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Long story short, is the energy value listed on the foods nutritional label a fixed value?

Food labels indicate macro-nutrients (carbs, proteins and fats). These values, in combination with the 4-4-9 system energy calculation, provides the total energy that 100g of that food can provide.
This submit the assumption that all of those macro-nutrients will be burnt.

Talking about proteins: this kind of nutrients can be used as energy or building block for our bodies.

Does this mean energy value listed on the nutritional food label is only the maximum energy intake? that is, energy calculation should undergo taking into account proteins burning.

Or it is inconsequential? that is, whatever proteins destiny, our body extracts exactly that amount of energy.

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  • It is a food label. It cannot know how your body is going to use it. Body is not 100%. Human feces has calories.
    – paparazzo
    Oct 28, 2018 at 16:27
  • @paparazzo my question is a little more specific to the proteins destiny
    – mattia.b89
    Oct 28, 2018 at 16:56
  • Food label can still not know how your body will use it. Since it does not assume how your body will use calories it is logical it will not assume how your body will use protein.
    – paparazzo
    Oct 28, 2018 at 17:01
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    You answered your own question with the link to the 4-4-9 system. Protein is counted as 4 Kcal/g. It's as simple as that. How much of that will be burned as energy and how much will be used to build proteins isn't known in advance, as @paparazzo has pointed out.
    – Carey Gregory is on strike ♦
    Oct 28, 2018 at 17:38
  • @mattia.b89, what do you mean by "protein density?"
    – Jan
    Oct 30, 2018 at 10:18

2 Answers 2

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Simplified from CALCULATION OF THE ENERGY CONTENT OF FOODS - ENERGY CONVERSION FACTORS (FAO.org)

Food energy can be:

  • Combustible or ingested energy = theoretical maximum energy content of a food measured using bomb calorimetry (the energy you ingest)
  • Metabolizable energy (ME) = ingested energy minus energy lost in feces by indigestible nutrients (the energy listed on nutrition labels)
  • Net metabolizable energy (NME) = metabolizable energy minus energy converted into heat due to dietary-induced thermogenesis

Comparison of ME (from food labels) and NME (potentially fattening energy) in different macronutrients (FAO.org, table 3.3.):

  • Protein (Calories/gram): ME = 4, NME = 3.2
  • Fat: ME = 9, NME = 9
  • Carbohydrates: ME = 4, NME = 4
  • Dietary fiber: ME = 2 (but wrongly counted as 4 on food labels), NME = 1.4
  • Alcohol: ME = 7, NME = 6.3

On food labels, dietary fiber is listed under carbohydrates as having 4 Cal/g, but its ME is only 2 Cal/g and NME only 1.4 Cal/g. So, a certain carbohydrate food that has a lot of fiber can have significantly less energy than stated on the food label.

The metabolic energy (ME) of individual amino acids in proteins can vary from 2 to 6 Cal/g (ResearchGate). Most proteins contain most amino acids, but in different proportions. I haven't found, so far, if this results in significantly different caloric value of various proteins, such as in beef and egg white protein, for example.

In conclusion, net metabolic energy (NME), which is potentially fattening energy, tends to be lower than metabolic energy (ME) stated on the food labels, at least for proteins, dietary fiber and alcohol.

The NME of certain nutrients can further differ due to personal factors, such as age, state of health, etc. and nutrient combinations (e.g., fiber can slightly inhibit the absorption of fat) (European Journal of Clinical Nutrition). So, Calories stated on the food labels are not fixed values, but, so far, I haven't found any evidence how could be this practically important.

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  • Talking about proteins: its energy contribute is the same regardless the fact their amino acids fate (fuel or building block)?
    – mattia.b89
    Oct 29, 2018 at 18:33
  • They are mainly proteins, not individual amino acids, that are used as building blocks, for example, in cell membranes. I can't say just now, which amino acids and in which percent are used in those proteins, but I don't have a feeling this information would tell you a lot about the energy value of different protein foods. Except if you have something very specific in mind. Another aspect is that diff amino acids contain diff amount of nitrogen that will be excreted as ammonia/urea, but again, not sure if this can be realistically important regarding energy content.
    – Jan
    Oct 29, 2018 at 18:41
  • Here fao.org/docrep/006/Y5022E/y5022e03.htm it says, the nitrogen content in different proteins varies from 13 to 19 percent.
    – Jan
    Oct 29, 2018 at 18:50
  • I could think: proteins burnt -> ME=4; proteins as building blocks -> ME=0 (simply because it is not burnt) or maybe a negative number (because the body needs to spend some energy to breaks proteins into amino acids)
    – mattia.b89
    Oct 30, 2018 at 17:24
  • Proteins/amino acids used as building blocks are simply not burned, so they contain most of energy the proteins in foods do. So, you do not need to think that their energy approaches zero. Your original question was if the energy stated on nutrition labels is fixed. It is not. For further discussion, I'd like to know what is the practical purpose of your question. Do you have some hypothesis about how to decrease caloric value of nutrients? If so, I suggest you to make some hypothesis, so we can know what we are discussing about. – Jan 11 mins ago
    – Jan
    Oct 30, 2018 at 18:12
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The definition of a Calorie (Note the big C, a food calorie is actually a kilocalorie, or 1000 small calories) is simply the amount of heat necessary to raise the temperature of 1 kilogram of water 1 degree Celsius.

To get this, originally foods were burned in a bomb calorimeter, and the calories determined by the rise in heat of the water. Now (At least in the United States), they are determined more indirectly using the Atwater system. (This was done to comply with federal labeling laws, that require it to be estimated from food components, so elements such as fiber were taken out).

The wikipedia entry is decent, and has links to pages pointing out the flaws in the system, and there is a short https://www.scientificamerican.com/article/how-do-food-manufacturers/ as well.

So as a summation, it is an estimation of the amount of food calories that are potentially in a food. As with anything, different people process in various efficiencies, so there is no true way to know how much actual energy you personally are getting out of a food item.

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