Obligatory disclaimer: I am not a medical doctor, and the content of this website was created for informational purposes only. Such content is not intended as a substitute for medical advice, treatment or diagnosis.
If you’ve ever been interested in diets or exercise, then it’s very likely that you’ve read about the energy content of the different macronutrients: carbohydrates, protein, fat and alcohol. Namely, that energy content comes at 4 calories per gram of carbohydrates, 4 calories per gram of proteins, 9 calories per gram of fat and 7 calories per gram of alcohol. Have you ever wondered how those numbers came about? In next week’s post we’ll talk about how that was done, but today’s post will deal with with how the energy content of a given food is determined.
Measuring the energy content of food: The Atwater system
The Atwater system, and its derivatives, is used to determine the available energy in food. Although there is some criticism to be made of the system, there have been no real alternatives proposed, and so it has become the reference.
Rather than go for the mathematical derivation of the Atwater system, like Wikipedia does, we’ll stick to its concepts and their development. That way we’ll be able to understand the system’s strong and weak points.
The basic idea is conservation of energy. As energy cannot be created or destroyed, then the energy we have available for our metabolism comes from our food. At the same time, some of that energy is lost through our excretions: urine, faeces, gases and secretions. Because of that, the energy that our body can use comes from the difference between the energy from our food and the energy that we lose from excretions.
Now, the energy in our food comes from the components of that food: carbohydrates, proteins, fats and alcohol. For the moment we’ll consider that our alcohol consumption is negligible to keep things simple. That means that our food energy intake is equal to the sum of our energy intake of carbohydrates, proteins, and fats. Simple, right? Atwater determined the energy content of carbohydrates, proteins, and fats through bomb calorimetry, so we will take such values as given.
Something similar can be said for the energy that we lose through our excretions. For example, we can consider the total energy lost through faeces as the energy of proteins, carbohydrates and fats that are excreted without being metabolically used. When it comes to energy loses through urine, these are measured from the amount of nitrogen in urine (one of the system’s caveats).
Considering all of that, Atwater ended with a formula that allowed him to calculate the energy content of a given food by knowing how many proteins, carbohydrates and fats were used by the body, and the energy content of such macronutrients. That formula is essentially a simple sum, where the energy of each macronutrient is multiplied by the amount of that macronurient that was digested.
Problems with the Atwater system
Remember how we said that carbohydrates have an energy content of 4 calories per gram? Well, that value is known as the heat of combustion (the heat released by burning a gram of a substance), and different sorts of carbohydrates have a different heats of combustion. In the grand scheme of things the variations in the heat of combustion for different carbohydrate sources are negligible for the average person, but if you want to get the numbers right (like scientists do) then you have to be a little more careful.
The same thing happens for proteins, where different proteins will have their own heats of combustion. Again, the difference between the heats of combustion is very small, but it is a problem to be wary of if you want to be as precise as possible. Unsurprisingly, fats have the same issue.
Atwater’s formula also depends on the amount of macronutrients that were actually digested, rather than ingested and then excreted through faeces. However, even if our diets were the same throughout our lives, the evidence shows that our faecal excretions are not, and so the energy we truly get from our food isn’t constant. Again, this not a problem for the average person who counts calories or is health minded, but for neurotic-number-crunching purposes the struggle is real.
Where the Atwater system gets things right
It’s very important to understand what the Atwater system does: it calculates the energy in food rather than measure it. Measurements always have some inaccuracies, and are reported as such. Calculations depend on measurements, and so they also come with their own inaccuracies.
Such inaccuracies are a part of the Atwater system. Whether they’re due to variations in how individuals digest food, errors in how food intake is measured, or variations in how the same meal is prepared on different occasions, the results of the Atwater system can only be as good as the measurements on which it depends.
However, the Atwater system is conceptually sound, and it works. Small variations not withstanting, the reality is that the Atwater system consistently gets the just-about-right results, and that is what actually matters.
As was stated previously, next week we’ll talk about bomb calorimetry, the method used to determine the energy content, which we now know is called heat of combustion, of carbohydrates, proteins, and fats. Hopefully that will make things much clearer regarding how food energy values are reported.
Hope you enjoyed this post, if you would like to see more content like it, check our Rebuttals to Fatlogic section.
See you next week!