The staging classification of heart failure (HF) was developed to highlight progression of disease among high-risk subjects (Stage A) to clinically overt HF (Stage C) and to emphasize the potential for HF prevention in its early stages. Although current recommendations among patients with Stage A HF focus on risk factor modification, including treatment of hypertension and cardiometabolic disease, the role of exercise training remains unclear. Obesity, cardiometabolic disease, and physical inactivity have all been linked to risk of HF, with specific predilection for HF with preserved ejection fraction (HFpEF). Previous studies demonstrate a graded, inverse relationship between leisure-time physical activity and HFpEF―but not HF with reduced EF―risk. 1 Furthermore, lower body mass index and higher levels of cardiorespiratory fitness are inversely associated with incident HF, yet only interval improvements in cardiorespiratory fitness―but not body mass index―are related to HF risk. 2 Because of the dearth of effective pharmacotherapies for HFpEF, these and other studies highlight the potential role that modifiable lifestyle efforts, including increased physical activity, weight loss, and regular exercise, may play in its prevention and treatment. In this context, the study by Hearon et al 3 in this issue of JACC: Heart Failure is a timely and important contribution. The investigators examined the effects of intensive lifestyle interventions among obese adults at high risk for HFpEF. Of 80 subjects randomized to 1 year of high-intensity interval training (HITT) versus an attention control group, 56 completed the intervention. Participants also received omega-3 fatty acid supplementation versus placebo, which had no effect on study outcomes. Subjects in the HIIT group had markedly improved cardiorespiratory fitness after 1 year, with an average increase of 4.46 mL/kg/min in relative peak oxygen consumption (VO2) and an increase of 0.43 L/min in absolute peak VO2 This increase of> 1 MET (assuming 1 metabolic equivalent= 3.5 mL/kg/min) is notable and clinically meaningful; extrapolated from observational data, we know that among men referred for clinical exercise testing, a 1 MET higher exercise capacity was associated with 12% lower mortality. 4

Although this magnitude of change in peak VO2 mirrors previous work by the same investigative group among patients with Stage B HF, 5 it represents a much larger improvement than previously seen in exercise training studies in patients with Stage C HFpEF, with the caveat that these latter interventions were largely shorter in duration, differed across types of exercise training, and enrolled older patients with lower baseline peak VO2. 6-8 Beyond showing improvements in peak VO2 with HIIT, the investigators conducted detailed phenotyping of cardiovascular structure, function, and body composition. Why is this important? We know that among patients with HFpEF, multiple deficits in both cardiac and extracardiac organ reserve can lead to its hallmark symptom―exercise intolerance. Quantifying specific organ contributions to exercise