According to Donald Layman PhD, professor of human nutrition at the University of Illinois, carbohydrates are the only nutrients that exist solely to fuel the brain and the body. I like metaphors and I like musicals—which have principal actors and supporting players. Think of macronutrients (carbohydrates being one of the three main types) as principal actors. They are vital nutrients that provide calories, which the brain and body use in relatively large amounts. On the other hand, think of micronutrients, including vitamins, minerals, and enzymes, as the supporting players. These substances are also important to both brain and body but are needed in smaller amounts. For example, the enzyme amylase helps break down carbohydrates into glucose—the main circulating sugar or glucose in human blood and the principal source of energy for cells that make up the human brain and body. For most living things, actually.