FGF21 and obesity due to melanocortin 4 receptor deficiency: Integrating wearable hemodynamic sensors with serial biomarker profiling (pre-/post-exercise) could enable adaptive algorithms—such as moderate continuous training for patients with HF with reduced ejection fraction who exhibit IGF-1 resistance versus carefully dosed high-intensity interval training for those with obesity who have HF with preserved ejection fraction when APN/FGF21 ratios indicate metabolic responsiveness—thereby balancing cardioprotection while minimizing pathological strain.