Heidelberg [Germany], May 7 : Obesity is known to raise the risk of colorectal cancer. Research from the German Cancer Research Centre (DKFZ) suggested that the significance of this link has probably been grossly underestimated. The reason is that many patients unintentionally lose weight before being told they have colon cancer.
Studies that only consider body weight at the time of diagnosis conceal the true relationship between obesity and the risk of colorectal cancer. The current study also shows that unintentional weight loss may act as a risk factor for colorectal cancer.
Obesity puts people at risk for a wide range of cancers. For example, in the cases of colorectal, renal, and endometrial cancer, this association is very clear. According to earlier estimations, obese people had a roughly one-third higher risk of developing colon cancer than people of normal weight.
"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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