The evolutionary role of nutrition and metabolic support in critical illness.

Maintenance of nutritional status is particularly challenging during critical illness. There is a common perception of a race against the clock to adequately feed the patient to prevent or minimize the sometimes catastrophic muscle wasting and general catabolic state that can result in the patient's deterioration. However, the course of critical illness may be separated into 3 phases, each with highly differing metabolic needs. The initial phase, in which the body attempts to fight the acute insult, is generally hypermetabolic. When the body fails to overcome the insult, it enters into a second phase, which is akin to hibernation. This stage is characterized by a functional metabolic shutdown triggered either by a lack of adequate energy supply or perhaps by the direct switching off of metabolism to spare excess use of a dwindling substrate and energy resource. Those strong enough to survive this phase enter into a period of recovery during which appetite returns, anabolism recommences, and organ function is restored. Nutrition should perhaps closely follow these nonlinear requirements, so as to avoid deleterious under- or overnutrition during the appropriate phase. This approach fits a teleologic argument that enabled many sick people to survive well before the advent of modern medicine and explains why catabolism still occurs despite adequate feeding.