The authors of the study say their data suggests that being aerobically fit does not protect obese people from premature death — at least, not young obese men. Indeed, their study found that “the risk of early death was higher in fit obese individuals than in unfit normal-weight individuals.”
Three decades of data
For the study, researchers at Umea University analyzed data collected from more than 1.3 million young men (average age: 18 years) who participated in mandatory Swedish military conscription between 1969 and 1996. At the time of their conscription, the men underwent various health-related measurements and tests, including an aerobic fitness test that had them cycle on a stationary bike until they became too fatigued to continue.
The men were followed for a median average of 29 years, until Dec. 31, 2012. During that period, additional data was collected, including on any acquired health conditions and on any ups (or downs) in the men’s socio-economic status. Deaths — and there were slightly more than 44,000 — were also recorded. The most common causes of those deaths were trauma, cancer, cardiovascular conditions and suicide.
An analysis of the data revealed that the men whose cycling-test scores put them in the highest one-fifth for aerobic fitness were 48 percent less likely to die prematurely of any cause compared with those in the lowest one-fifth.
Interestingly, the researchers found that high aerobic fitness was more strongly associated with a reduction in the risk of death caused by alcohol and narcotics abuse (an 80 percent reduced risk) or by suicide (a 59 percent reduced risk) than by cardiovascular disease (a 45 percent reduced risk).
These associations weakened only slightly when the researchers adjusted the data for possible confounding factors, such as whether the men had asthma or some other common medical condition at the time of their conscription, or what their income and educational levels were 15 years later.
Adjusting for weight
The researchers then analyzed the data again, this time adjusting for body weight. They found that although higher aerobic fitness was associated with a significantly reduced risk of death from any cause in both normal-weight and in overweight men, the reduced risk was not as significant for those who were obese (those with a body mass index, or BMI, of 30 or greater).
In fact, the obese men whose fitness scores placed them in the highest one-fourth of aerobic fitness were 30 percent more likely to die during the study’s follow-up period than normal-weight men who scored in the lowest one-fourth.
“These results counter the notion that the ‘fat but fit’ condition does not increase mortality risk,” the study’s authors write.
Caveats and implications
This study is an observational one, so it can’t prove cause and effect. It has other limitations as well. The men’s physical fitness was measured only at the time of their conscription, for example, and the study does not adjust for smoking, an important confounder. (The researchers could not adjust for this variable, as no data was collected on the men’s smoking habits.)
Another obvious limitation is the study’s demographics. It involved only men, and therefore it’s not clear that the results would pertain to women, too. Furthermore, few men were followed past the age of 50. Most deaths related to sedentary behavior and obesity occur at older ages.
Still, the study’s findings are interesting. “Despite the limitation posed by the observational nature of this study,” write its authors, “these results suggest that low BMI early in life is more important than high physical fitness, with regard to reducing the risk of early death.”
You can find an abstract of the study, which was funded by the Swedish Research Council, on the website of the International Journal of Epidemiology, but the full study, unfortunately, is behind a paywall.