Being overweight or obese significantly increases the risk of developing colon cancer, breast cancer or liver cancer, and is a major risk factor for 10 other types of cancer, a new study has found.

Until recently, researchers believed that 2% to 8% of all cancer cases were attributable to excess body weight. But a new study by a team led by Hermann Brenner, an epidemiologist with the German Cancer Research Center  (DKFZ), suggests that this was a serious underestimate.

According to the study, up to 11.5% of cancers may be attributable to being overweight.

The DKFZ researchers "have not only updated existing risk estimates, but have also questioned whether the link between excess weight and cancer may have been underestimated until now," Michael Leitzmann, an epidemiologist at the University of Regensburg, told DW.

Leitzmann, who was not involved in the study, researches diet, body weight, physical activity and the development of chronic diseases at the university's Institute of Epidemiology and Prevention, and has advised German Cancer Aid.

The abdominal-fat 'trigger'

The DKFZ team changed its research methods for the new study. Previous research used body mass index (BMI), which is calculated based on height and weight, as an indicator of excess weight. But BMI does not distinguish between fat and muscle mass, and does not provide information on how fat is distributed throughout the body.

This is important, because not all fat is the same. Fat cells that accumulate around the abdomen are more than just a passive store for excess calories, unlike those in other parts of the body. "They trigger what is known as a chronic inflammatory metabolic state," Leitzmann said.

Excess abdominal fat functions like a gland that releases various signaling molecules, known as adipokines. These travel through the bloodstream to different parts of the body, where they influence other metabolic processes — including cell growth and division, as well as the inhibition of programmed cell death. "These are all mechanisms that can contribute to the development and subsequent growth of tumors," Leitzmann said.

The DKFZ researchers therefore focused not only on BMI but also on waist circumference and the waist-to-hip ratio. Data from 458,543 women and men from the UK Biobank, which holds one of the world's biggest health datasets, were included in the study. The participants were followed for and average of 12 years, during which 50,000 new cases of cancer were identified.

Understanding previous underestimates

The researchers wanted to rule out a potential methodological bias in previous studies: the fact that cancer can lead to weight loss even before diagnosis — that is, while it is still developing.

That suggests that previous studies failed to identify a link between obesity and cancer because the participants had already lost weight because they had developed cancer by the time they entered the study.

For this reason, the team excluded the first few years following the participants' enrollment from the analysis. Only cancer cases that occurred seven years after the start of the observation period were considered — and that made the statistical link between being overweight and cancer clearer.

Women at risk

The study suggests that the proportion of overweight women who develop cancer is higher than that of men, but Leitzmann urges caution when interpreting these findings. Further studies with a similar design are needed to see whether the results can be replicated.

Leitzmann offered a theory on why being overweight might have a more severe impact on women. Being overweight is blamed, among other things, for breast, uterine and ovarian cancers. "These types of cancer, which affect women, are hormone-dependent," Leitzmann said. For example, the hormone estrogen can stimulate the growth of breast cancer, and excess abdominal fat can contribute to elevated estrogen levels.

Most people are now aware that obesity increases the risk of cardiovascular disease or Type 2 diabetes, Leitzmann said. Conversely, this also means that people who lose weight also reduce their risk of heart attacks, strokes and diabetes.

Though the DKFZ study emphasizes the increased risk associated with obesity, Leitzmann said the findings were also a reason for optimism: "To me, this clearly indicates that, by preventing obesity, we will be able to make a greater contribution to cancer prevention than previously assumed."

This article was originally written in German.