Heart failure (HF) is a global public health menace which is associated with increased hospitalization, morbidity, mortality and high economic costs. However, HF which has ischemic heart disease as a common underlying disease, is a preventable condition. Cardiorespiratory fitness (CRF) is an index of habitual physical activity levels and is considered the gold standard for aerobic exercise capacity. Cardiorespiratory fitness, an important component of physical fitness and directly measured by peak oxygen uptake (pVO₂), and ventilatory efficiency slope (VE/VCO₂ slope), has been identified as one of the most important predictors of health outcomes and survival. We have previously reported that directly assessed pVO₂ is strongly, independently and inversely related to a lowered risk of cardiometabolic conditions such as HF,^, atrial fibrillation,^, ventricular fibrillation, diabetes as well as mortality from cardiovascular diseases (CVDs).^, Our recent population-based study based on UK Biobank suggests that CRF is a strong risk indicator for mortality and addition of CRF to a conventional risk factor score improved the overall discrimination of mortality risk and, more importantly, the predictive value of CRF varied across levels of some relevant risk factors, including age, sex, and smoking. This is an indicator of the potential of CRF to be a valuable risk assessment tool in clinical practice.

It is well-known that obesity, as measured by body mass index (BMI), is related to the development of cardiovascular outcomes. However, the combined effect of obesity and CRF on the risk of future HF still requires further study. It is of clinical relevance to understand if CRF attenuates the association of obesity with later risk of HF due to other underlying CVDs. The majority of previous studies on higher BMI, other obesity parameters, and HF risk have not taken into account the differences in CRF levels. Though high BMI is a risk factor for HF, there are findings of a non-linear relationship between BMI and CVD outcomes in HF patients which indicates an unusual relationship. In patients with established HF, accumulating evidence suggests that overweight and obese (higher BMI) individuals have improved survival compared to those with normal BMI, a known concept which has been called the “obesity paradox” or “reverse epidemiology”. Some mechanisms have been proposed to explain the HF obesity paradox which includes HF therapy being more effective in obese patients. It has been reported that CRF might play also play a role by either mitigating or negating the “obesity paradox”.^,

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