system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why modern humans have to be so careful about diet (balancing carbs, proteins, fats, and ensuring that we have certain vitamins), when early humans wouldn't have had to put any thought into it?
gpt
Because now we can eat what we want. In ancient times, if you just tried to eat as much of the tastiest food you could get, you'd be fine. You would hardly ever over-indulge in a serious way, because food was scarce, and fat, sugar, salt etc were hard to get. For example, the easiest way to get sugar would be fruit, which is only available for part of the year, and isn't very bad for you. Sometimes you'd get honey, but that's very rare and hard to get. So you'd never suffer from over-consumption of sugar. Same with basically every nutrient that we currently over-consume. The reason we never evolved a feeling of "ok that's enough fat/sugar/salt" is because they were rare enough that we never needed to cut back.

In the ancestral environment, we didn't need to put much thought into what we ate because what we thought didn't have much effect - we ate what we could get or we starved.