Obesity is known to increase the risk of colorectal cancer. Research from the German Cancer Research Center (DKFZ) suggested that the importance of this link may have been greatly underestimated. This is because many patients lose weight unintentionally before they are told they have colon cancer.
Studies that only consider body weight at the time of diagnosis hide the true association between obesity and colorectal cancer risk. The current study also suggests that unintentional weight loss may act as a risk factor for colorectal cancer. Obesity puts people at risk of many types of cancer. For example, in cases of colorectal, renal and endometrial cancers, this association is very clear. According to earlier estimates, the risk of developing colon cancer was about a third higher in obese people than in people of normal weight.
“However, these studies have so far not taken into account that many affected people lose weight in the years before colorectal cancer diagnosis,” says Hermann Brenner, epidemiologist and prevention expert at the German Cancer Research Center. “This has led to a substantial underestimation of the risk contribution of obesity in many trials.”
To assess the magnitude of this bias, the Brenner researchers evaluated data from the DACHS study*. The nearly 12,000 study participants included in the current evaluation provided information about their body weight at the time of diagnosis and also reported their weight in the years prior to diagnosis (measured at 10-year intervals).
Based on body weight at the time of diagnosis, no indication of an association between body weight and colorectal cancer risk could be established. The picture was quite different, however, when the researchers looked at participants’ earlier body weight: Here, a strong association was found between overweight and the likelihood of developing colorectal cancer, which was highest 8 to 10 years before diagnosis. was clear.
Study participants who were excessively overweight — referred to as obese — were twice as likely to develop colorectal cancer during this period as those of normal weight. Marko Mandić, first author of the study, said, “If we had only looked at weight at baseline, as has been done in many previous studies, we may have completely missed the link between obesity and an increased risk of colorectal cancer. “