Our cells have evolved in a manner that has made carbohydrates the human body’s primary energy source; in fact, the entire metabolic system begins with glucose. While most of the cells in our bodies can adapt, at least temporarily, to using lipids and proteins as energy sources, our brain cells require glucose. This means that when blood glucose levels fall too low, the brain cells shut down and we faint. By fainting, the flow of blood and glucose to the brain increases.