Aerobic fitness is generally tied to a longer life, but the same can't be said for obese people, according to new research.
In a study of nearly 1.3 million young Swedish men, the least fit normal-weight men were still less likely to die over the next few decades than the most fit of the obese men.
The new findings suggest it's more important to be a normal weight at a young age than it is to be fit, said senior author Peter Nordstrom, of Umea University in Sweden.
He and his colleagues write in the International Journal of Epidemiology that the concept of fitness compensating for obesity has emerged in recent years. Evaluation of the "fat but fit" concept could have implications for public health.
About 40 percent of the world's adults were overweight in 2014, according to the World Health Organization (WHO). Of those, 13 percent were obese, which is defined as a body mass index (BMI) - a measure of weight in relation to height - of 30 or more. (Calculate your BMI here: http://1.usa.gov/1ooHYzU.)
As BMI increases, so does the risk of cardiovascular disease, diabetes, musculoskeletal problems and some cancers, the WHO says.
The new study started tracking the men at age 18 and followed them for an average of 29 years. In addition to weight and height, the researchers measured the men's fitness by seeing how long they could pedal on a bicycle before getting tired.
During the follow-up period, 44,301 participants died.
The researchers found that men who originally placed in the top-fifth of fitness at the beginning of the study in 1969 were about half as likely to die as those who placed in the bottom-fifth of fitness.
When the researchers separated the participants based on BMI, they found a decreasing benefit of fitness on risk of death as BMI increased.
For example, among normal-weight men, those in the top half of fitness at the start of the study were about 34 percent less likely to die during follow up than those in the bottom half. That benefit shrunk to 28 percent among overweight individuals and 26 percent for obese people with a BMI between 30 and 34.
The benefit of fitness on death risk disappeared for severely obese people with a BMI above 35.
Nordstrom told Reuters Health by email that the results argue against the so-called obesity paradox, which suggests being heavier may be somewhat protective against poor health.
However, he can't explain why fitness wasn't associated with the risk of death among obese people in the new study.
Nordstom said the new findings "do not mean that obese people should not exercise."
He said the next step would be to experiment with interventions to determine the effects of exercise instead of relying on observational data, which can't prove cause and effect.