Malnutrition remains a prevalent and critical risk factor in patients undergoing cardiac surgery. Malnutrition significantly impacts cardiac surgery outcomes, exacerbating infections, delayed healing, prolonged ventilation, and mortality through preoperative deficits and postoperative stress. Early screening and intervention are critical. Recent studies emphasize that beyond classical preoperative fasting and postischemic stress states, factors such as sarcopenia, micronutrient deficiencies, and altered body composition significantly influence outcomes. Validated tools like GLIM criteria, NUTRIC Score, and Johns Hopkins Hospital Nutrition Support Score enable preoperative risk assessment. Advanced methods, including bioelectrical impedance and muscle ultrasound, detect sarcopenia to tailor therapy. Early enteral nutrition (within 12–24 hours) is standard, targeting 25–30 kcal/kg/day and 1.3–2.0 g/kg/day protein; supplemental parenteral nutrition is added if goals are unmet. In pediatric cases, high metabolic demands and feeding issues heighten malnutrition risk. Early enteral feeding with cautious advancement and fortified formulas is preferred; parenteral nutrition is used for delays. Routine pharmaconutrients (e.g., glutamine, arginine) are not recommended, but omega-3 emulsions show promise. Evidence-based nutritional protocols in perioperative and ICU care enhance recovery, reduce complications, and improve outcomes in adult and pediatric patients.