Some studies have found that people with overweight or obesity seem to survive colorectal cancer longer than people at lower weights. That unexpected pattern is sometimes called the “obesity paradox.”
New research suggests the explanation may have less to do with higher body weight being protective and more to do with weight loss that happens before cancer is diagnosed.
In a study of more than 5,500 people with colorectal cancer, researchers found that substantial weight loss before diagnosis was linked to worse survival. Once they accounted for that earlier weight loss, much of the apparent survival advantage associated with higher BMI disappeared.
The findings, published in JAMA Network Open, do not suggest that gaining weight improves survival after colorectal cancer. Instead, they show why a person’s weight history may matter more than a single measurement taken at diagnosis.
Researchers followed 5,521 people with colorectal cancer for a median of about five years. They looked at participants’ body mass index, or BMI, at diagnosis as well as changes in weight during the years leading up to it.
At first, people with overweight appeared to have somewhat better overall survival than people whose BMI was in the healthy-weight range.
But cancer can cause weight loss before a person knows they are sick. That means someone who weighs less at diagnosis may have lost a significant amount of weight because of the developing disease.
When researchers accounted for weight loss before diagnosis, the apparent survival advantage linked to higher BMI was largely eliminated.
The pattern was especially noticeable among people who had lost more than 10% of their body weight before their cancer diagnosis. That amount of weight loss was associated with worse survival.
This can create a misleading picture. Instead of lower body weight causing poorer outcomes, an undiagnosed cancer may cause weight loss while also being linked to worse outcomes. Looking only at BMI at diagnosis can therefore make higher body weight appear more protective than it really is.
The study does not show that losing more than 10% of body weight is a specific sign of colorectal cancer. It also cannot prove that the weight loss itself caused poorer survival. Researchers studied people who had already been diagnosed with colorectal cancer and looked back at their earlier weight changes.
BMI has other limitations, too. It does not distinguish between fat and muscle or show where body fat is stored. That can be especially important during cancer, when loss of muscle may affect strength, treatment tolerance and overall health.
The authors said the findings should not be interpreted as a reason for people at a healthy weight to gain weight after a colorectal cancer diagnosis. They also do not suggest that current recommendations for cancer survivors should change.
Instead, the study suggests that changes in weight over time may provide more useful information than BMI measured at a single point.
For researchers, taking prediagnosis weight loss into account may also help clarify how body weight relates to cancer survival and prevent illness-related weight loss from being mistaken for a benefit of higher body weight.
The study was supported by the German Research Council, the German Federal Ministry of Education and Research, German Cancer Aid, the German Federal Ministry of Research, Technology and Space and the Horizon Europe TMA-MSCA Doctoral Networks program.