I know that chemical energy of food is converted to heat energy to be used by the body in many metabolic reactions. Specifically, I know that the breaking down of bonds is what releases energy.

Does that mean that some of the energy of food is lost during digestion as enzymes breakdown large food molecules into smaller soluble ones? Or is it the common concept of losing energy during digestion because not all the food is absorbed and some are egested?

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    $\begingroup$ How do you know that energy is required for digesting food? Enzymes are biocatalysts i.e. they just enhance the speed of digestion of food. Without enzymes too, all your food will get digested, but will take more time. $\endgroup$ Feb 17 '17 at 12:47
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    $\begingroup$ THere is plenty of overhead cost from digesting food. It takes energy grind up that food into small bit so that it can be digested and absorbed. To move food bolus through the digestion system via pertaltic contractions. And it takes energy to make those enzymes. And it take energy to maintain the immune system to control any parasites that came with said food. It also takes energy to breakdown anti-metabolites (compounds that prevent digestion and absorption) that come with most living tissue. $\endgroup$
    – JayCkat
    Feb 17 '17 at 13:04
  • $\begingroup$ virtually all of the food is digested, most of your poop (that isn't water) is actually dead gut bacteria, chemically resistant fibrous material makes up most of the rest. $\endgroup$
    – John
    Feb 17 '17 at 15:24
  • $\begingroup$ @another'Homosapien' Enzymes aren't free, and they aren't the only contribution to digestion. $\endgroup$
    – Bryan Krause
    Feb 17 '17 at 21:04
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    $\begingroup$ Good question. But it is not true that heat energy is used by the body to drive other reactions. Heat is the portion of energy that is lost. $\endgroup$
    – Roland
    Feb 18 '17 at 8:32

Typically, catabolic reactions such as the hydrolysis of food stuffs into their monomers is an exothermic process. This includes conversion of proteins into amino aids, starchs into monosaccharides and fats into fatty acids plus glycerol. As an example, hydrolysis of a peptide bond linking the amino acids in a protein liberates 8-16 kJ/mol of energy. https://en.wikipedia.org/wiki/Peptide_bond

So, that means a certain amount of the energy in food is liberated in the gut. But this is very minimal compared to the amount yet to be liberated, and harvested, as the many other bonds in the monomers are further broken down during cellular respiration.

A bit of research from a few different sites suggests the gut temperature is not meaningfully higher that the core body temperature measured in, say, the anus or vagina. https://en.wikipedia.org/wiki/Human_body_temperature Any heat liberated from digestion in the gut must be slow enough to be absorbed into the bulk of the body without accumulating in the gut.

  • $\begingroup$ Also, this release of some heat is what makes digestion spontaneous, otherwise (if $\Delta$G is 0 too) enzymes would require ATP for digestion to provide activation energy to the molecules. Anyways, nice answer, +1! $\endgroup$ Feb 18 '17 at 5:08

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