PURPOSE OF REVIEW: A discrepancy has emerged between experts' recommendations on how to feed ICU patients according to their requirements using parenteral nutrition, if enteral nutrition is not reaching the target. This review describes the differences in the recent guidelines issued by the American Society of Parenteral and Enteral Nutrition (ASPEN) and the European Society of Clinical Nutrition and Metabolism (ESPEN) regarding these aspects. RECENT FINDINGS: ASPEN/Society of Critical Care medicine (SCCM) experts hesitate to recommend the administration of parenteral nutrition to nonmalnourished ICU patients receiving some but not an adequate amount of enteral feeding during the first 7-10 days after admission. ESPEN guidelines recommend to compensate the deficit by adding parenteral nutrition after 24-48 h. These recommendations are mainly based on observational studies showing a strong correlation between negative energy balance and morbidity-mortality. SUMMARY: The energy deficit accumulated by underfed ICU patients during the first days of stay may play an important role in ICU and hospital outcomes for long-staying ICU patients. To reach calorie requirements by artificial nutritional support without harming the patient is still a subject of debate. Future studies, some already on their way will clarify this discussion.