And that’s all I can reasonably explain about the basics of fat digestion and absorption while still keeping it basic. With that, I have covered the absorption of all 3 macronutrients (the basics of it, at least)! YESSS! The next blogpost will be: MORE BASICS, because I like to know that stuff. After all, you are probably dying to know what happens after the macros have been absorbed. That’s when the real fun starts (and things get complicated … mostly complicated, so extra fun! (for me, maybe not for you (to bad ;-p)))!
This is part 1 of a 3-part series about how the three macronutrients (carbohydrates – proteins – fats) are absorbed by the body. Here I explain how digestive enzymes break down food and how the intestines take up nutrients into the body. The textbook ‘Medical Physiology’ 2nd edition by Boron & Boulpaep is the source of this information.
– Inside the stomach, protein is broken down by the enzyme pepsin into smaller pieces, called peptones.
– Pepsin is derived from pepsinogen, which is made by chief cells in the stomach lining.
– Stomach acid made by parietal cells in the stomach lining transforms pepsinogen into pepsin and creates an acid environment to optimize the function of pepsin.
– The stomach lining is protected from stomach acid and pepsin by a layer of alkaline mucous made by mucous cells. The alkaline environment neutralizes the acid and inactivates the pepsin.
Food entering the stomach provides an additional signal to promote the production of acid and digestive enzymes.
ECL cells are so-called endocrine cells, because they release (crine) histamine inside (endo) the body, whereas parietal and chief cells release their product on the outside (exo) of the body. Technically, any part of the body that is not behind a layer of epithelium is internal. The digestive tract is technically a single long tube, which is lined by epithelium of some kind and is therefore considered external to the body.
Histamine may be familiar to some in a different context. Histamine is an important messenger for the immune system en is the main cause of allergic symptoms. Generally speaking, histamine signals for an increase in inflammation. But in the stomach it appears to have a different function.
Brief summary: ECL cells produce histamine, which makes parietal cells produce more acid. ECL cells therefore indirectly increase the acidity of the stomach content.
Somatostatin is also made by cells in the small intestine (duodenum) as response to a lower pH when the stomach content gets enters the small intestine. Endocrine release of somatostatin can be transported by blood to provide signals elsewhere in the body, not just the immediate environment around the endocrine cell.
That is some of the information I gathered from my book on human physiology regarding stomach acid regulation and protein digestion. My book is at least 5+ years old by now and mentioned that several unknown cells and signals are also suspected to affect the stomach. And, of course, my descriptions are simplified for your understanding ;).
Regardless, I hope you can now appreciate a bit more of the complexity involved in a seemingly simple process. This is a common theme, in my experience, when studying anything in-depth.
|Figure 1: Protein digestion|
Digestion of carbohydrates and fats starts in the mouth by mixing in enzymes to break carbohydrate chains (amylases) or fats into fatty acids (lipases). These enzymes remain functional in the stomach, with the addition of enzymes to break proteins down into small amino acid chains (peptidases). The digestive enzymes are made by cells in the lining of the stomach (epithelium). The stomach lining contains many alcoves (crypts) and is covered by a thin layer of gel or slime (mucus).
|Figure 2: Pepsinogen and pepsin|
|Figure 3: Pepsin inactivation|