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Non-biologist speaking here. As you can probably see from the title, I'm having trouble wrapping my head around concepts related to the body's energy balance amd the role of macronutrients. Let me explain in more detail.

If a person consumes excessive amounts of calories for a certain period of time (i.e. more than they need) they will start to gain weight in the form of fat. Conversely, an extended caloric deficit will lead to weight loss. So far, so good.

Now, each macronutrient has a certain amount of calories per gram of nutrient - if I'm not mistaken 1 g of protein has 4 kcal.

On the other hand, according to different sources that I've been reading lately, the body does not really use protein for energy gain unless in extreme circumstances. In other words, carbohydrates and fat can readily be "converted" to ATP for energy gain while the conversion of protein into glucose and then ATP is very inefficient. At least that's my understanding, if there's already a misconception here feel free to correct it.

Proteins are mostly used to build and repair different structures in the body. I will use the reparation and buildup of muscle tissue as an example. And here comes my main question: if proteins aren't being "burned" by the body to extract energy in the form of ATP, why are the calories consumed in the form of protein really relevant? In other words, if the protein that I eat is being used by the body to build stuff such as muscle, haemoglobin etc. and not being burned, why do I care about the calories that protein has? Why will excess protein calories make me fat?

I obviously "know" that calories from protein matter, but I don't understand why this is the case. I feel like I'm really not grasping something essential here. I hope that the question is clear and that an interesting discussion could follow. If anyone could give a more detailed explanation of what's going on in the body in this scenario, that would be much appreciated.

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    $\begingroup$ Relevant to what? Please provide context, i.e. a scientific source in which the energetic values of different foodstuffs are presented with implied relevance to something. Be aware that this site cannot provide dietary advice as questions about medicine and health are specifically off-topic. Also note that this is a question and answer site, not a discussion site, nor is it intended to replace standard textbooks in which metabolism is described for those wishing to know "what's going on in the body". Please cite the texts that you have consulted. $\endgroup$
    – David
    Feb 13 at 21:14
  • $\begingroup$ I tried to give an answer "in the spirit of your question," so to speak, but feel free to ask for more biochemistry $\endgroup$
    – That Guy
    Feb 14 at 1:24

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Preamble
I had not intended to answer this question as I felt it lacked the clarity required for this site. However I now think that it is necessary to correct the assertion that “protein has to be converted into glucose to generate energy”, especially as this is echoed by the answer from @ThatGuy. I shall clarify the question by interpreting the word “relevant” primarily to mean general “purpose” or “utility”, and shall consider this in relation to the Food and Agriculture Organization of the United Nations (FAO), one scientific body that has published values for energy values of nutrients.

Summary

  • The dietary nutrients carbohydrate, fat and protein can have different metabolic fates in the body depending on individual circumstances. The assumption that “protein is mostly used by the body for purposes other than extraction of energy” does not apply in many circumstances.

  • Energy values for food are concerned only with the situation in which the nutrient is used to produce its maximum biological energy yield as ATP, using the appropriate metabolic pathways.

  • These values are not simple bomb calorimeter values. They are calculated separately for protein, carbohydrate, and fat, taking into account their different metabolism. For proteins this takes into account the different metabolism of the 20 amino acids, although it should be emphasized none of these need to be converted to glucose to generate energy.

  • There are several different purposes to which these values can and are used, but the FAO is primarily concerned with their use to assess basic nutritional needs and their fulfilment in the countries of the world in which malnutrition may occur.

The FAO and standard energy values for nutrients
As mentioned above, I will consider this question in relation to the FAO, the Food and Agriculture Organization of the United Nations, a body that has produced energy values of different nutrients — values that appear to be used by government organizations and commercial food groups in many countries. The source of my information is this report on the subject, produced in 2003, to which the reader is referred for fuller details.

