The problem is that your brain only runs on glucose. It cannot use fat as a metabolic fuel
(your muscles or liver, for example, can run on either).
(All dietary sugars are converted into glucose in the gut wall).
In situations where there isn't enough glucose (e.g. because you don't eat enough),
there is a metabolic process called gluconeogenesis which converts some amino acids
into glucose.
But it's not terribly effective; it's like driving your car in reverse. You can do it, but it's
awkward.
In situations of extreme glucose depletion, your brain can run on ketones.
For some reason these are called ketone bodies in the literature.
Ketones come from fat breakdown, and again it is very inefficient.
In order to get enough protein your body will sacrifice any fat within you to make
ketones.
This is a starvation situation.
Low carb diets are designed to create these two scenarios to some extent.
They provide lots of protein for these unusual metabolic processes, but no carbs, which
forces you into gluconeogenesis or even ketosis.
Full story:
https://www.quora.com/Does-the-human-body-need-carbohydrates?no_redirect=1
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