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Normal physiology of nutrition 

Normal physiology of nutrition
Chapter:
Normal physiology of nutrition
Author(s):

Annika Reintam Blaser

and Adam M. Deane

DOI:
10.1093/med/9780199600830.003.0201
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date: 25 October 2020

Energy is derived from three major categories of macronutrient—carbohydrate, lipid, and protein. While energy requirements to maintain stable weight can be estimated, it is uncertain if meeting these energy requirements improves outcomes in the critically ill. In health, excess energy is stored via non-oxidative metabolism and during periods of inadequate energy delivery catabolism of storage products occurs. Both storing and using the stores cost energy, each may require up to quarter of energy contained in stored nutrient. Excess carbohydrate stored as glycogen is easily available, albeit in a limited amount. Storage of lipid is the largest energy repository, but requires complex metabolism and is limited by low oxidative capacity. Protein catabolism normally contributes less than 5% of energy requirements, but during periods of inadequate energy delivery or increased catabolism there is a marked increase in endogenous protein breakdown.

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