It is a myth that the brain can only use glucose for fuel. Glucose is the preferred fuel, however it can adapt quite easily to using ketones as fuel.
“The brain’s first choice for energy is glucose. However, when glucose is not available, the brain uses ketone bodies. Using ketones meets the energy requirements of the brain and maintains its proper function.” (Amiel, S.A., "Organ Fuel Selection: Brain," The Proceedings of the Nutrition Society, 54(1), 1995, pages 151-155.)
"Ketones are particularly important for the brain. When glucose is not available, the brain turns to the alternative energy source of ketones. Ketosis (elevated ketone levels in the blood) is normal during fasting, after prolonged exercise and when a high-fat diet is consumed." (Mitchell, G.A., Kassovska-Bratinova, S., Boukaftane, Y., et al., "Medical Aspects of Ketone Body Metabolism," Clinical and Investigative Medicine, 18(3), 1995, pages 193-216.)
"Most tissues in the body can readily use fatty acids for fuel just as easily as glucose. There are a few tissues such as the renal medulla, red blood cells and one or two other that can only use glucose. However, those cells essentially make their own glucose by recycling lactate (produced from glucose metabolism) back into glucose.
The brain is in its own weird category. Under most conditions, it relies exclusively on glucose. And while it can’t use fatty acids directly, it can use a fatty acid derived fuel in the form of ketone bodies. After roughly three weeks of adaptation to using ketones, the brain may only need 25 grams/day of glucose or so, which can be made by the body (in the liver and kidney) from sources such as lactate, pyruvate, amino acids and glycerol." (www.bodyrecomposition.com/nutrition/carboh