The dangers of being obese are well documented. Though it’s associated with heart disease and diabetes, some believe obesity doesn't always equal being unhealthy. A new study published in the International Journal of Epidemiology challenges this belief, suggesting that even if an obese person exercises frequently, their risk of early death is still higher than a person at an average weight.

"The interaction of aerobic 25 fitness and obesity [...] with regard to the risk of all-cause mortality remains unclear," researchers wrote. "In recent years, the concept of ‘fat but fit’ has emerged, implying that high fitness can compensate for obesity. Further evaluation of this hypothesis would be of importance and could have implications with respect to public health."

Given few studies have examined the link between aerobic fitness and health in younger populations, researchers analyzed a cohort of over one million Swedish men an average age of 18 between 1969 to 1996; they were followed either until the date of death or the study's end, Dec. 31, 2012. In order "to test whether the association between aerobic fitness and death was linear," participants completed a high-intensity cycling test at baseline and were monitored for mortality and obesity later in life.

The results showed that men in the highest fifth of aerobic fitness had a 48 percent lower risk of death from any cause compared to men in the lowest fifth; the strongest associations were observed for deaths related to suicide and abuse of alcohol and narcotics. Men of a normal weight were at a lower risk of death compared to obese individuals in the highest fifth of aerobic fitness, regardless of their own fitness level. 

Researchers unexpectedly observed a strong association between low aerobic fitness and deaths related to trauma, which co-author Peter Nordstrom said they can't explain.

"We could only speculate, but genetic factors could have influenced these associations given that aerobic fitness is under strong genetic control," Nordstrom added in a statement.

The relative benefits of high-level fitness may be greater in obese people, but in this study, the positive effect of high fitness was reduced with increased obesity. And in those with the highest level of obesity, there was no significant benefits of aerobic exercise at all.

The study was limited in the sense that it only included men and only looked at relative early deaths. The study concludes, however, by saying these findings ultimately challenge "the currently held idea that obese individuals can fully compensate mortality risk by being physically fit."

Source: Hogstrom G, Nordstrom A, Nordstrom P. Aerobic Fitness in late adolescence and the risk of early death: a prospective cohort study of 1.3 million Swedish men. International Journal of Epidemiology. 2015.