I am quite sure that you know the elementary math of dieting:
weight gain = calories in – calories out
The same of course for weight loss with negative values on both sides. It is the kind of math I have on the hate list of this blog. Not because its content, which I can neither verify nor deny. But because of its limited usefulness.
It is like answering the question „Why is this room so crouded?“ with „Because more people went in than went out“. This is wise, but useless. A related math joke is about a professor who came early to his lesson, and standing outside observed 10 students entering the empty room and 11 leaving it. His conclusion: „If one more student enters, the room will be empty.“
Another joke of the same kind: Two persons in a balloon get lost. They see a man on the ground and shout to him „Where are we?“. After a long thought the man answers: „In a balloon!“. The balloon riders conclude that this must have been a mathematician, because his answer took a very long time, and then was completely useless.
Admittedly, there are such philosophers – even among mathematicians, I might add. However, I often observe the contrary, namely scientists of other areas applying math blindly. If it is math, it must be accurate. The theories then fall apart as soon as we see that none of the assumptions are ever fulfilled in real life.
We could add to the balloon story that the two persons in the balloon were most likely economists. Because (1) they were able to get lost in a balloon ride, and (2) they did not ask a precise question. But we should hesitate to let the joke end like this, since (1) can happen to all of us, and (2) is not true, if common sense is applied.
Back to dieting. The correct questions must be, of course: Why do people eat more than they need, and why do they move less than is good for them? I took that idea from the book „Why we get fat“ by Gary Taubes. I believe the dieting equation is a good example of how mathematics can be useless.
If you are reading this for a dieting advice, I cannot help you. I am neither a doctor, nor a specialist on the human metabolism. Read the book! My summary would be to avoid extensive sweets and those cheap filling side dishes, in favor of quality food like vegetables, fruits, meat, birds, or fish, and not to be afraid of fat. But again, I have no right to give advice here.