The obesity paradox, where high patient body mass index (BMI) is correlated with improved overall survival in patients with cancer, has had few plausible explanations to date.1

It’s understood, for example, that overweight and obese patients are more likely to develop cancer, with a 2014 study showing that BMI was associated with 17 of 22 evaluated cancers, including liver and colon cancers.2 BMI is also associated with all-cause mortality: a 2013 study published in JAMA found that individuals with a BMI of 35 or higher had a hazard ratio for death of 1.29 compared with people with a normal BMI.3

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Furthermore, a 2006 study published in The New England Journal of Medicine found that overweight and obese people, even those who were otherwise healthy or never smoked, had a higher risk of death than those with a normal BMI.4

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Yet despite these findings, obese patients with cancer tend to survive for longer, on average, than others. A study published in February of this year, for example, showed a hazard ratio for death of .50 among patients with colorectal cancer who had a BMI of at least 30 compared with others.5 Conflicting findings, however, remain and a 2018 analysis of 22 clinical trials did not find consistent results linking BMI to improved survival — though the correlation appeared to hold for male patients.6

Given that weight loss in cancer settings, including in patients with colorectal cancer, is associated with a higher risk of all-cause mortality, it’s possible that retaining a high BMI throughout treatment reduces the risk of death from other cancer-related issues, such as cachexia. But conflicting research exists that suggests that this reasoning alone is not sufficient, including a 2017 study that found that weight loss after a colorectal cancer diagnosis also reduces the risk of cancer-related death.7