Study: Obesity is risk factor for colorectal cancer

When studies simply take into account body weight at the time of diagnosis, the true link between obesity and the risk of colorectal cancer is obscured. The new study also demonstrates that inadvertent weight reduction may serve as a precursor to colorectal cancer

The risk of colorectal cancer is known to increase with obesity. This association has likely been severely underestimated thus far, according to research from the German Cancer Research Centre (DKFZ). The cause is that a lot of people accidentally lose weight prior to receiving a colon cancer diagnosis.

When studies simply take into account body weight at the time of diagnosis, the true link between obesity and the risk of colorectal cancer is obscured. The new study also demonstrates that inadvertent weight reduction may serve as a precursor to colorectal cancer.

A wide variety of malignancies are at risk because of obesity. This connection is notably evident, for instance, in the cases of colorectal, kidney, and endometrial cancer. Previous estimates indicate that the risk of colon cancer in obese people is around one-third higher than in normal weight people.

"However, these studies have so far not taken into account that many affected people lose weight in the years before their colorectal cancer diagnosis," says Hermann Brenner, epidemiologist and prevention expert at the German Cancer Research Center. "This has led to the risk contribution of obesity being significantly underestimated in many trials."

To assess the magnitude of this bias, Brenner's researchers evaluated data from the DACHS study*. The nearly 12,000 study participants included in the current evaluation had provided information on their body weight at the time of diagnosis and had also reported their weight in the years preceding diagnosis (measured at 10-year intervals).

On the basis of body weight at the time of diagnosis, no indication of a relationship between body weight and colorectal cancer risk could be established. The picture was quite different, however, when the researchers looked at the participants' earlier body weight: Here, a strong correlation between overweight and the probability of developing colorectal cancer was found, which was most pronounced 8 to 10 years before diagnosis. Study participants who were highly overweight - referred to as obese- during this period were twice as likely as those of normal weight to develop colorectal cancer. "If we had only looked at weight at baseline, as has been done in many previous studies, we would have completely missed the link between obesity and increased risk of colorectal cancer," said Marko Mandic, the study's first author.

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