A slightly less dumb idea would be to tax food by calorie content. In ages of famine, that tends to be the main factor in its cost, after all, and when obesity is rampant, it could make people more aware of their diet. Artificial sweeteners and other low-energy alternatives would become way more viable. Of course, this is impractical as it would inadvertently cause massive misrepresentation of calorie count (even if the <5 cal/serving ≈ 0 cal loophole is closed).