This multicenter, randomized controlled trial aims to evaluate whether early initiation of supplemental parenteral nutrition (SPN) (on day 4 of ICU admission) compared to late initiation (on day 8) can reduce 28-day all-cause mortality in critically ill patients. Secondary objectives include assessing the effects of early SPN on long-term functional outcomes (2-year EQ-5D-5L scores), mortality at various timepoints (ICU, 28-day, 90-day, 180-day, and 2-year), ICU length of stay, serum prealbumin levels (days 8 and 28), skeletal muscle measurements (diaphragm thickness, biceps, quadriceps, and rectus femoris cross-sectional area), incidence of ICU-acquired infections, adverse events (e.g., hyperglycemia, hypoglycemia, dyslipidemia, liver dysfunction), and ventilator-free days within 28 days. Eligible patients are adults (≥18 years) with at least one organ failure (SOFA score ≥2) within 24 hours of ICU admission, expected to remain in the ICU for \>72 hours, and unable to reach 50% of caloric target via enteral nutrition (EN) within the first 72 hours. A total of 946 patients will be enrolled and randomized to receive either early SPN (day 4) or late SPN (day 8) in addition to standard EN. Follow-up will include in-hospital assessments and telephone follow-ups at 28, 90, 180 days, and 2 years. The study hypothesizes that early SPN may improve short-term survival and nutritional/muscle status, though its impact on long-term quality of life remains to be determined.