The brain requires a constant supply of energy from glucose- without this we would fry. In fact about 20% of glucose manufactured goes directly to our brains , this equates to about 500 kcals of glucose every day.
The body stores limited amounts of glucose in the muscles however this glucose supply is strictly for the muscle use- in order to allow us to speed away form danger (muscle nergy demands) the only other place we store glucose is in the liver and any excess glucose is turned into fat and we end up wearing it. Any excess calories consumed and not burnt as energy is stored whether carbohydrate, protein or fat itself. Should the brain become starved of energy or glucose it will convert stores from the liver -we only have between 1- 3 days (maximum) unless our body gets hold of glucose before we die.
How ironic that we depend upon glucose to live and yet we only have a limited amount stored in the body to be able to convert into energy for the brain? So how did the hunter gatherer survive? Going for long periods without food ?
Something called the Krebs cycle
The reason a starving person can live for 40-60 days without food is because the human body is cleverly designed to turn its body fat into ketones which are the four source of energy supply ( along with carbohydrates, fat and protein), so the more fat you have on your body the longer you are able to survive without food. Glucose (stored carbohydrate) would only sustain us for a few days and protein would mean muscle wasting and de-habilitation.
So cleverly quite naturally in the absence of glucose in the liver the body will take body fat stores and convert them into glucose to be able to feed the brain. This process is called Ketosis.
This is what happens in the absence of carbohydrates, which is why when people limit carbs on these low carb diets – they lose weight and burn their body fat.
In order to sustain life on earth for millions of years we had to be able to survive long periods of famine where we would go for days without food, we certainly didn t have access to snacks every 3 hours and yet we still had to have the energy to wander looking and hunting down our next meal.