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What kinds of chemical species that do not belong to the category of fats, carbohydrates or proteins can the body metabolise?

Clarifications: By metabolise, I mean extract energy from (e.g. as ATP). The chemicals would be ingested as “food” regardless of whether they occur in natural dietary nutrients.

I already know of one, ethanol.

At a wild guess, I suspect the body can extract energy from acetate and citrate too.

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    $\begingroup$ @classy_BLINK I'd be very impressed if you could find me somebody who could ingest enough of those to obtain food energy from them without instantly ODing. $\endgroup$
    – Ingolifs
    Jul 12 at 0:58
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    $\begingroup$ I suspect this question needs a little more detail to be answerable. Are you specifically after things that a reasonable user might think of as 'normal food'? We metabolise pyruvate to release energy, and you can buy it as a supplement. If pyruvate is in, then we can start with a list of most of the intermediate metabolites in the Krebs cycle. $\endgroup$
    – bshane
    Jul 12 at 1:35
  • $\begingroup$ Also: human bodies only? $\endgroup$
    – bshane
    Jul 12 at 1:43
  • $\begingroup$ @bshane Yes human bodies, and no it doesn't necessarily have to be 'normal food'. Sounds like the intermediates of the krebs cycle would count. I'm interested in anything a human can extract a non-negligible amount of food energy from without dying in the process. By 'non negligible' I mean at the very least something that would stave off death for a starving person in the short term. $\endgroup$
    – Ingolifs
    Jul 12 at 2:57
  • $\begingroup$ I see two ways to approach this. First, molecules used as part of respiration to store energy as mentioned by bshane. Alternatively, it may be worth considering environmental context (e.g. what other energy sources would humans naturally intake; think chemosynthesis in the deep ocean). Sure, supplementing a human body with pyruvate and/or friends may fit the bill, but (as hinted by classy_BLINK, it would seem likely that having that as a sole (or majority) form of energy intake would probably cause significant dysregulation of metabolism. $\endgroup$
    – Greenstick
    Jul 12 at 20:48
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There are two significant energy sources for humans besides fats, carbohydrates, and protein listed in this USDA report that I found:

  • Organic acids: these are compounds like citric acid and malic acid that make up a small but significant portion of the energy content of many fruits and some vegetables. The energy content is in the range of 2-4 Calories per gram, so slightly less than from protein or carbohydrates.
  • Alcohol provides about 7 Calories per gram.
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