ABSTRACT
Critically ill patients have a characteristic metabolic response to injury whether
their illness is secondary to trauma, burns, systemic infection, severe head injury,
or sterile inflammatory processes. This response is associated with an increase in
metabolic rate and rapid loss of fat and muscle mass; if prolonged, it has several
adverse effects, including immunosuppression, decreased or delayed wound
healing, loss of muscle strength, and diminished activity. Nutrition support in
the early stages of critical illness may be necessary to mitigate these potential
adverse effects. Of note, the metabolic response to critical illness is very different
from simple or uncomplicated starvation in which loss of muscle is much slower
and visceral proteins may be spared for a long period. This is because individuals
who suffer from simple starvation are hypometabolic and, therefore, have
decreased caloric demands compared with normal individuals. This is in contrast
to critically ill patients, who typically are hypermetabolic and have increased
caloric demands compared with normal individuals. In addition, starved individ-
uals have adaptive responses that lead to decreased glucose utilization, decreased
gluconeogenesis, and consequently a slower rate of protein loss. These adaptive
responses do not occur or are limited in severe illness and, therefore, net protein
loss can be high and compromise host defense mechanisms if nutrition support is
not initiated early in the disease process.
