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Nutrition: macronutrient metabolism 

Nutrition: macronutrient metabolism
Nutrition: macronutrient metabolism

Keith N. Frayn


August 28, 2014: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.


Many nutrients and metabolites also regulate metabolic pathways rapidly through G protein-coupled receptors.

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date: 28 September 2021

Food intake is sporadic: for most people it occurs in three major boluses each day. Energy expenditure, however, is continuous, with variations during the day that bear no resemblance to the pattern of energy intake, except that over some reasonable period of time (a week or more) the two will, in most people, match almost exactly. Therefore the body has developed complex systems that direct nutrients into storage pools when they are in excess, and that regulate the mobilization of nutrients from these pools as they are needed. The situation is analogous to the fuel tank of a car and the throttle that regulates fuel oxidation, except that in the car there is just one fuel and just one engine: in humans there are three major nutrients and a variety of tissues and organs, each of which may have its own preferences for fuels, that vary with time. Carbohydrate, fat, and protein (made up of amino acids), are the three sources of energy which are variably stored and assimilated from food each day. The fact that we can carry on our daily lives without thinking about whether to store or mobilize fuels, and which to use, attests to the remarkable efficiency of these control systems....

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