The fallacy here is that a human body is a machine with well-defined intake and output.
How would one even know what is "exactly" the right amount?
Over a year we consume 1 million calories. If one were just 0.1% off of systematically eating more (or less) than required then according to your model one would end up being 200 pounds fatter or 200 pounds leaner ... do you think that anyone can regulate up to 0.1% accuracy to what they actually need?
eating a little more or less has absolutely not discernible effect the body adapts to it.
So a 5% calorie deficit or surplus would have a fairly small effect during a single year, but over a decade or a lifetime the effect is huge.
2 - 2.5 million calories a day is normal for people with a normal level of physical activity.
I was always taught that the body uses up its glycogen stores before burning fat. But if that were true even a small calorie deficit would gradually use up our glycogen stores and we'd be walking around without any after that.
Plus before losing any fat you'd have to experience a glycogen crash/wall which most small deficit dieters don't experience.
Generally what fuel your body burns depends on the intensity of work you are doing, so if you're walking you may be burning 80%+ fat, but if you're running a 10k you may be 80% glycogen.
However, if you eat too many carbs at a meal (beyond what you need in the next few hours), your body needs to get that excess out of your bloodstream, so it will do a combination of: - fill your muscle/liver glycogen stores (limited size) - burn any excess carbs in preferences to stored fat - convert any excess carbs to stored fat
Once it's got rid of the excess carbs from your bloodstream, it will return to burning whatever the normal ratio of fat:carbs is for you.
Consequently, if you always eat too much carbs, you'll gradually pile on the fat, unless you're doing large amounts of training which depletes your muscle/liver glycogen, e.g: elite athletes.
It may not be possible to know or calculate the exact intake and output values, but those values are not needed. Your body gives you cues.
> eating a little more or less has absolutely not discernible effect the body adapts to it.
Yes, the body adapts to it by storing or eliminating fat (among other things).
> Yes, the body adapts to it by storing or eliminating fat (among other things).
No, it compensates by changing your metabolic rate mostly. You eat less, you will feel more lethargic, you eat more and you will fill (and be) more active
Fast-forward 20-ish years, and I’m still just as tall but I don’t have any problem finding pants that fit me. Sigh.