Ketogenic diets and intermittent fasting have gained attention for their potential to improve cardiometabolic health, yet their direct effects on human cardiac metabolism, myocardial perfusion, and insulin sensitivity remain incompletely defined. This presentation will highlight findings from studies employing positron emission tomography (PET) to quantify myocardial substrate utilization and perfusion, alongside hyperinsulinemic euglycemic clamp experiments that assess organ‑specific insulin sensitivity. Together, these approaches reveal how dietary modulation of systemic fuel availability reshapes cardiac energy metabolism and insulin action across tissues. The emerging data provide mechanistic insight into the cardiometabolic consequences of ketogenic and fasting‑based interventions and underscore the need for further research to determine their long‑term clinical relevance.b