For several years, researchers have struggled to explain the obesity paradox. This is the observation that, after having their cardiovascular disease diagnosed, people who are overweight or obese live longer than people who have a healthy weight.

How is it possible that extra pounds provide extra years of life? The answer, it turns out, is they don’t.

A study published in the journal JAMA Cardiology shows what’s really going on: People who are overweight or obese are receiving cardiovascular disease diagnoses at younger ages. Although they spend more years living with the disease than their slimmer peers, they do not live longer overall. Indeed, one of the main effects of carrying around too much weight is that you get fewer years of disease-free life.

A team of researchers led by Dr. Sadiya Khan of Northwestern University’s Feinberg School of Medicine examined data from the Cardiovascular Disease Lifetime Risk Pooling Project. They pulled medical information on 190,672 Americans who did not have cardiovascular disease at the beginning of the study and followed them for at least 10 years. Altogether, they provided 3.2 million years of health data.

By looking at people’s health over a longer period of time — not just after a diagnosis — the true significance of the obesity paradox comes into view. Khan and her colleagues found that the higher the BMI, the greater the lifetime risk of some type of heart problem. For example, compared to middle-aged men with a normal BMI, the risk of a heart attack was 18 percent higher for men who were overweight, 42 percent higher for obese men, and 98 percent higher for morbidly obese men.

For middle-aged women, the risk of a heart attack was 42 percent higher for those who were overweight, 75 percent higher for those who were obese and 80 percent higher for those who were morbidly obese.

They concluded: a normal weight was associated with a longer life overall.