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It is stated here that in human nutrition, micronutrients are nutrients required generally in less than 100 mg daily quantities whereas macronutrients are required in gram quantities. It is widely stated that our macronutrients are carbohydrates, lipids, and proteins. How was it shown that these are the human macronutrients and that we don't have other macronutrients?

One answer to this similar question on Quora suggests that ethanol or ketones could be considered macronutrients. Ethanol is mentioned in this article, though ketones are not. Other sources I saw didn't clarify.

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  • $\begingroup$ there aren't, there are four, carbohydrates, lipids, proteins, and water. Also ethanol and ketones are not even essential nutrients much less macronutrients. you are getting downvoted for not reading your own source. $\endgroup$
    – John
    Feb 10 at 3:30
  • $\begingroup$ What about nucleic acids? $\endgroup$ Feb 10 at 6:20
  • $\begingroup$ i read the sources john, but thanks for the input $\endgroup$
    – imrobert
    Feb 10 at 13:54
  • $\begingroup$ if nobody knows, does anyone know a resource where i could hunt this down? $\endgroup$
    – imrobert
    Feb 14 at 18:10

1 Answer 1

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Summary
The question is based on the misconception that the term “macronutrient” originated as a scientific definition, to which entities satisfying this definition were assigned. In fact it was a default term used as a distinction from compounds falling into the earlier category, “micronutrient”, and was used initially to encompass the three specific classes of food that had been established to be sources of energy through decades of nutritional research. Hence there was no question of whether there “should” be more macronutrients. The term is not a scientific definition, and today is used in different ways so that anyone using the term needs to clarify what it should be taken to mean in that particular context.

Food Energy and the history of Nutrition
Scientific studies of nutrition, dating the late 18th century, were initially concerned with chemical structure, metabolic fate and energy produced by different foods, and this is described in a short readable article by Ned Stafford in Nature (2010) 468, S16–17. By the end of the 19th century protein, fat and carbohydrate had been established as the chemical fuels that supplied energy from the diet, and the energy values (in terms of the, now obsolete, calorie) had been established by Atwood, using his respiration calorimeter.

Micronutrients
The next phase of nutritional research was concerned with constituents of the diet that did not supply energy, but were nevertheless essential for health. The first of these were the vitamins, organic molecules that in many cases turned out to be cofactors for enzymic reactions. The term, vitamin, introduced in about 1912, was a contraction of “vital amine”, although the chemical description “amine” does not apply to some of the essential organic molecules later assigned to this class.
Later still came the discovery that certain non-organic molecules were essential dietary components, especially metal ions such as copper or selenium, sometimes known as ‘trace elements’ or ‘mineral nutrients’. These were not termed vitamins because of their non-organic nature, but the need to refer to all non-energy supplying dietary requirements together led to the employment of the portmanteau term, ‘micronutrient’. The first mention I can find of this in relation to human nutrition was in a US Senate report of 1954, although it had been employed extensively in relation to plants from 1940, and the word is most frequently used in a botanical context.

The introduction of the word ‘Macronutrient’
The first recorded use of the word ‘macronutrient’, in relation to human nutrition, was in a 1968 article in the Archives of General Psychiatry. The term only appears in the title of the article (“Comparison of macronutrients in the diets of psychotic and normal children”), although it is clear from the tables therein that it refers to carbohydrate, protein and fat. It seems likely that it was (or had already been) coined by analogy with — and in contrast to — micronutrient, as it was shorter and appeared more elegant than ‘energy-producing food’. Indeed the Oxford English Dictionary entry includes this contrast:

1968– Physiology.
A nutrient belonging to one of the three major food groups (carbohydrates, proteins, and lipids) required by animals, esp. as opposed to a vitamin or mineral.

It should also be mentioned that the term had been in agricultural/botanical use since 1942, but embracing a quite different set of compounds (nitrogen, phosphorus, potassium etc.).

Distinguishing scientific definitions from descriptions
In my opinion the root problem in the original post is confusion between terms that are scientific definitions from those that are merely descriptions. For example, a chemical term like amine is rigorously defined and indicates a class of chemicals to which any with a structure satisfying its definition (‘contain a basic nitrogen atom with a lone pair’) will be assigned. Hence any list of amines will be expanded by inclusion of newly discovered compounds. In contrast, scientific descriptions (at least in this context) start with a collection of known entities and assign a name to encompass them. This, I submit, is the case with ‘macronutrient’. It not a definition, but a term assigned to carbohydrates, proteins and fats, as a group.

Thus, one answer to the question:
“How is it know that there are only three macronutrients…?”
is
“Because that is what the word was introduced to describe”

But…
…context is all. As already stated, in plant science the term has a quite different meaning, and those who feel that they should regard the term as a definition, redefine it to exclude ethanol, for example, on the basis that it is not essential. In practice, all that is necessary is to make it clear how one is going to use the term. The FAO food and nutrition paper 77 — Chapter 2, for example, merely states “Analytical methods for alcohol, which can be a significant source of energy in some diets, polyols and organic acids were not discussed, and hence no recommendations for methods are made.”

So an alternative answer to the question:
“How is it know that there are only three macronutrients…?”
is
“Because that is how macronutrient has been defined in this context.”

or, as Humpty Dumpty said:

“When I use a word it means just what I choose it to mean….”
Lewis Carroll — Alice in Wonderland

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  • $\begingroup$ Thank you! This historical context is elucidating, and I did not realize macronutrient is a non-technical descriptor of molecules where we usually get our food energy. This led me to this page (en.wikipedia.org/wiki/Food_energy) which states, "However, the direct calorimetric method generally overestimates the actual energy that the body can obtain from the food, because it also counts the energy contents of dietary fiber and other indigestible components..." It also discusses polyols and organic acids, and I have this follow-up question: What gives me food energy? $\endgroup$
    – imrobert
    Feb 22 at 18:54
  • $\begingroup$ @imrobert — Your concerns about simple bomb calorimetry are understandable, and were the subject of another question on this list, which I recently addressed in this answer. $\endgroup$
    – David
    Feb 22 at 21:30
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    $\begingroup$ @imrobert — A few changes to that: "The amount of energy a food molecule gives me is the net amount of energy it produces from the TCA cycle, fatty acid breakdown and glycolysis." 1. "Net amount", because it may use some energy (as in the elimination of N from amino acids), 2. "Produces from" as your present formultion makes it sound as if the TCA cycle needs energy, that food produces. In fact the TCA cycle is a way of breaking down 3C molecules in a way that converts their energy (ultimately) to a useful form, rather than heat. 3. To cover carbohydrates and lipids. $\endgroup$
    – David
    Feb 24 at 18:49
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    $\begingroup$ @imrobert — But I would say, more technically “The amount of energy a food molecule gives me is the net amount of energy, in the form of ATP, that can be produced from oxidation reactions in the body.” Just as the heat energy from burning wood is produced by its oxidation, the energy for movement, growth etc is produced from oxidation of foodstuffs. Although a little is generated directly in particular reactions, most is generated in two-step process. First the reactions use an oxidizing agent NAD, which is reduced to NADH in the process. cont... $\endgroup$
    – David
    Feb 24 at 18:58
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    $\begingroup$ ...cont Then there is a complex system in the mitochondrion, where the energy in NADH is used to form ATP from ADP through reoxidizing it to NAD. Not so simple. Explaining that is what put bread on my table for many years. $\endgroup$
    – David
    Feb 24 at 18:59

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