Energy value from food: is it the maximum avail?
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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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 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.
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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