The idea of calories and balance between intake and output has sound logic. While learning about the topic I came across the Atwater system. I was expecting (naively) that measuring calories was something we deduct from how the body utilises energy. So a kcal, should’ve been something that creates this precise output in the body, one way or another. I was very surprised to see that this is something that is being measured outside.
I feel now stuck because I don’t understand how we’ve got from that system to human nutrition.
Is there something that I could read to understand a bit better how this all works? What are some interesting things that you discovered while learning about this?
So in essence a calorie is a measurement for energy, with joule being the more commonly used metric in physics and calorie being more commonly used in nutritional science.
Now, we do know from the first law of thermodynamics, that energy can not be created or destroyed but only change its for. For example a fast car goes fast due to a high output of energy through its tires. When it pushes the brakes there is a lot of friction between the tire and the street so two energies working against each other, leading to the creation of lots of thermal energy in the process of stopping a car.
Now similarly, foods contain energy. This energy can be found in fats, carbohydrates, protein and alcohol. In your body, a lot of chemical processes lead to these calories in these macros to being transferred to ATP, as well as heat. We can indeed measure these calories indirectly, especially through the input of Oxygen and output of Carbondioxide as well as urea (which gets produced during protein turnover), which even enables us to calculate the exact micronutrients content (as fats, carbs and alcohol need set amounts of oxygen and produce set amounts of carbon dioxide and protein producing set amounts of urea)
You could measure all of these three components and then calculate the total calories from that. Alternatively, you could actually burn the food (I am not kidding) and measure the released heat. As said, the energy in foods can not be destroyed. In this case, it would be completely released as thermic energy. Knowing that 1 Cal can raise the temperature of 1kg of water by 1°C, you can thereby calculate the total calories of food as well.
In essence it’s less about what happens to the human body, as its nothing but simple conversion of energy from one form to the other.
Hope this helps in understanding it a bit better. Generally most scientific books on nutrition will have similar explanations so you might want to look into those for better understanding