Physiology of Digestion and Absorption

Digestion is the chemical breakdown of ingested food by enzymes secreted by glandular cells in the mouth, chief cells in the stomach, acinar cells in the pancreas, and intestinal mucosal cells. (1) The main components of the human diet are carbohydrates (roughly 50% daily intake), lipids (roughly 20-40% daily intake), and proteins (roughly 12-15% daily intake).


Carbohydrates

Roughly 50% of carbohydrate intake is in the form of starch, 30% is in the form of sucrose, and the remaining is a mixture of lactose, maltose, glucose, fructose, and nondigestible carbohydrates such as cellulose. The average daily intake is 300 grams. Carbohydrates are digested by the enzyme amylase which is secreted by both salivary glands and the pancreas. The absorption of the sugars is mainly in the proximal small intestine. (2)


Proteins

The average daily intake of protein is 70-100 grams with endogenous secretion adding 50-60 grams per day. (1) Proteins are essential for the amino acids that they provide as the building blocks for human life. The liver can synthesize all but 8 essential amino acids, which are: tryptophan, lysine, methionine, phenylalanine, threonine, valine, leucine, isoleucine (two essential for children- histidine and arginine). The digestion and absorption of proteins is more complex than carbohydrates. 


The process begins in the acidic environment of the stomach where parietal cells secrete hydrochloric acid, lowering gastric pH, and denaturing the proteins. The presence of food also causes the stomach’s chief cells to secrete pepsinogen, which converts to pepsin, which hydrolyzes the protein’s peptide bonds. As the protein’s move into the duodenum, pancreatic proteases and enterocyte enzymes carry out luminal digestion of the proteins. The peptides are then absorbed in the jejunum and ileum.


Lipids

Dietary lipids consist of phospholipids, sterols, hydrocarbons, waxes, triacylglycerols, and fat- soluble vitamins A, D, E, and K. Lipids are insoluble in water making digestion and absorption very different from proteins and carbohydrates. In order to be absorbed by enterocytes, lipids must be partitioned into water soluble micelles which are formed by bile salts, phospholipids, and cholesterol in bile and products from enzymatic digestion of lipids. Lingual and gastric lipases initiate the breakdown of lipids. (2) The majority of lipid digestion is completed by pancreatic lipases mixed with the emulsified fat droplets in the distal duodenum, which forms the micelles that can be absorbed through the mucosal brush border.


Fluid and Electrolyte Absorption


Everyday roughly 8-9 L of fluid reaches the small intestine from a combination of oral intake and endogenous secretions (gastric juices, pancreatic juices, intestinal secretions, bile, saliva). Nearly 98% of this fluid will be absorbed by the gastrointestinal tract, about 80% by the small intestine, and 18% by the colon. (2)


Absorption of water is mainly a passive movement across the epithelial membrane in response to osmotic and hydrostatic pressures. When food enters the duodenum, there is rapid osmotic equilibration with the osmotic pressure of plasma- water is absorbed from hypotonic solutions and is secreted into the lumen in response to hypertonic solutions. (1)  Patients with a short bowel ending in a duodenostomy or jejunostomy, with no more than 100-150 cm in adults, have inadequate distal absorptive bowel to compensate for the proximal secretory small bowel.


It is also important to consider, when working with short bowel and intestinal failure patients, that certain nutrients and electrolytes are only capable of being absorbed in certain locations of the gastrointestinal tract. Folate is primarily only absorbed in the duodenum, with limited absorption in the jejunum. The distal ileum is the only section of the small intestine with the capability of absorption of vitamin B12. Also, the absorption of bile acids is specific to the distal ileum and functional or structural loss of this part of the small bowel may lead to malabsorption of bile acids.


Vitamins

Vitamins are organic compounds we cannot synthesize and are vital to metabolic function. We can get them from our diet as well as from the bacteria in our intestine. Bacterial production and uptake by the colonic mucosa has been shown for vitamins: K, B1 (thiamine), B2 (riboflavin), H (biotin), folate, and pantothenic acid. Also, Vitamin B3 can be synthesized from the amino acid tryptophan. All other vitamins we require must be digested and absorbed.



References:

1. Langnas AN, Goulet O, Quigley EMM, Tappenden KA. Intestinal Failure Diagnosis, Management, and Transplantation. Malden, MA: Blackwell; 2008.

2. Tappenden KA. Pathophysiology of Short Bowel Syndrome. Journal of Parenteral and Enteral Nutrition. 2014;38(1_suppl). doi:10.1177/0148607113520005.