PURPOSE OF REVIEW: A recent and large multicentre study reports that ICU patients receive less than half of the recommended energy requirement. This review aims at clarifying whether underfeeding is scientifically justified or sustained by evidence-based medicine. RECENT FINDINGS: There is evidence that optimal nutrition improves clinical outcome of critically ill patients. The deleterious effect of overfeeding ICU patients is now well acknowledged, but underfeeding is not scientifically justified in ICU patients. Total energy expenditure in ICU patients is variable and methods to predict resting energy expenditure are questionable in these patients. SUMMARY: There is a need to measure energy expenditure in clinical practice. When not possible, the current guidelines on artificial nutrition (i.e. 25 kcal/kg per day) should be applied in order to limit underfeeding.