A paper in the International Journal of Epidemiology seeks to dismiss the concept of 'fat but fit' and instead suggest that the protective effects of high fitness against early death are reduced in obese people. 

The detrimental effects of low aerobic fitness in older people have been well documented but few studies have investigated a direct link between aerobic fitness and health in younger populations. A study in Sweden followed 1,317,713 men for a median average of 29 years to examine the association between aerobic fitness and death later in life, as well as how obesity affected these results. The subjects' aerobic fitness was tested by asking them to cycle until they had to stop due to fatigue.

Men in the highest fifth of aerobic fitness had a 48 per cent lower risk of death from any cause compared with those in the lowest fifth. Stronger associations were observed for deaths related to suicide and abuse of alcohol and narcotics. Unexpectedly, the authors noted a strong association between low aerobic fitness and also deaths related to trauma. Co-author Peter Nordström has no explanation for this finding: "We could only speculate, but genetic factors could have influenced these associations given that aerobic fitness is under strong genetic control."

The study also evaluated the concept that 'fat but fit is ok'. Men of a normal weight, regardless of their fitness level, were at lower risk of death compared to obese individuals in the highest quarter of aerobic fitness. Nevertheless, the relative benefits of high fitness may still be greater in obese people. However, in this study the beneficial effect of high aerobic fitness was actually reduced with increased obesity, and in those with extreme obesity there was no significant effect at all.

With the limitation that the study cohort included only men, and relative early deaths, the authors say it disputes the notion of 'fat but fit.'