Also you may find it interesting to know that carbohydrates are not essential nutrients in humans, but fats and proteins are. The body can obtain all its energy from protein and fats. NOw., before you come back and say the the brain needs carbs for energy, this is not true. It needs glucose. The body can make the glucose it needs for the brain and neurons from a few of the amino acids in protein and also from the glycerol backbone in triglycerides.
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Glucose is the form of sugar that travels in your bloodstream to fuel the mitochondrial furnaces responsible for your brain power.
Glucose is the only fuel normally used by brain cells. Because neurons cannot store glucose, they depend on the bloodstream to deliver a constant supply of this precious fuel.
This blood sugar is obtained from carbohydrates: the starches and sugars you eat in the form of grains and legumes, fruits and vegetables. (The only animal foods containing a significant amount of carbohydrates are dairy products.)Brain Energy Demand
Your brain cells need two times more energy than the other cells in your body.Neurons, the cells that communicate with each other, have a high demand for energy because they're always in a state of metabolic activity. Even during sleep, neurons are still at work repairing and rebuilding their worn out structural components.
They are manufacturing enzymes and neurotransmitters that must be transported out to the very ends of their– nerve branches, some that can be several inches, or feet, away.
Most demanding of a neuron's energy, however, are the bioelectric signals responsible for communication throughout the nervous system. T
his nerve transmission consumes one-half of all the brain's energy (nearly 10% of the whole body's energy). Dr. Norberto Cysne Coimbra M.Sc., Ph.D., Laboratory of Neuroanatomy and Neuropsychobiology, Faculty of Medicine of Ribeirăo Preto of the University of săo Paulo; Neuroscience Art Galleries
Complex vs. Simple Carbohydrates Complex carbohydrates are like time-release capsules of sugar. Simple carbohydrates are more like an injection of sugar.
Complex carbohydrates tend to be in natural foods – and have long chains of sugar molecules that the liver gradually breaks down into the shorter glucose molecules the brain uses for fuel. In natural foods, the cell walls are made of cellulose fiber that resists digestion, slowing the breakdown and the subsequent release of sugars into the bloodstream, kind of like the way a time-release capsule works.
Simple carbohydrates are found in most processed or refined foods and some natural foods. These carbohydrates have short-chained sugar molecules and, because they break apart quickly, enter the bloodstream quickly. Sugary foods--including corn syrup, fruit juices, and honey--contain glucose that is absorbed directly through the stomach wall and rapidly released into the bloodstream, almost as quickly as if delivered by syringe.
Most of us have discovered that thinking can be tiring, even exhausting. As the primary source of energy in the human brain, glucose can be rapidly used up during mental activity.
Some interesting research has shown that mental concentration actually drains glucose from a key part of the brain associated with memory and learning – underscoring just how crucial this blood sugar is for proper brain function.Glucose, Learning and Memory - Study
Psychology professor Paul E. Gold has researched the stability of glucose levels in the brain. Working with Ewan C. McNay , they found that as rats went through a maze, concentrations of glucose declined in the animals' hippocampus , a key brain area involved in learning and memory – even more dramatically so in older brains.
Except under conditions of starvation, it was thought that the brain always had an ample supply of glucose. "While this is the case in terms of consciousness, the new findings suggest that glucose is not always present in ample amounts to optimally support learning and memory functions," said Gold, who is director of the Medical Scholars Program in the University of Illinois College of Medicine. 
