That "delta_weight = calories_in - calories_burned" expression is absolutely technically correct--the best kind of correct. But... Some forget that calories_i

That "delta_weight = calories_in - calories_burned" expression is absolutely technically correct--the best kind of correct. But... Some forget that calories_in is the output of a function with multiple input variables, and calories_burned is also the output of a function with multiple input variables, plus whatever your wrist-worn fitness monitor says you burned directly through exercise. If you stand in the freezing cold for 15 minutes, and then go back inside, and you still have some brown fat left after infancy, it will add to calories_burned for a little while. If you are insulin resistant, and you cut carbs out of your diet for a while, that will tend to deduct from calories_in, because the foods you are now eating impact your competing appetite and satiety functions differently, such that satiety is winning more often now. If you exercise vigorously, your calories_burned rises by when you burned directly, but you will also increase metabolism for a few hours afterward, and the fitness monitor won't tell you that, but it will also boost your appetite function, and your food scale might catch that.