Energy content from carbohydrates, fats, alcohol and proteins

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On previous posts we’ve talked about the energy content of food and the standard method by which it is determined. In that post we talked about how one must account for the amount of carbohydrates, proteins and lipids that a meal contains, and how much of those macronutrients our body actually digests. By knowing that last quantity, we can then add up all the energy coming in the form of carbs to the energy in the form of proteins, and the one in the form of lipids. In this post we will therefore learn a) the energy content of carbohydrates, proteins, alcohol and lipids and b) how those energy contents were determined.

Caloric densities / energy contents / energy yields of the four macronutrients

For this section we’ll use the following reference: Purves, W. K., Sadava, D. E., Orians, G. H., & Heller, H. C. (2001). Life: the science of biology. Macmillan. Anyway, let’s just get to the numbers, shall we?

  • Carbohydrates provide 4.52 calories per gram, usually rounded to 4 calories per gram.
  • Fats provide 9.5 calories per gram, usually rounded to 9 calories per gram.
  • Proteins provide 4.1 calories per gram, usually rounded to 4 calories per gram.
  • Alcohol provides around 7 calories per gram

It’s worth noting that these numbers are average ones. For example, olive oil (a fat) provides 9.47 calories per gram while beef fat provides 9.5 calories per gram. In other words, and as we’ve said before, there are small variations between similarly cooked and prepared meals when it comes to their energy content. However, those variations are small enough so that the numbers used in our prior list are accurate, as the following reference shows: Merrill, A. L., & Watt, B. K. (1973). The Energy Values of Foods, Basis and Derivation. USDA Handbook 74.

How were those numbers determined?

By watching the world burn. Ok, let’s take this a little more seriously.

Indeed, the energy yields of the four macronutrients were determined by burning them up and determining their heat of combustion, the fancy term for the heat generated when burning something. In other words, a known quantity of each of the four macronutrients was used as fuel for a fire that would heat up a known quantity of water. Since we know how much energy it takes to change water’s temperature, then we’re also able to determine how much energy was used up during the combustion. That energy would then correspond to the energy contained in each of the four macronutrients.

This method is also used to determine the heat of combustion of other substances, like coal, wood, and hydrogen, as the tables in Wikipedia show. In a later post we’ll talk about calorimetry and how this process is done in more detail, but for the current topic it seems to not be essential.

In closing

Now we have a better idea of how the energy yields of the four macronutrients were determined and what their values are. In future posts we’ll talk about calorimetry, for completeness sake, as well as of leptin and how to determine someone’s BMR.

Hope you enjoyed this week’s post, if you would like to see more like it, check our Rebuttals to Fatlogic section. See you next week!