I’ve been meaning to write this for a while and I’m not sure if it will be a series or not but wevs. If you have any questions feel free to message me I will be happy to answer, or at least try, I obviously don’t know everything but there is a lot of misinformation going round and I thought I would give a scientific overview.
Firstly, what are calories?
Calories are a way of measuring energy. We eat food and it provides us with energy to breath and dance and frolic etc, the amount of energy the food provides is the calories. Another meausure of energy is Joules but we will stick with calories since they are more commonly used when talking about food.
There are 4 types of calories:
Carbohydrates = 4 calores/gram
Protein = 4 calories/gram
Fats = 9 calories/gram
Alcohol = 7 calories/gram (Yup booze has calories!!)
When we eat more calories than we need the body stores it. We can write this simply as:
(Calories Into Body) - (Calories Calories Out of Body) = (Calories Stored In Body)
Calories get into the body through eating. Calories get out of the body mostly by being converted to heat and CO2 by respiration (which we colloquially call “burning” calories). A small percentage of calories don’t get absorbed and are pooed out, this is usually a very small fraction but can increase a lot if for example you have the shits.
This formula cannot be circumvented, energy cannot be created or destroyed, if energy goes into the body and doesn’t come out it must still be in the body.
How are the calories stored?
When more energy is going in than coming out it has to be stored. The body has a number of ways of doing this. The first storage system the body uses is glycogen, this is stored in muscle and the liver. This usualy accounts for up to 8% of the liver mass and maybe 1-2% of muscle mass. This is maybe 3-4 pounds in total in an adult. 1 pound of glycogen (which is about 80% and 20% actual glycogen) is about 500 calories so most adults are storing 2000 calories of glycogen at any time. Glycogen levels fluctute fairly quickly, it is a fairly fast acting enery storage mechanism. It increases after a meal and then starts to decrease until the next meal. Think of it a bit like RAM in a computer. Because 500 calories is 1 pound of glycogen this is why bodyweight flutuates so much on a daily basis.
For longer term storage the body uses fat. If, after a meal all of the glycogen is full then the exess energy is stored as fat. Around 3,500 stored calories weighs 1 pound (Fat is really energy dense but not vloume dense). Fat is used much more slowly, the body only starts to use fat once glycogen stores become low.
Another type of energy storage (although it isn’t storage per se) is muscle. Muscle is “built” in response to need, so if you lift lots of weights your body goes, “oh crap” I need more muscle to lift this shit! Now obviously the muscle can only be built if we have the right stuff to build it. That stuff is protein. 1 pound of muscle has between 600 and 800 calories in it. It also uses energy (by respiration) to build, although not that much.
Now while all calories eaten can be stored as glycogen and fat (the body can convert them) only protein can become muscle, the body can’t convert fat into protein. I will point out that converting protein into fat is not efficient and does have an energy cost, the body finds it much easier to just store fat!
So that is the basics on storing energy, next time I will talk about energy defecits (weight loss).