As a skinny person who eats like crap, BMI isn't a sole indicator of health, but being over a certain weight range does take a toll on the human body. And the obesity paradox in medicine is about warped data - people who are overweight have a lower mortality rate and higher chance of recovery in things like strokes and heart attacks because overweight people get their first heart issues and strokes at a far younger age than people who haven't been overweight in their lives.

If both of us were to get the exact same kind of cancer at the same time and both lose 25 kg in radiation treatments, naturally I would be far more likely than he to die from it, but the odds of that happening are not the same.