On a standard high carbohydrate diet the brain is dependent on glucose. If carbohydrates are restricted (or you fast for an extended period) the body will initially break down protein from muscle for conversion into glucose. However, this response is only temporary because over time the body metabolism changes over to ketones.

Ketones are produced directly from fat and have the ability to cross the blood-brain barrier and feed the brain. Ketones may also have anti-inflammatory, anti-ageing and anti-cancer effects.

How quickly does this process of fat adaptation take? At least several weeks and this process is interrupted with the addition of dietary sugars so you cannot safely cheat!