Our diets must contain essential fatty acids, such as omega 3. It must also contain essential amino acids. These acids cannot be synthesised (made) by the body and must be consumed in the diet, to maintain optimal health.
There are no essential sugars or starches (carbohydrates). None. Your body and brain do require some sugar (glucose) to function, but they do not need you to eat any carbs to function. They can produce as much sugar as they need, from scratch. This is known as gluconeogenesis. There is no absolute requirement to consume carbohydrates. Gluconeogenesis is a process by which glucose is created from a non-carbohydrate substrate, such as fat (glycerol) or protein (amino acids). The brain consists of fat and cholesterol. Any new glucose (or ketone) formation can be done in situ – within the brain itself.
Your brain can happily thrive on ketones (from fat) for energy.
The human adult brain uses, on average, 20% of the total energy available. Our brains are significantly larger than those of other mammals. Humans continue to generate ketones even when they have more than enough protein to supply the brain’s needs with glucose. Other animals sacrifice their lean tissue (muscle), if necessary, but we do not. It seems feasible that ketosis is a natural state for a large brain. Our ancestors frequently had little access to carbohydrates and would need to hunt and survive on game.
The brain needs some glucose to function, but it does not require us to eat glucose (carbohydrates) to function.
In clinical environments, ketogenic diets are widely used to control epilepsy, with a significant reduction in seizure frequency seen. There is some preliminary evidence that ketogenic diets can also help with other neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease. In these disease states, ketones appear to be a superior fuel to glucose.
You would eventually die without dietary fat or protein, but you would survive perfectly well without any dietary carbohydrates.