[Jimmie]

>But, no one has ever put on weight on a calorie deficit. NEVER.

I want to be a pedant and correct that to "body fat" instead of "weight". A change in diet can stress the body into retaining extra water weight, causing a temporary increase in weight.

[EternalFury]

There are definitely temporary adaptations and self-preserving mechanisms that slow down the process, but the laws of thermodynamics still hold.

Also, the calorie deficit does not even need to be large. 100-200 calories a day will already make a bit change in the long run.

Part of the problem is that people want results fast. Again, faster than even the laws of thermodynamics would permit.