If you’ve ever tried to calculate the total calories contained in food by the macronutrient content but have then become unstuck with your final figure, this article could explain why.
The Atwater system  is used in the food industry to determine the total calorific value of food by employing the 4-9-4 method. This system applies energy conversion factors to the macronutrients carbohydrate, fat, protein and fibre. The average values of energy are expressed as the number of calories per 1 gram of the macronutrient. The Atwater general factor system includes energy values of 4 kcal per gram (kcal/g) (17 kJ/g) for protein, 4 kcal/g for carbohydrates and 9 kcal/g (37 kJ/g) for fat . Alcohol is also technically considered a macronutrient and contains 7 kcal/g (29 kJ/g) . For instance, if you have a food that contains 20g protein, you would multiply 20 by 4 to give 100 calories supplied by protein in that food.
A more extensive general factor system has been derived to include organic acids, used in food preservation, at 3 kcal/g (13 kJ/g) and polyols at 2.4 kcal/g (10 kJ/g) .
These figures were originally determined by using a bomb calorimeter and measuring the heat of food combustion and the resultant amount of energy the food produces .
There are a number of shortfalls when using the Atwater system to determine the total calorific value of a food. The energy conversion factors are estimates and are therefore likely to be associated with some inaccuracy when compared to the direct assessment method, bomb calorimetry. The energy conversion factors for macronutrients as a whole are not clear cut.
The calorie content of individual amino acids (AAs) were found to be different when calculated by correction of heat combustion . However, the protein conversion factor of 4 calories per gram was derived as an average for the energy yield of all amino acids . This means that if a food contains a large amount of the amino acid phenylalanine, which yields 6.7 kcal/g (28 kJ/g) , a relatively high energy value compared to other AAs, then the overall energy value of the food may be higher than what is derived by calculation using the protein conversion factor. The table below shows the heat of combustion produced by each AA used as the energy conversion factor.
Figure 1: Heat of combustion of amino acids 
|Amino acid||Heat of combustion (kcal/g)|
The assumptions based on carbohydrates are even more problematic. Firstly, the conversion factor does not distinguish between sugars, starch and dietary fibre. For example, monosaccharides have combustion heats of around 3.75 kcal/g (16 kJ/g), disaccharides 3.95 (17 kJ/g) and polysaccharides 4.15–4.20 kcal/g (17–18 kJ/g) . There are also sugar alcohols to consider, organic compounds that are derived from sugars – also known as polyols – all with a varying energy conversion factor; for example, xylitol provides 2.4 kcal/g (10 kJ/g) whereas glycerol provides 4.3 kcal/g (18 kJ/g) and erythritol is 0 kcal/g . However, 2.4 kcal/g (10 kJ/g) is a general rule of thumb for sugar alcohol conversion factors excluding erythritol .
By way of example, if a food was predominantly made up of monosaccharides, the energy conversion factor of 4 kcal/g may result in an overestimation of overall calories. Secondly, the Atwater method does not account for variants in fibre and resultant calories. Fibre can be partially degraded and absorbed in the large intestine and is assumed to be 70% fermentable , thereby providing some metabolisable energy. The extent of this degradation depends on the individual and the source of fibre. There is currently no clear-cut data to provide guidance on how to factor in the influence of fibre, although 2 kcal/g (8 kJ/g)  is typically used as the conversion factor for fibre within the food industry.
Fatty acids also differ in their heat of combustion; however, the difference is relatively small. Long-chain triglycerides have a value of 9 kcal/g whereas medium-chain triglycerides (MCTs) have a value of 8.3 kcal/g (35 kJ/g)  and salatrims, which are used as reduced-calorie fat substitutes , have a value of 6 kcal/g (25 kJ/g) . Albeit, a figure of 9 kcal/g is used as the standard within the food industry as a conversion factor for fat.
Generally, energy conversion for nutrients in the US is the same as it is for the UK and Europe. However, carbohydrates are calculated differently in the US from the method ‘carbohydrate by subtraction’ . The resulting ‘total carbohydrate’ value contains sugars, starch and fibre . For nutrition labelling in the EU, carbohydrate is defined as ‘available carbohydrate’, which does not include the fibre component and is instead derived by calculating the sum of sugars and starches in the food .
For now, the use of energy conversion factors provides a method for estimating the available energy intake, although their limitations cannot be disregarded. Contrarily, it is worth considering the significance of these shortcomings and application to real life. Although there may be slight variations in the energy yield of a food by calculation versus measurement by heat of food combustion, the differences may be so small that the values become negligible.
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