I know kcal = 9fat mass + 4 (carb mass + protein mass)
However, according to https://fdc.nal.usda.gov/fdc-app.html#/food-details/168462/nutrients ,
The raw salad has 23 kcal, 2.96g protein, 0.39g fat, 3.36g carbs, which according to the formula, it actually 6 extra kcal. Where does the whopping 25% come from!?
According to https://fdc.nal.usda.gov/fdc-app.html#/food-details/171955/nutrients ,
The cod has 82 kcal, 17.8g protein, 0.67g fat and no carbs and according to the formula it has 4.77 less kcal. Where did the kcals dissapear? What’s happening?
The ELI5 explanation is that how you make a food changes how much of its nutrient stock is actually available for digestion, and how effectively the body will digest it.
So there are specialists whose job is to figure out more precisely how many calories a food will give you. The formulas involved are a lot more complex & there are more of them than just the very simple one you gave.
However, we use the simple one because it tends to get us close enough for exercise and weight control purposes; we don’t really need to know whether someone uses 2000 or 2300 calories per day to recommend a very effective diet for them.
Additionally, any systematic biases will probably cancel out. Let’s imagine for a second that our simple formula will consistently underestimate the calories in oily foods. Your formula might say you use 3000 calories a day, and the packaging of your foods might estimate it at 3500!
But does that really matter, if your diet is at least approximately similar each week? No; if you start eating a 300 calorie deficit under either method (formula vs given kcal), you’re going to get pretty similar results. As long as the bias is systematic, it actually doesn’t particularly matter.
So the simple, ideal-lab-conditions-estimate formula is valid enough for effective use, hence its popularity. The ‘true’ value could be closer to the one on the packaging, but it’s more annoying to use, and who says all the calorie estimators are equally skilled, anyway? May as well just use the simple one.
Not all fat is exactly 9 kcal per gram, and not all carbs and proteins are exactly 4 kcal per gram. That is just an approximation. Those factors are referred to as the “Atwater general factor system”.
There is also the “Atwater specific factor system” which uses more specific values for different fats/carbs/proteins and therefore is more accurate.
You can read more about this here: https://www.fao.org/3/y5022e/y5022e04.htm
I don’t think counting every single calorie is healthy, or is going to make the difference. Don’t worry about the exact number of calories and macros you are eating each meal, you just need to have a good idea about it. Plus 6 or less 6 calories is nothing.
The measurements imply a higher precision than exists.
Food label numbers are ± 20% to accommodate product variation.
Be careful. High precision food tracking can interfere with enjoyment and healthy eating.
to add - Layne Norton said that what you read on the back of a product might differ like 10-40% (can’t remember exactly) from what it actually is.
Scary stuff. Eat basic food with as few ingredients as possible.