What did the FAO envisage that the results of their work would be used for?
To quote from Section 1.1 of the report:

  • assessment of the energy needs of countries… living under different circumstances;
  • assessment of food availability within regions and countries;
  • assessment of the potential ability of available food supplies to meet a country’s … needs,
  • assessment of individuals’ diets (although the recommendations are not meant for this purpose));
  • as a basis for food labelling, with implications for consumer information/education about specific foods, regulatory compliance regarding nutrient content and claims, and trade.

This should answer the original poster’s concern with the “relevance” of energy values. The purpose envisaged was clearly with energy requirements in countries in which energy malnutrition was a problem, not affluent Western countries suffering from easily preventable diseases caused by overconsumption of food (although they are not irrelevant in the latter).

Did the FAO take into account the different metabolic fates of different energy supplying nutrients?
Yes. It was constituted of personnel who, unlike the poster, were biological scientists with a sound understanding of the physiology and biochemistry of metabolism. To quote from section 1.3 of the report:

“Energy requirement recommendations remain “theoretical” and of little practical value until they can be related to foods… Two pieces of information are needed in order to translate individual foods, and ultimately diets, into energy intakes that can be compared with the requirement recommendations… …amounts of components must be converted into energy content using an agreed set of physiology-related factors that correspond to the energy-producing potential of the components in the human body. Thus, in order to make accurate estimates of energy intake, it is essential to have energy conversion factors for each component that denote the energy per gram for that component. However, it has long been recognized that the energy contents of protein, fat and carbohydrate differ, both inherently in the compounds themselves and owing to their different digestion, absorption and metabolism.”

So the scientists on this body were well aware of the problems in producing energy values for different nutrients, and the paper considers proteins, fats and carbohydrates separately and in detail in sections 2 and 3 of this report. For each of these (and for different sources of the same constituent) they produced separate values of various measures, with final values in Section 4.

However the value of 17 kJoules per gram protein (=4kcal/g)) quoted by the original poster is the FAO value of Net Metabolizable Energy (NME) — i.e. the energy produced in circumstances when all the protein amino acids are oxidized to produce ATP. This is relevant to individuals in protein equilibrium but in net energy deficit.

(The overall energy values they suggest for fat is 37 kJoules/g, and for carbohydrate 17 kJoules/g).

Summary of metabolite interconversions in obtaining metabolic energy from proteins, fats and carbohydrates
Pathways for energy generation from nutrients

The diagram above is my own attempt at a biochemical summary for the situation when carbohydrate, fat or protein are being solely used to generate energy. Although some ATP is generated directly in one pathway, most of the carbon skeletons are oxidized in the TCA cycle, shown. Not shown is the NADH produced in the TCA cycle and its oxidation in the electron transport chain to produce a proton-motive force which drives the conversion of ADP to ATP — a molecule that can be considered as a source of metabolic energy.

In the energy-deficit scenario carbohydrates are converted to glucose, then to pyruvate in glycolysis (with some direct generation of ATP) and then to Acetyl CoA which for oxidation in the TCA cycle.

In this scenario fat (triglyceride) is converted to fatty acids and glycerol, the latter feeding into glycolysis, and the former being oxidized to Acetyl CoA, and hence to the TCA cycle.

In this scenario, in which they are not required for protein biosynthesis, each of the 20 amino acids from protein undergoes different metabolic conversions, as shown. All these can lead to the TCA cycle, and, as is evident, none involve conversion to glucose†. (The removal of nitrogen from the amino acids, actually consumes energy, but this is allowed for in the FAO calculations.)

Analogy (for those that find them useful)
If I buy a gold trinket in an antique shop it will (in Britain at least) bear a hallmark indicating the purity of the gold. If I am interested in keeping the piece until the price of gold increases and then selling it for its gold content (perhaps after melting it down) then the hallmark information is of primary relevance to me. If, however, I am interested in the trinket because of its beauty or because it may be a rare piece from a famous designer, the hallmark, although reassuring, would have little “relevance” to my purpose.

Even shorter analogy (to demonstrate my sense of humour)
One man’s meat is another man’s poison.


