We now know that obesity is a risk factor for a whole range of cancers. The association is particularly highlighted in the case of endometrial cancer, kidney cancer, or colorectal cancer. Thus, people who are obese have a risk of colorectal cancer that is about 30% higher than people of normal weight.
What is the true contribution of obesity to the risk of colorectal cancer?
To assess the extent of possible current bias, the researchers assessed weight data, at the time of diagnosis, but also years prior, from 12,000 participants. This analysis reveals:
- no consistent relationship between body weight at diagnosis and risk of colorectal cancer;
- On the other hand, there is a strong relationship between being overweight and the future risk of colorectal cancer.
- This association is strongest 8 to 10 years before diagnosis;
- Participants who were overweight or obese during the 8-10 years prior to diagnosis had a 2-fold increased risk of developing colorectal cancer in the future;
- Finally, a large number of participants with colorectal cancer lost involuntary weight prior to diagnosis;
- Unintentional weight loss of 2 pounds or more in the two years prior to diagnosis was 7.5 times more common in participants with cancer than in controls.
During this period, cancer was already present, but its symptoms were not yet noticed. Unintentional weight loss can also be an early indicator of cancer or other diseases and should be investigated.
In conclusion, taking weight into account only at the time of cancer diagnosis does not identify this link between obesity and an increased risk of colorectal cancer, explains lead author of the study, Dr. Mandyk, a researcher at the DKFZ. However, obesity is already a major factor, years ago, for colorectal cancer in the future.