Technically, food calories ARE bullshit. Mostly because they’re presented as a useful fact about the food, but they’re almost always a rough estimation. Warning, science ahead!
So, there are actually two types of calorie: the small or gram calorie, and the large or food calorie. Most old science textbooks use the gram calorie, which is the amount of energy required to raise one gram of water by one degree celsius. The large calorie is the amount of energy required to raise one kilogram of water by one degree celsius. So if you ever see a “calorie count” that seems to be 1000 times too big or too small, there is a good chance that someone slipped up and used the wrong calorie.
Science doesn’t use calories anymore; they’re Imperial units. One of the more sensible of the Imperial units, except for that whole ‘there’s two of them with the same name’ bit. Science now uses the SI unit, the joule. 1 small calorie is 4.184 J.
Looping back to food: the idea of printing calories on the package is to tell you how much energy is in the food. You CAN scientifically calculate the potential chemical energy of an organic substance using a tool called a “bomb calorimeter”. To excessively simplify, this looks like a pressure cooker in a water bath with a lot of tubing attached. You put the item in the chamber, usually you add a heap of oxygen and maybe a precise amount of water, and then you take a while to burn it to death in an extremely thorough fashion, and measure how the temperature and pressure changes.
This is useless for food, because our body does NOT directly turn food into a cloud of heat and gases. We need to know how much chemical energy was in the food to start with, and then how much is in the waste that comes out the other end, so we can figure out how much got digested. This is, in theory, what is listed by the calories on the package.
The only way to even ATTEMPT a truly accurate measurement of food energy is to feed a known quantity of the food to a volunteer and measure what comes out the other end. This is very tricky, because you can’t just starve someone until their digestive tract is empty before you start (this is both unethical and can also send them into ketosis, which will ruin your data). Different people will digest different amounts of the food. The same person can digest different amounts of the food at different times based on levels of micronutrients and physical activity and hormones etc. We DO have some good data, painstakingly gathered, but it’s a little on the rough side.
A scientist called Atwater took all this data, and lots of careful study about the chemical composition of food, and came up with a way of estimating the amount of digestable energy in food. His system just used the amount of carbohydrates, proteins and fats in a food. Later ones consider things like whether it is a plant or animal protein (because we digest those very differently).
It’s still extremely flawed, and scientists KNOW it is flawed. For example, it can’t really distinguish between “roughage” carbohydrates, sugary carbs or starchy carbs, but those digest in completely different ways. It also doesn’t consider food interactions, which is a huge part of digestion.
The main point calorie labelling was always meant to be comparative. It was so you could look at product A and compare it to product B and go “okay, so this one is a lot more energy-dense” and choose accordingly. It’s not objective. You cannot count your calories and say “I have consumed this much energy today.”
You cannot count your calories and say “I have consumed this much energy today” and if you try that your answer is guaranteed to be completely and utterly wrong.
“Calories in vs calories out” does not work when you cannot possibly know how many calories went in!
(You also can’t measure how many calories went out, but that’s a different rant. Those charts of how many calories you burn doing [x] are also wrong, but admittedly they’re less-wrong.)
And that is why calories are bullshit.
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