[dbrunsek]

Sorry but your perception of laws of thermodynamics is wrong. No one needs to eat the same amount of calories every day - your metabolism does not simply reset at midnight. It's a difference in energy consumption vs. expenditure that builds in a longer period of time that affects energy balance (positive or negative) and finally your weight change.

Eating less doesn't interfere with having diabetes or hypothyroidism. And 90% of people don't have these conditions.

[zrail]

joe_the_user above states it much better than I can. The body is not a static science experiment in a lab. For people who's bodies don't naturally regulate themselves properly, the body is an active agent that is fighting tooth and nail against what the person knows to be best. For someone who's body is physiologically normal I'm sure it's really difficult to conceive this situation