system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Biologically we have evolved to have carbohydrates as our main source of energy. Why are they such a taboo in the health and dieting scene?
gpt
We actually work on sugar and we can break down carbohydrates to obtain that sugar.  We store the excess in fat that we can use later.  

For the vast majority of the history of life, obtaining food was a major activity that we had to do in order to get enough energy to live and it took energy to do that.  For hundreds of millions of years, life walked that fine edge between getting enough energy to live while using less energy to get that food.  One of the big survival factors was being able to take advantage of abundances of food by storing it so we could get through times where we couldn't get any (i.e. fat).

With civilization and cooperation, we've made is far less energy intensive to obtain food but it's still too new of a concept for evolution to have possibly caught up.  So we have to consciously fight the instinct to consume the abundance of food that lots of people have access to because the mechanisms that used to help us survive (storing excess food as fat) eventually reaches a point where our bodies never evolved to handle the constant higher levels of fat for extended periods.

Focusing on avoiding obvious carbohydrates is a fairly straightforward way for us to self-regulate.