Dear Udi - and anyone else who’s interested. Let’s start with definitions.
A calorie, or kilocalorie, is a unit of some form of energy (the energy required to increase the kinetic energy of the water molecules in a volume of water). A joule, or kilojoule, is also a unit. Specifically, it’s a unit of work. A joule, roughly, is the amount of work done by a force to move an object through a given distance. And, so what?
Well. When you’re sitting around doing nothing, you’re not moving anything, so the work, in joules, is zero. Because zwift only calculates ‘pizza slices’ based on work done, you don’t have to subtract a ‘baseline’ amount of energy from your overall calculated energy output at the end of your ride, because your ‘baseline metabolic work’, is zero. This is basic physics.
Heartrate calculations, on the other hand, require a subtraction of a baseline energy amount. That’s because your heart is always pumping, in order to keep cells respiring. So, any heartrate increase above that baseline for a given period of time, is correlated with the amount of energy (‘calories’) expended for that time. But: you’ll need to subtract the amount of energy that would’ve been expended just for keeping them cells of yours alive!
As far as the energy listed on food labels: obviously it’s an estimate. But it’s the only estimate consumers have. So it’s a good place to start. And measuring calories in, calories out, if done systematically, is the only sure-fire method to have a reasonable measure of the ‘energy budget’ of one’s body. Exceptions would be if you had thyroid dysfunction, for instance.
Triglycerides require beta oxidation in order to be used for cells to make proteins etc. So that’s why fat is not a direct source of energy. Glucose is the only immediate source, and protein, fat, and alcohol, must all be converted into suitable-length carbon chains.
Eating fat won’t help you change your body composition, regardless of leptin and grhelin and satiety, because you’ll be forced to replace foods with 4kcal per gram, with foods that have 9kcal per gram. That is: you’ll forego lesser energy-dense foods for more energy-dense ones. So, even if you are sated more often, your less frequent meals will rely on more energy-dense items anyway, so it’ll require even more calculation to find out if there’s some advantage. (This last point is a priori true. No degree in nutrition needed. Until 9=4, substituting lipids for carbs won’t work. Which is why counting calories is the sensible strategy).
Is changing body composition in a favorable direction difficult? Yes. Can you measure things to gain a greater grasp of where to change things in order to aid favorable changes? Yes. Is it a hassle? Yes. But so is pulling off the highway to fill your petrol tank when the gauge shows an empty tank. Guesswork leads to disappointment. Calorie counting is guesswork, but it’s systematic guesswork, and can be a reliable, replicable and informative method to achieve things.