The calories in, calories out, eat less, move more for weight loss has some flaws.

Among the flaws is that even though a pound of sawdust (fiber) would have calories, it would not have the same hormone effect (insulin and glucagon to name a couple) as a pound of sugar. Or a pound of ribeye steak. Or a pound of fat. So that is one problem with the idea. Not all carbon forms that would burn in a scientific instrument and provide a caloric number are metabolized in the body and utilized or metabolized at all. Another problem is not all sources of energy may be measured and accounted for easily (to satisfy the first law of thermodynamics). For example a person in ketosis is likely getting rid of energy in the way of ketones in their urine and acetone in the breath. So in some metabolism states energy is actually "wasted" as far as utilization goes and that energy is not easily measured outside a lab setting. The same can be said for brown fat vs white fat. Brown fat creates additional body heat (to keep our organs warm in cold weather) and in certain metabolism states white fat can act more like brown fat. There is a lot more to it that what I just stated, but I am getting carried away and digressing from the point meant to be made.

WHAT we eat can matter just as much as how much we eat.

So different metabolism utilizes energy differently, not just what goes to fat storage. Hormones matter. Different foods create different uses of hormones.

Here is a doctor I ran across this morning that does not go into all the stuff I mentioned above. He keeps it in a lot lighter presentation format. In only about six minutes in ordinary language. Dr Eric Westman.

https://www.youtube.com/watch?v=sJ5hkBBLBl4

More on how hormones control our fat accumulation/disposal but in a much more technical discussion centered on uric acid and fructose. Dr Perlmutter.

https://www.youtube.com/watch?v=QZ6jPCcFNa8