Glucose is the body’s main energy substrate which typically provides over 50% of the total needed energy of a typical meal. It is the only usable energy source for some parts of the body like the red cells and brain. It is completely broken down by many tissues of the body into carbon dioxide, one of the waste products of the body’s metabolic activities. The body gets its glucose supply from dietary carbohydrates and internal glucose stores.
The amount (concentration) of glucose in the blood is dependent on how much the body is fed and how much glucose it uses. This concentration is subject to intense control. In the medical setting the blood glucose concentration of an individual informs the healthcare provider how well and the rate at which the body uses its blood glucose. There are normal concentration limits for when the body is in a fasting state and when the body has been fed. Below or above these normal limits, especially when constant, tells that the body abnormally utilizes glucose and needs to be properly investigated and managed. The amount of blood glucose falls to the normal pre-meal levels in four hours after a meal and when fasting continues, the body resorts to its glucose stores to provide itself with energy. This is an adaptive change for the body to maintain a balanced state. After about 72 hours of fasting, proteins (amino acids and glycerol) and fats (ketones) become the main glucose sources.
When fasting is prolonged and all glucose sources are depleted, starvation starts and eventually comes coma and/or death.
Glucose is needed for our survival (of course with other nutrients). Too much of it presents dire consequences and so does too little of it.