system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is a ketone and how does it vary from glucose to create the positive effects health benefits claimed to be had of a low-carb or Ketogenic diet?

I've looked around online a little and can't find an answer to this question exactly. I keep finding the same generic answer that when fat metabolites it creates ketones and then depending if Im on a site about a low-carb diet or one about diabetes then it varies on how good or bad ketones are. I understand carbs breaking down to sugar or glucose and the body burning that as energy and when carbs and sugar aren't present the body burns fat and it creates ketones that can help with mental clarity, lower blood pressure, lower cholesterol and seemingly lots of other positive benefits but why does that happen and what is it about ketones that create this better effect on the body? Essentially, ELI5 what is a ketone?
gpt
We're talking about specific ketones called ketone bodies. Ketones are a class of small organic molecules that even include some types of sugar.  Ketone bodies are a sub-category of three of them, which are related to how the body processes fat.    What happens is when there isn't enough blood sugar, the body breaks down stored fat.  The result of fat breakdown can go through what's called the kreb's (or citric acid) cycle, which is a pathway that the body would normally use in burning sugars as well.  However because of the chemistry involved there might not be the right chemical balance to run that cycle.  For example inside the liver when its trying to make more blood sugar, a process called gluconeogensis.  Instead the products from breaking down fatty acids can be converted to ketone bodies.    Some people believe that because some of the same enzymes are used to form ketone bodies as to create cholesterol that ketogenesis would limit the ability to make cholesterol, however they occur in different places in the body and are regulated interdependently there's no direct reason for cross-over.    

All this goes on in the liver as it produces sugar from fat, this stuff is essentially a byproduct.  The brain needs sugars or ketone bodies that can get past the blood-brain barrier in the way large fatty acids can't.  Most of the body's tissues can just handle burning fatty acids directly and aren't responsible for producing extra sugar, they go through the citric acid cycle and the electron transport chain to produce energy like normal.  Almost any cell, except the liver itself and those without mitochondria like the red blood cells, can gain energy from ketone bodies, but for the most part they prefer just using fatty acids themselves, and ketogensis is limited by other factors like the production of acetone which is toxic and the fact that they acidify the blood.  The body can manage those things reasonably well unless there's another problem, like diabetes leading to diabetic ketoacidosis.  

As for all the other supposed benefits, well maybe it helps with mental clarity,  high blood sugar can cause sleepiness.   But its unlikely to be anything overly drastic.   However the main benefit is probably that it burns fat, which many people want.