Your body needs a constant supply of energy in order to complete the many daily metabolic and biochemical processes it undergoes. Your body has preferred energy sources and uses these substances in order if sufficient amounts are available. First, your body uses carbohydrates. This is the most preferred energy source. Carbs. are converted to glucose. Glucose is vital to your existance. The digestion of carbs. begins in the mouth by the action of an enzyme called salivary amylase. Digestion of carbs. is pretty much completed by the time they reach the duodenum(first part of the small intestine). Starches are carbohydrates so they undergo this same process. If and only if your body lacks sufficient carbs. for use as energy, then the body will start to digest fats, fats are digested in the small intestine. Thirdly, if you lack enough carbs., and fats to provide energy, then and only then the body will begin to digest protein to use as fuel. Proteins are digested in the stomach and the small intestine, this is a catabolic reaction because proteins are broken down into their basic units called amino acids.
Bile is manufactured in the liver and stored in the gallbladder. Bile is necessary to help digest fats. The process of fat digestion is called emulsification.