking — which causes about 170,000 cancer deaths per year — as a preventable cause of cancer.
The team studied more than 900,000 people nationwide for 16 years to provide the first definitive understanding of the role of obesity in causing cancer. With only a few exceptions, being overweight increases the risk of virtually every form of cancer and, the more overweight you are, the greater the risk.
Men in the highest weight groups were 52 per cent more likely to die from cancer than those of normal weight, while women in the highest weight groups were 62 per cent more likely to die from it.
“Many of us had believed that these relationships were present before, but they have never been so clearly shown,” said Dr Robert J. Mayer of the Dana Farber Cancer Institute in Boston. “This is an epidemiological landmark.”
Researchers have only a limited understanding of what causes the excess cancers. Fat tissue, for example, leads to an overproduction of estrogen and other steroid hormones that play a crucial role in breast and other endocrine-related cancers. Obesity also leads to excess insulin production and an increase in insulin-like growth factor receptors, which have previously been associated with cancer.
Obesity is also linked to gastroesophageal reflux — the eruption of stomach acids into the esophagus. That, in turn, has been shown to increase the rate of various esophageal cancers, whose incidence has grown tremendously over the past two years. But scientists still have much to learn about the biological links, said Dr Eugenia E. Calle of the cancer society, who led the study.
In addition to causing cancers, obesity also exacerbates their effects. The obese are often reluctant to visit doctors, either because of shame or simple difficulties in travelling for appointments.
Detection of cancers is often more difficult in the obese because they are obscured by excess tissues, and treatment can be a problem because fat absorbs chemotherapeutic drugs.
An estimated 30 per cent of Americans are considered obese and another 35 per cent are considered overweight. Previous studies have suggested that 11 per cent of deaths from heart disease in men and 14 per cent of cases in women are caused by obesity alone.