Digestion, Absorption, & Metabolism of Macro-Nutrients (e.g., Carbohydrate, Protein, & Fat)

The digestion process begins at the mouth and oral cavity, including the tongue, teeth, and salivary glands. Next, the pharynx, specifically the epiglottis part of the larynx, esophagus, specifically the walls of the alimentary canal, guides the food to be digested. Then, the stomach, small intestine, and large intestine finish digestion and eliminate the unusable waste. Some other organs help and assist in the digestion process; the liver manufactures bile needed for the digestion of fats, the gallbladder releases a hormone to contract and squeeze bile, and the pancreas releases digestive enzymes and buffers to neutralize the acidity of the chyme in the small intestine (Colbert et al., 2019a).


The process of digesting begins at the mouth or when the act of ingestion occurs. The teeth, tongue, and saliva work together to physically break food down in a process called mastication. As the chewed food continues down the esophagus into the stomach, the process of digestion breaks food down into small molecules for the digestive tract to absorb. This process involves the secretion of acids, enzymes, and water to break the food down physically and chemically in the intestine and becomes ready for absorption into the blood. Lastly, the waste materials are prepped for excretion through defecation through the anus (Sizer & Whitney, 2016).


There are many different secretions of digestive chemicals needed in the process of digesting. Again, beginning in the mouth, the three salivary glands can produce 1–1.5 liters of saliva per day and not only aid in digesting but help keep your mouth clean. When eating, some antibodies, buffers, ions, and enzy

mes (amylase) are released. Amylase is a general name for digestive enzymes that help speed the chemical breakdown of foods. Plyalin is an example of a specific enzyme that can chemically break down starches into simple sugars. Most digestion in the stomach occurs in the pyloric region; this is also where chyme moves through to the small intestine. The stomach muscles move in three directions to help mix the food with gastric juices from the gastric glands. Gastric juice is another general term for combining different chemicals. In this case, it is the combination of hydrochloric acid and pepsinogen. When these two gastric juices combine, they form pepsin used to break down proteins. The primary organ that is involved in digestion is in the small intestine and uses several digestive enzymes and two hormones. The bulk of nutrient absorption near 80% occurs in the small intestines’ columnar epithelial cells (microvilli). Bile from the gallbladder is used to assist in the breakdown of fats. The pancreatic juice has the enzymes needed to break down nucleic acids and proteins. To break down sugars, the enzymes lactase, maltase, and sucrase are needed (Colbert et al., 2019a).


In this letter from our digestive tract, we would receive a plea of adaptations in how and what we eat. Our digestive tract would be asking for deeper self-awareness to listen to what our body might be need. We are asked to use our new knowledge of digestion when eating, drinking, sitting, or gaining weight. All of this is requested to help aid the digestion process and to prevent unnecessary harm to our digestive tract. Bad eating habits like eating too fast can cause gas, overeating can cause heartburn or damage to the esophagus, and gaining weight can constrict the stomach making it harder to digest. To alter these symptoms eat smaller portions, eat slower, stay active, and limit alcohol and soda. To elaborate on one of the many digestive disorders, let’s focus on lactose intolerance.

A simplified description of lactose intolerance is when the body has a deficiency of a small intestinal enzyme called lactase. Without a sufficient amount of lactase, the body cannot digest lactose which is sugar from dairy products. These undigested sugars cause bloating and diarrhea (Colbert et al., 2019a, p.)

After reading an article on what the author called a surface understanding of lactose intolerance, I have never understood lactose maldigestion better. Lactose intolerance is a common food sensitivity. In fact, infants produce lactase-phlorizin hydrolase to digest human milk, but this lactase production begins to decline months into life. Lactose cannot be absorbed in the small intestine until it is broken down by lactase, and if there is a decrease of the enzyme, then lactose can not be turned into monosaccharides, galactose, or glucose. Bacterial fermentation occurs when the undigested lactose reaches the colon, producing gastrointestinal symptoms (Jansson-Knodell et al., 2020, p. 1502).

All ethnicities are affected by lactose intolerance. In North America, Asian Americans are the most affected, second being Native Americans, then African Americans, Hispanics, and lastly Caucasians. The most practical test for lactose maldigestion is the Hydrogen breath test. This test requires the patient to overnight fast and then consume lactose syrup equivalent to 32 ounces of milk. The individual then exhales into a bag; the breath is then analyzed for hydrogen. The higher the level of hydrogen, the more likely the patient has lactose maldigestion (Jansson-Knodell et al., 2020, p. 1502).

Depending on the severity of the symptoms for the individual, a minimum dose of 12g up to a maximum amount of 18g of dairy should be consumed. Altogether avoiding dairy could cause nutrient deficiencies, and it is recommended that the exclusion of lactose is unnecessary (Jansson-Knodell et al., 2020, p. 1502).

Stay MOTOvated!! Live MOTOvated!!


Sizer, F., & Whitney, E. (2016). Nutrition: Concepts and Controversies — Standalone book (14th ed.). Cengage Learning. https://content.ashford.edu/books/Sizer.2199.17.1

Colbert, B., Ankney, J., & Lee, K. (2019a). Anatomy, Physiology, & Disease: An Interactive Journey for Health Professionals (3rd ed.) [E-book]. Pearson. https://etext-ise.pearson.com/products

Jansson-Knodell, C. L., Krajicek, E. J., Savaiano, D. A., & Shin, A. S. (2020). Lactose Intolerance. Mayo Clinic Proceedings95(7), 1499–1505. https://doi.org/10.1016/j.mayocp.2020.04.036

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