Macronutrient Digestion

The human body is fascinating, with a digestive tract that begins in the mouth. Each macronutrient (carbohydrate, fat, protein) is digested, absorbed, and metabolized differently. The nutrients require various actions and body parts to accomplish each task in the steps to giving the body energy and vitality. 

The body needs to break down carbohydrates into glucose to be able to use it, so this process begins with enzymes in the saliva that change starch to maltose. This starch breakdown halts in the mix of stomach juices, but is picked up again to continue in the small intestine. Help is provided from the pancreas and starch is further split into disaccharides, small polysacchardies, and then other enzymes aid in absorption. "The absorbed carbohydrates (glucose, galactose, and fructose) travel in the bloodstream to the liver, which can convert fructose and galactose to glucose. The circulatory system transports the glucose and other products to cells" (Sizer & Whitney, 2012, p. 123).

There is not much that takes place in the stomach when a person ingests fat, besides the separation from other watery mixtures. The body relies on bile, which is released from the gallbladder, to aid in the mixture of the fat and water in the small intestine. Eventually, the pancreas aids with a fat-digesting enzyme that can break down the fats for absorption to take place. Bile helps the fat be absorbed in the digestive tract lining too. When a person does not have enough carbohydrates to use as fuel in the body then fat may be used instead. Healthy fats that are metabolized aid the body in healthy cholesterol levels and reduce inflammation. Fat also provides cushion for organs, in addition to fuel for muscles to work.

Proteins provide the body with amino acid that are vital to many different functions in the body. "Digestion of protein involves denaturation by stomach acid, then enzymatic digestion in the stomach and small intestine to amino acids, dipeptides, and tripeptides" (Sizer & Whitney, 2012, p. 197). These amino acids are released into the bloodstream to be used as the body needs it, such as replacing cells or even new ones. Other roles that protein aids in include forming molecules for the immune system, regulating hormones, and fluid balance. 


Sizer, F., & Whitney, E. (2012). Nutrition: Concepts and controversies, MyPlate Update (12th ed.). Mason, OH: Cengage Learning.