A controversial idea called "the obesity paradox" holds that people who carry excess weight may actually live longer than their trimmer counterparts. But one researcher now says that this paradox may really boil down to the idea that — at any body weight — a longer life depends on a person's level of cardiovascular fitness.

In other words, an obese person with a high level of fitness would be expected to live longer than a non-obese person who is not fit.

Although the obesity paradox may seem counterintuitive, numerous epidemiological studies have shown that among people who have heart disease those who are overweight or obese tend to have a better prognosis than those who are leaner.

But a person's weight — whether it's from their body fat or their lean tissue — is not the only factor that plays a role in their survival, said Dr. Carl Lavie, the medical director of cardiac rehabilitation and prevention at the John Ochsner Heart and Vascular Institute in New Orleans. Lavie spoke here yesterday (Nov. 9) at an annual meeting called the American Heart Association Scientific Sessions. Lavie is also the author of the book "The Obesity Paradox" (2014, Hudson Street Press).

There is increasing evidence suggesting that fitness may in fact play a stronger role in survival than "fatness," Lavie said. [Extending Life: 7 Ways to Live Past 100]

For example, a 2014 meta-analysis published in the journal Progress in Cardiovascular Diseases found that fitness was more important than fatness when it came to long-term mortality. The researchers found that fit people, whether they were a normal weight, overweight or obese, all had similar survival rates, whereas unfit people were twice as likely to die.

When people's fitness levels are factored into their health, the obesity paradox actually disappears, Lavie said.

Another study, which Lavie co-authored, found that neither BMI nor waist circumference nor percentage of body fat affected mortality rates when fitness was considered. The people considered "fit" in the study all had good survival rates, he said. 

Lavie said that what the researchers considered fit in the studies was not, in fact, a very high level of fitness. Rather, people's risk of dying during the study period increased if they fell into the lowest category of fitness, he said. Anyone whose level of fitness was above that level had better chances of survival, he said.

The findings may help people who need encouragement get more fit, Lavie said. Knowing that going from a sedentary lifestyle to getting light, regular physical activity could significantly cut risk could help people to get motivated, he said.

Of course, the existence of the obesity paradox does not suggest that people should try to gain weight in order to live longer.

There is still overwhelming evidence that supports the importance of obesity in the development and progression of many diseases of the heart, Lavie said.

In addition, Lavie stressed that becoming more physically active and losing weight are both important means of improving cardiovascular fitness.

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