digestion prior to wls

The Science of Supplementation: Digestion Prior to WLS

November 8, 2017

Welcome to a multi-part series on vitamin supplementation and the science supporting it for post-operative patients. In this article, I will be discussing a general overview of digestion prior to WLS. Future articles will feature individual vitamins and minerals, and their importance in our bodies.

Digestion Prior to WLS: The First Part of Digestion

We all know how digestion starts: food must enter our mouths and undergo mastication or chewing. While in our mouths, salivary amylase and lingual lipase are enzymes that begin to further break down carbohydrates and lipids (fats), respectively.

When we swallow, chewed food, also called a bolus, travels down the esophagus into the stomach, where hydrochloric acid begins to break down proteins. Hydrochloric acid also works to break down the enzyme pepsinogen into pepsin, a strong enzyme that works to digest protein. Gastric lipase is an enzyme in the stomach that works to break down lipids.

The stomach also produces intrinsic factor, which is important for the absorption of vitamin B12, but more will come on that later when I discuss the B vitamins. While some nutrients can be absorbed through the stomach, such as copper and iodine, the majority of nutrients are absorbed in the small intestines, made up of the duodenum, jejunum, and ileum, and the large intestines, as seen in the image below.


Once in the small intestine, further chemical digestion occurs when the pancreas secretes digestive enzymes and bicarbonate. Bicarbonate is important as it is a base that helps neutralize the acidity of the stomach contents as food enters into the small intestines.

In the pancreatic juice, an important group of enzymes are proteases, which further the breakdown of proteins into smaller particles called peptides. Peptides must be further broken down by enzymes produced by the small intestines, peptidases, into free amino acids (the building blocks of protein) and small peptides. Free amino acids and peptides with four or less amino acids are able to be absorbed directly into the intestines.

The Pancreas

The pancreas also secretes pancreatic lipase, which digests larger lipids called triglycerides into smaller particles called monoglycerides and free fatty acids, which can be absorbed directly into the intestines. However, this digestion will not occur successfully without bile acids, which are found in bile and act as emulsifiers. This is similar to how an egg yolk acts as an emulsifier to keep the water and oil together in mayonnaise.

In digestion, the bile acids help keep lipase, a water-soluble enzyme, and the larger lipids bound together for digestion and absorption. Bile is produced by the liver and secreted by the gallbladder in response to food. Bile is important in the absorption of lipids and fat-soluble vitamins, as a majority of bile acids secreted into the small intestines are absorbed back into the body in the large intestines.

Bile is also important for the excretion of waste products such as cholesterol through elimination in the feces. If the gallbladder is not present, the liver continues to secrete bile but it is not released in higher amounts in response to food, which can lead to symptoms of fat malabsorption in some people.

Also included in the pancreatic juices is pancreatic amylase, an important enzyme that breaks down certain carbohydrates into smaller particles for absorption. As larger starch molecules are broken down, the smaller particles are higher in osmolarity, which is the concentration of particles in water. With higher osmolarity items, the intestines will pull additional water from the body to dilute hyperosmolar concentrations. As the particles are absorbed, the osmolarity stabilizes and the body can then reabsorb the water.

Dumping Syndrome

This is an important reaction to note in those who experience dumping syndrome. Larger particles that break down into smaller particles with high osmolarity can trigger an increase in intestinal osmolarity, causing water to “dump” into the intestines.

Immediate symptoms of this increase in water include nausea and vomiting, diarrhea, abdominal cramping, rapid heartbeat, and low blood pressure, which can exhibit as light-headedness, sweating and/or flushing.

Approximately one to three hours afterward, some people can experience reactive hypoglycemia (low blood sugar), which can present itself via fatigue, dizziness, rapid heartbeat, confusion, and sweating and/or flushing; some people may even pass out and will need medical attention.

The small intestines also produce additional enzymes that help further break down starches into individual sugar molecules called monosaccharides. These enzymes include maltase, sucrase, and lactase. Lactase is the enzyme responsible for digesting lactose primarily found in dairy products; when people are lactose intolerant, it is often because their bodies do not produce enough lactase to digest the dairy products they consume.

The Large Intestine

The final part of digestion occurs in the large intestine, where large amounts of reabsorption occur, particularly for water and electrolytes. Gut bacteria will break down remaining products such as fiber; this breakdown can lead to the production of gas. Certain foods have been found to produce more gas, as our bodies are not able to breakdown the majority of particles prior to reaching the gut bacteria in the large intestines.

Gut bacteria have also been found to synthesize vitamin K and some B vitamins; however, most of the B vitamins are then excreted in the feces rather than absorbed in the large intestines. Most importantly, the large intestines are where feces are formed, as undigested particles combine with mucus and bacteria.

This concludes part one of this series. In my next article, I will be addressing the changes in digestion after WLS and how vitamin and mineral supplement recommendations are created. Please stay tuned!

Digestion Prior to WLS References

digestion prior to wls


Bec McDorman, MS, RDN discovered her passion for health and wellness after undergoing Roux-en-Y Gastric Bypass surgery in 2010 to lose more than 100lbs. Bec has received her masters from Cal Poly Pomona and completed her dietetic internship at Johns Hopkins Bayview Medical Center. She has reached her goal of being a registered dietitian so she can help pre- and post-op bariatric patients with their journey.

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