The brain and the rest of the body can run exceptionally well on ketones, which is the fuel produced by metabolizing fat. This is the fuel humans have thrived on for most of our history. True, there are a couple of functions of the body that require glucose. The liver is well capable of synthesizing the needed glucose from protein, a precess call gluconeogenesis. Consumption of carbohydrates and excess protein is not required at all for this process.