To our knowledge, this is the first study to demonstrate that the association between fitness and health may be similar if not augmented in individuals with severe obesity as compared to mild obesity and extends previous research done in populations with normal weight, overweight and mild obesity. Further, we suggest that these differences in health risk by fitness may be related with differences in waist circumference.
High fitness is commonly defined in the health literature as being in the top 80% of an age- and sex-category as this is the threshold often reported as being associated with the greatest gains in health benefits [1, 2]. This means that the 10% prevalence of high fitness observed in those with severe obesity is 8 times lower than what would be expected in the general population. Within the present study, fitness was predicted using symptom-limited treadmill testing. Therefore, some individuals may have stopped due to problems such as musculoskeletal pain which is shown to be more likely in populations with obesity  rather than cardiovascular fatigue. Also, certain medications that are more prevalent in populations with obesity, such as beta blockers, can influence heart rate and blood pressure and increase the likelihood of experiencing early cardiovascular fatigue during exercise . Given that our sample comprised individuals with higher levels of obesity who are likely to have more health problems, our ability to accurately identify patients with high fitness may have been reduced. Nevertheless, the concept that an individual can present with high fitness despite severe obesity is a novel observation that has important clinical and public health implications.
Several studies have demonstrated a positive association between obesity and metabolic risk factors, as well as an inverse association between fitness and metabolic risk [2, 3]. These observations are largely limited to populations consisting of mainly normal weight, overweight and mild obesity [1, 2]. Borodulin et al.  and Lee et al.  demonstrated that there was a stronger association between CRF and systolic blood pressure with increasing levels of adiposity. Conversely, data from the Aerobics Centre Longitudinal Study suggest that the association between fitness and blood pressure may be weaker in those with greater obesity . We extend these findings to demonstrate that the benefits of fitness for most of the metabolic risk factors are similar for all obesity classes. The lone exception was that those with severe obesity, individuals who were fit were less likely to have pre-clinical or clinical hypertension than those were unfit.
The health effects of fitness are suggested to be mediated in part through the positive health benefits of engaging in regular physical activity . Physical activity has been shown to improve fasting glucose by increasing the rate of glucose uptake in skeletal muscle  and improve lipid metabolism through increases in lipoprotein lipase in both skeletal muscle and adipose tissue . As individuals with severe obesity are more likely to have deteriorations in these metabolic risk factors, it may not be surprising that the benefits of fitness may also extend to those with severe obesity. Nevertheless, in our study and others [18, 21, 22], obesity was more strongly associated with metabolic health risk than fitness. However, it should be of note that within the fit individuals, those with severe obesity did not have significantly elevated glucose, blood pressure or lipids as compared to those with mild obesity. This supports the potentially important health benefits of having a high fitness level, particularly for those with severe obesity.
The beneficial effects of fitness on health may also be attributed to differences in abdominal obesity. Wong et al.  report that in men the differences in visceral obesity with high fitness are reduced with increasing BMI and may be completely abolished at a BMI of ≥35 kg/m2. This would suggest that the benefits of fitness should be attenuated with those with moderate and severe obesity. However, our results in women and to a lesser degree in men, demonstrate that there was a greater difference in waist circumference between fitness groups in the moderate and severe obesity group than the mild obesity group: an observation that mirrors our results for metabolic health. These differences may in part steam from the sex or other demographic differences between the ACLS and WMC populations studied, but may suggest that fitness may be particularly important for women with obesity. However, these primary observations need further investigation for confirmation.
The strengths and limitations of our study warrant mention. Although this study used a larger sample size with higher levels of obesity than previous studies, the cross sectional design of our study does not allow us to infer causality. Also, we were unable to adjust for other factors such as physical activity, ethnicity, smoking status, and education as these variables were not consistently reported within our clinical population.