When it comes to weight loss, almost everyone focuses on calories — a measure of energy — and some people love to quote the First Law of Thermodynamics as if it told the complete story. But calories are almost impossible to measure accurately in either the intake or expenditure direction. Why is it not more common to focus on measuring mass?
For example, every week I could take my daily average weight and also measure my total intake of food and drink by weight. If my weight goes up or stays the same, consume slightly less by weight over the following week; otherwise, keep intake the same. This way, I'm not counting on some laboratory's bomb calorimeter, or on some population-based formula for my resting metabolic rate, or on the "calories burned" reading from a treadmill. I'm not saying those numbers are meaningless, but I suspect for many people they feel abstract and hard to connect with their goals. For the approach I described, all I need is a good bathroom scale and a good kitchen scale.
In theory this plan should be infallible. It should work if your diet is carnivore, keto, or vegan. Doesn't matter if you are constrained by religion, by food sensitivities, or by anything else you can think of. Doesn't matter if your lifestyle is active or sedentary. And if anything you'd probably want to maximize the calories you consume, which seems more psychologically appealing to me than constantly minimizing them. Note that you'd apply all the same healthy-eating principles as you would if you chose a calorie-based regimen.
Don't get me wrong. Mass-based thinking is still "law of conservation" thinking and doesn't address the hard problems any more than calorie-based thinking. If laws of conservation told the complete story, no one would be unwillingly overweight, because they already have the answer. The hard problems, as with many things in life, are around why we behave the way we do despite knowing what we know.
I don't know how to solve those problems, especially considering they are different for everybody. For me, satiety has historically been a big issue, but there have been other factors as well. Maybe weight loss drugs will be the answer, at least for some of us. We'll see, I guess.