Why do humans need carbahydrates?

Your brain requires an uninterupted flow of oxygen and blood glucose to function. The brain needs large amounts of the sugar, glucose, which is its primary energy source. The mature, healthy, mammalian brain uses only glucose to obtain energy. A second function is the breakdown of glucose contributing to the formation of the neurotransmitters.

The carbohydrates we use as foods have their origin in the photosynthesis of plants. They take the form of sugars, starches, and cellulose. The best source of glucose is carbohydrates (another source is protein). If you do not eat carbohydrates you will, usually, not have enough glucose. If you do not have glucose your brain begins to starve. When your brain is starving you do not think clearly and everything becomes secondary to a sugar source to your glucose starved brain. Lack of glucose energy to the brain can cause symptoms ranging from headache, mild confusion, and abnormal behavior, to loss of consciousness, seizure, and coma. Severe hypoglycemia [low blood sugar (low glucose)] can cause death.

Not all carbohydrates are the same, some are simple sugars like monosaccharides or disaccharides while some are complex carbohydrates like starches or polysaccharides. A simple sugar like sucrose or table sugar is made up of glucose and fructose. It is disaccharide composed of two monosaccharides. Starches are polymers, long repeating chains, of glucose. Plants convert excess glucose into starch for storage. Starch is hundreds to thousands of glucose units long. This is why we eat things like wheat, corn or potatoes, they offer a large slowly digestible source of glucose that is a necessity to us.

"Glucose is the major substrate that sustains normal brain function. When the brain glucose concentration approaches zero, glucose transport across the blood-brain barrier becomes rate limiting for metabolism during, for example, increased metabolic activity and hypoglycemia."

"Although insulin is not supposed to play a role in the brain, recent evidence in both lab animals and humans suggests that the hormone may be needed for normal brain functions, including learning and memory; if so, defects in the brain's ability to use insulin could lead to anything from mild memory loss to Alzheimer's disease. The idea is still controversial, however, partly because no one yet knows exactly how insulin might affect brain neurons. Some researchers suggest that the hormone chaperones glucose to brain neurons and thereby helps them maintain their energy production, in which case, memory loss might result when brain cells lack insulin and become glucose starved. Others hypothesize that insulin has other beneficial roles, such as spurring neuronal growth and inhibiting the formation of brain lesions called neurofibrillary tangles that characterize Alzheimer's. If insulin's role in cognition can be pinned down, though, the work might one day point the way to drugs that could reduce memory loss in both Alzheimer's and normal aging."