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On the Medical Sciences StackExchange site I asked Can a calorie be neither protein, carb, nor fat? and got a very helpful answer, which was that ethanol (in alcoholic drinks) is caloric but neither a protein, nor a carb, nor a fat. I then asked a follow-up question as a comment: Where can I find a list of all such substances? That is, substances that are caloric but neither protein, nor carb, nor fat. The person who answered my question recommended I post the question to this SE site.

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    $\begingroup$ A quick answer is ethanol from beer wine etc. Just to complement the answer by theorist. $\endgroup$ – Alchimista Jan 26 at 8:13
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If you go to https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:01990L0496-20081211&qid=1580028914722&from=EN you will see a fairly extensive list of caloric compounds (screenshot pasted below). This list is probably not comprehensive -- likely there are many compounds we (which includes our gut bacteria) can metabolize for calories. Rather, it's limited to componds that are caloric and approved for use in food. [It also doesn't include compounds that are naturally found in foods, and may be caloric, but are only present in very small quantities, e.g., nucleic acids.]

You can see that, in addition to fat, proteins, carbohydrates, and ethanol, the table lists polyols, organic acids, salatrims, and fiber.

Some explanation of the table may be helpful:

Polyols are artifical sweeteners. They include lactitol, maltitol, mannitol, sorbitol, xylitol, erythritol, glycerol, hydrogenated starch hydrolysates, and isomalt. [Though note that while erythritol is a polyol, it is listed separately as non-caloric. And while glycerol, aka glycerin or glycerine, is a sweetener, it is also used in food for other purposes.]

Organic acids include acetic acid (the main component of vinegar, other than water), as well as citric acid, ascorbic acid, and malic acid (the latter three are found in citrus fruits).

Fiber refers to dietary fiber, which is the type of fiber we can digest completely.

Salatrims are "short and long chain acyl triglyceride molecules"; they are a type of low-calorie fat substitute.

Source: Consolidated text: Council Directive of 24 September 1990 on nutrition labelling for foodstuffs (90/496/EEC) Select: 6 CELEX number: 01990L0496-20081211 Author: Council of the European Union Date of document: 11/12/2008

enter image description here

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    $\begingroup$ Minor point: Digestible fiber structurally is a carbohydrate. A more general answer is that we can metabolize an enormous range of compounds, as long as they are composed primarily of carbon, hydrogen, oxygen and nitrogen. A little bit of sulfur or phosphorus is okay too. Some of these compounds will not be net sources of calories, but you have to address that case by case. $\endgroup$ – Andrew Jan 26 at 2:57
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    $\begingroup$ @Andrew As the OP was interested in the types of compounds that could be caloric, I thought it made sense to use dietary categories, which distinguish dietary carbohydrates (sugars and starches) from dietary fiber, in part because, on average, they have significantly different caloric values. Having said that, it is a good point that, structurally, dietary fibers are carbohydrates. At the same time, it is worth noting (also a minor point) that not all dietary fibers are purely carbohydrates. Carageenan contains sulfur, and chitin contains nitrogen. $\endgroup$ – theorist Jan 26 at 3:26
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    $\begingroup$ @BuckThorn Thanks for the fix, and the bibliographic info. I incorporated both into my post. $\endgroup$ – theorist Jan 26 at 18:25
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Alcohol!!! It has 7kcal per gram 😁

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