A high body mass index (BMI), a measure of weight relative to height, has been linked to cancer, diabetes, and cardiovascular diseases, and that has led to guidelines based on low BMI, despite it being a population-level statistical correlation with little individual clinical relevance.  A large weightlifter may not be unhealthy while a thin person with a terrible diet is no role model.

A new study in JAMA Oncology increases confusion about what BMI does and does not tell us because it finds a statistical correlation between high BMI and cancer outcomes.  In clinical trials of atezolizumab, a common immunotherapy treatment for non-small-cell lung cancer (NSCLC), there was improved responsiveness to the drug in those with a high BMI.

It may not be so much an "obesity paradox" as a misuse of statistics using BMI, and that creates a lot of confusion.

Of the 1434 participants studied in the new paper, 49 percent were normal weight, 34 percent were overweight and 7 percent were obese. Patients with high BMI (>25 kg/m2) in four clinical trials had a significant reduction in mortality with atezolizumab, apparently benefiting from immune checkpoint inhibitor (ICI) therapy.

Surviving cancer that a poor lifestyle may have caused is no reason to be happy about being fat. The WHO estimates at least 2.8 million people die each year as a result of being overweight, including adverse metabolic effects o risk factors such as blood pressure, triglycerides and insulin resistance. Risks of coronary heart disease, ischemic stroke and type 2 diabetes mellitus increase steadily with increasing BMI.