The brain normally changes in some characteristic ways as it ages. For one thing, its blood supply decreases. For another, the number of neurons, the brain cells that specialize in transmitting information, diminishes as people age. When the number of nerve cells in on part of the cerebral cortex diminishes, hearing and speech are affected. Losses of neurons in other parts of the cortex can impair memory and cognitive function. When the number of neurons in the hindbrain diminishes, balance and posture are affected. Losses of neurons in other parts of the brain affect still other functions. Some of the cognitive loss and forgetfulness generally attributed to aging may be due in part to environmental, and therefore controllable, factors---including nutrient deficiencies.