First off, why is this information interesting? I think it is a prime example of how we tend to think too rigidly and simplistically about things like macronutrients. It also highlights a lack of good science in something so simple.
(Let me state for simplicity sake, I'm going to use the term calorie when we are actually talking about kilocalories obviously.)
I started this poll based on a simple statement from a biochem class I had over 15 years ago. The instructor made the simple statement,
"Protein has 5 calories per gram, however approximately 20% of those calories are lost due to energy required to digest the protein."
Studying this further has lead to a deeper understanding of this over simplified statement, the answer is much more complex.
First, the definition of a food calorie (kilocalorie):
The food calorie approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C
Simple enough, however here is where things get strange:
Conventional food energy is based on heats of combustion in a bomb calorimeter and corrections that take into consideration the efficiency of digestion and absorption and the production of urea and other substances in the urine. These were worked out in the late 19th century by the American chemist Wilbur Atwater.
About the Atwater system:
The Atwater system or derivatives of this system are used for the calculation of the available energy of foods. The system was developed largely from the experimental studies of Atwater and his colleagues in the later part of the 19th century and the early years of the 20th at Wesleyan University in Middletown, Connecticut. Its use has frequently been the cause of dispute, but no real alternatives have been proposed. As with the calculation of protein from total nitrogen, the Atwater system is a convention and its limitations can be seen in its derivation.
Basically what Atwater did is combust carbs, proteins, and fats and measure the energy produced. Then he tried to determine how much energy was lost in digestion. Sound rather arbitrary and likely to vary greatly? It is. Here is his formula:
Metabolisable Energy= (Gross Energy in Foods) -(Energy lost in Faeces and Urine)
He then measured the "apparent digestibility" of carbs, proteins, and fats and derived a system for calculating faecal energy losses. Urinary losses were calculated from the nitrogen found in urine.
If there isn't enough uncertainty here, it gets much crazier. For the "gross energy" of the carbs, proteins and fats:
Atwater collected values from the literature and also measured the heat of combustion of proteins, fats and carbohydrates. These vary slightly depending on sources and Atwater derived weighted values for the gross heat of combustion of the protein, fat and carbohydrate in the typical mixed diet of his time. It has been argued that these weighted values are invalid for individual foods and for diets whose composition in terms of foodstuffs is different from those eaten in the USA in the early 20th century.
And then it gets even better, for the "apparent digestibility" of macronutrients:
Atwater measured a large number of digestibility coefficients for simple mixtures, and in substitution experiments derived values for individual foods. These he combined in a weighted fashion to derive values for mixed diets. When these were tested experimentally with mixed diets they did not give a good prediction, and Atwater adjusted the coefficients for mixed diets.
And FINALLY, even more room for error, for the urinary energy lost:
The energy/nitrogen ratio in urine shows considerable variation and the energy/organic matter is less variable, but the energy/nitrogen value provided Atwater with a workable approach although this has caused some confusion and only applies for subjects in nitrogen balance.
By this time, if you are still following (and not laughing too hard), you have to be thinking that someone in more modern times
must have reworked this stuff and figured out something more precise. Turns out, not really, there are some tidbits however:
Merrill and Watt derived a system whereby specific calorie conversion factors for different foods were proposed. This takes cognizance of the fact that first the gross energy values of the protein, fats and carbohydrates from different food sources are different, and second, that the apparent digestibility of the components of different foods is different.
This system relies on having measured heats of combustion of a wide range of isolated proteins, fats and carbohydrates. It also depends on data from digestibility studies, where individual foods have been substituted for basal diets in order to measure the apparent digestibility coefficients for those foods. This approach is based on the assumption that there are no interactions between foods in a mixture in the intestine, and from a practical view point, such studies with humans are difficult to control with the required accuracy.
I find this obvious lack of accuracy, and lack of practical application to true dietary habits completely hilarious, but also par for the course in our field. So lets get back to our POLL!
THE ANSWER IS: 5.65*
*but only for certain protein sources, different protein sources vary lol.
I will continue this thread with more interesting info on where this energy is lost, why it might not be lost in every individual, and why the one person who posted
3.2 is correct in certain situations!