A model of two primary forms of energy-containing molecules (fatty acids and glucose) and different pathways through which they can travel once in circulation in the bloodstream. These energy-containing molecules enter into circulation from the intestine following a meal. They then pass through the liver, the body’s primary metabolic machinery in charge of converting one form of energy to another based on metabolic demand. As this energy continues through circulation, it may be stored as fat in adipose tissue, stored as glycogen in muscle or liver, or eventually oxidized to produce ATP in the mitochondria.