The brain runs on glucose, a form of sugar that the body converts to energy and uses to do all of its normal functions. For the brain, not enough glucose means not enough energy to perform all the processes of a brain: learning, remembering, communicating, concentrating, recalling, connecting, analyzing. Junk food that is high in fat results in a body full of fatty acids. These fatty acids, in effect, clog the brain's ability to retrieve and process glucose into energy. This slows the metabolism of glucose and causes the brain not to function as well as it does in normal conditions.

Don't assume that if the brain functions from sugar, eating low-fat but sugary junk food should be good for it. If you eat junk food that contains a lot of sugar or simple carbohydrates--such as soda, white bread, pastries or potato chips--you might feel a bit more energetic and alert for a little while just after you consume the food. But although the brain gets a sudden surge of energy from that sugary food, the sugar form is so simple that it is quickly consumed by the body and the brain is left without additional fuel. A better choice for your snack foods is anything made of complex carbohydrates, such as fruits or whole grains. Those complex carbohydrates are broken down and absorbed slowly by the body, which means there is an ongoing, steady supply of glucose to the brain.

Neurotransmitters are the chemicals in the brain that allow one part of your brain to communicate with another part of your brain. We know that biochemical imbalance--that is, a lack of sufficient neurotransmitters--can cause miscommunication between parts of the brain and that miscommunication often results in depression. Junk food can be one of the causes for a lack of neurotransmitters. The components of junk food--high sugar, simple carbohydrates and high fats--actually interfere with or decrease the levels of the neurotransmitters that affect our moods. You may feel happy while you're eating your favorite junk food, but if you do so often enough, you will actually be reducing your capacity for pleasure by reducing the level of mood-lifting neurotransmitters in your brain.