†The misconception about protein and glucose may have arisen in the following way. In extreme starvation the primary metabolic consideration is to supply the brain with glucose, on which it is absolutely dependent. To do this muscle protein is broken down to amino acids, some of which (e.g. those in the diagram that go to pyruvate) can be converted to glucose in the liver by the reversal of glycolysis (gluconeogenesis). However in a normal diet this is unlikely to happen because there will be a plentiful supply of glucose from carbohydrate, and it is certainly not the scenario in which energy values of protein are calculated.
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  • $\begingroup$ Thank you for the detailed answer, this cleared up some of my misconceptions. In retrospect I should not have asked this question on this site because it is too "unscientific", or at least I should have formulated it more precisely. $\endgroup$
    – ilovemaths
    Feb 25 at 14:45
  • $\begingroup$ When I asked why the calories from protein are "relevant", I meant why their energy value of 4 kcal/g should be counted when keeping track of consumed calories (in a diet for example). My reasoning was that bodybuilders, for example, require high amounts of protein merely to build their muscle mass – therefore the caloric value of their protein intake is irrelevant as they do not metabolize this protein to gain energy but rather build body tissue with it. $\endgroup$
    – ilovemaths
    Feb 25 at 14:46
  • $\begingroup$ On the other hand the reasoning often promoted in dieting tips is that they need to keep track of every calorie they consume – regardless of its origin – to avoid eating more calories than they need, otherwise they will gain fat. Apparently this misunderstanding is very common since it is very often asked on numerous forums. I resorted to this forum not because I wanted dieting tips but rather because I wanted to understand the mechanisms behind protein metabolism in this scenario, but you are right, this question probably doesn’t belong here. I apologize $\endgroup$
    – ilovemaths
    Feb 25 at 14:46
  • $\begingroup$ Side note: A biochemistry professor answered this question (asked by someone else, not me) on a non-scientific forum called quora (quora.com/…, answer by Mark Roseman). I also found his answer very interesting and in case anyone with the same misunderstanding as me is reading this, go read Prof. Roseman's answer and the comments to his answer on that site. $\endgroup$
    – ilovemaths
    Feb 25 at 15:02
  • $\begingroup$ I should also note that your answer sparked a deeper interest in biochemistry. I found some very good textbooks that I am hoping to study soon if I have the time to do so. I know that comments shouldn't be used to say "thank you" here on stack exchange, but in this occasion I'll do it anyway: Thank you David for catalyzing my interest in a very fascinating scientific discipline! $\endgroup$
    – ilovemaths
    Feb 25 at 15:27
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Think about it this way: if you're compiling a budget or asset report then you want to know about cash flows - income vs. expenditures - regardless of what form that "cash" might take. Sometimes, you want to know how much "cash at hand" you have, and you'll count only the most liquid assets, but often you'll also need to know the total value of any assets that can be converted into cash at all - not just stocks and bonds, but real estate, let's say, and a lot of other possible in-betweens.

In that they are used here (in the context of nutrition), calories are basically like energy currency with varying degrees of liquidity. You might have some assets, like carbs, which are like publicly traded stock shares that can be fairly easily converted to ATP with little commission. Proteins, then, might be more like specialized assets. But the crucial point is, proteins are still energy assets. When you are evaluating your wealth for the purposes of taxes or to take out a mortgage, the bank/IRS will still want to know your net worth considering any property contributing to that net worth. Because proteins can be converted to energy, they still add up to the overall energy budget.

Imagine you have a financial portfolio where you're investing more into stocks (proteins) than what is optimal for your growth targets. The same way that if you have excess cash in your financial portfolio (after meeting your investment and savings goals), you might invest it into more stocks or bonds, the excess energy provided by proteins will need to be stored somewhere.

In the body, consuming more protein than needed for muscle repair and other functions leads to excess calories. Since the body sees this as an "excess cash flow," it converts proteins into glucose (sort of- the details are complicated) and stores it as fat (which is, in our parable, like the savings account).

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