I think your advice is naive and does not solve anything. Its like saying "Money in / money out" is as absolute a fact as it gets. so I say "You have two options: Earn more, spend less, pick the one you like".

That advice might be true, bit its also totally useless in solving poverty.

The hole argument for different diets is based on alternative theories of how different foods affect your body and/or mind.

To take absurd example, maybe carrots move energy from building fat cells into a process that converts your blood to carrot juice. You could eat the same amount (in calories) of carrots and something else, spend all the energy and still end up with a different body.

That's what the debate is about. Just telling people to run more or eat less will simply not work. Running (Sports) by it self, is essentially proven to not be effective for weight lost. You can actually train fat people to run marathons.