This is something that gets obscured in today's medical reporting--everyone, as they get older, will get sick for some reason and die at some point. The biggest risk factor for heart disease and cancer isn't BMI, cholesterol, blood pressure, etc....it's age. And to a large extent, the incidence of those diseases is random; doing everything right lowers your risk of disease, but not to zero, and usually not to that small a fraction of the risk for the general population. There is no magic set of behavior that will prevent disease; the closest is obvious behavior to prevent cancer like "don't smoke" and "don't zap yourself with radiation on a daily basis."
But there's a big difference in confirmation bias. When a person with good habits and numbers has a random heart attack at age 60, people will be shocked and sympathetic, thinking it's abnormal. When a fat person has a random heart attack at age 60, people will think it's entirely the fat person's fault. In reality, the fat person's risk of heart attack is only slightly higher; most people think that his risk is drastically higher, and his heart attack will confirm the bias while a "healthy" person's heart attack will be discounted as not fitting the pattern.
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