It has recently been recommended that parenterally fed, critically ill patients should receive considerably less energy than the 36 kcal.kg(-1).d(-1) customarily received in earlier years and that mixed amino acid infusions not exceed 1.5 g.kg(-1).d(-1). The implications of these recommendations should be considered carefully, especially for patients with low body weight. Any sizeable reduction in energy provision will lead to negative energy balance in at least some patients, and negative energy balance is known to increase protein requirements. The optimal rate of amino acid delivery for underfed, critically ill patients is not well defined and could well exceed 1.5 g.kg(-1).d(-1). In addition, there are good reasons to suspect that the safe protein requirement of severely underweight, critically ill patients is >1.5 g.kg(-1).d(-1), even when adequate energy is provided.