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To move around and do the things you like to do, you need energy. Macronutrients—fat, carbohydrates, and proteins—provide this energy, and the amount of energy they provide is measured in calories. When macronutrients are broken down in the body, they are used in three ways: they can produce energy, they can be pooped out, or they can be stored as fat or glycogen in muscle, fat, and the liver to be used later. One of the main points here is energy production. When macronutrients are broken down, they help to create ATP (adenosine triphosphate), the molecular currency of the body. You will learn more about this amazing molecule in ssss1, How Fuel Provides Energy. For now, think of ATP as one of the big guns of cellular processes—for example, it is used during muscle contractions and nutrient absorption in cells.

The GI tract (a.k.a. the gut) starts at the mouth and ends at the anus. This is one long tube, around 25 feet in length, that contains bacteria and its own nervous system known as the enteric nervous system. When you think about it, at the most basic level, you are a walking tube, with a brain to help make decisions on how to protect this tube and what to put in it. Weird, eh? Let’s use a delicious food as an example of how food is processed as it moves through the body. How about a red, juicy, sweet-smelling strawberry? The pathway of digestion starts here, with the scent of the food. Your mouth begins to water at the scent and the anticipation of the texture and taste. The saliva comes from this anticipation of digesting the food.