Food Digestion and Absorption
o Digestion begins from the buccal cavity with the help of salivary amylase then proceeds the digestion process in the stomach where with hydrochloric acid and other gastric juices continues the process of digestion and then small intestine further digests the food with the enzymes secreted by the pancreas and liver.
o The digested food is converted to the smaller particles for the villi present in the small intestine absorbs easily into the bloodstream.
o In large intestine, the food is mostly digested.
o The bacteria present in the intestine takes action of digestion over the remaining digestible food.
o Water with vitamins minerals is all absorbed here and the undigested food is removed from the body as feces by the process of egestion through the anus.
What is Absorption?
o The process through which the small intestine with the help of villi present in the lining of walls help absorb the food particles into the bloodstream.
o Blood and circulatory system help the digested to reach every cell of the body.
o Liver helps in detoxification of the amino acids, vitamins, salts, sugars, glycerol that is carried by the blood.
o Liver also stores the nutrients and then process after detoxification.
o This complete process is considered as the process of absorption and can occur through five mechanisms.
o Five mechanisms of absorption are active transport, passive diffusion, facilitated diffusion, co-transport and endocytosis.
Different mechanism of absorption are as follows;
1. Active Transport: substances move from the lower concentration to higher concentration through the cell membrane and requires pumps. Proteins in the cell membrane help as the pump and utilizes ATP as the energy for the transport.
2. Passive Diffusion: movement of substance from higher concentration to lower concentration.
3. Facilitated Diffusion: it is also the movement of substances from higher concentration to lower concentration but uses a carrier protein for the transportation.
4. Co-transport: molecules move from higher concentration to lower concentration that helps to charge the molecules of lower concentration to transport to the higher concentration.
5. Endocytosis: cell membranes consume the substances and requires energy for this kind of transportation. ATP acts as energy.
Absorptions of Nutrients
1. Carbohydrate Absorption
o Carbohydrates are digested to monosaccharides and absorbed as monosaccharides by the small intestine.
o Small intestine efficiently can absorb 120gm per hour, where indigestible fibers are eliminated as feces.
o The monosaccharides transport by co-transport via the protein carrier into the epithelial cells.
o It is then transported to the capillaries by facilitated diffusion.
2. Protein Absorption
o Most proteins are absorbed by the duodenum and jejunum part of the small intestine by active transport. But the carrier differs for the proteins, most of the protein are transported via sodium carriers whereas short amino acid chain transport actively.
o Absorption into epithelial cells, proteins are broken into amino acids then transported into blood capillary via diffusion process.
3. Lipid Absorption
o 95% lipids are absorbed with the help of bile in the small intestine. Bile salts also help in the absorption of the products of lipid digestion along with the lipid absorption.
o Water soluble short chain fatty acids are absorbed directly into the enterocytes by simple diffusion and transported into the blood capillaries in similar way as monosaccharides and amino acids.
o Micelle is formed for the monoglycerides with the help of bile salts and lecithin as it is not easily soluble in the chyme and water.
o Micelle has hydrophilic surface for the water environment and hydrophobic surface for fatty acids.
o Lipids require micelle formation for the absorption in the epithelial cells.
o Micelles enter the microvilli and are absorbed by the simple diffusion process.
o Monoglycerides are incorporated into triglycerides after absorption into the epithelial cells.
o Chylomicron (water soluble lipoprotein) is formed when triglycerides is mixed with phospholipids and cholesterol has a protein coat.
o Chylomicrons are released from the cells after being processed by the Golgi apparatus, as the chylomicrons cannot pass through the blood capillaries, it enters into the lymphatic vessels through lacteals.
o Chylomicrons enter the circulatory system through thoracic duct into the subclavian vein.
o Triglycerides are broken down by the lipoprotein lipase enzyme into glycerol and free fatty acids.
o These are then stored in adipose tissue as fats or used up as energy for the cells.
o Remaining chylomicron are combined with protein to form lipoprotein that helps in the transportation of cholesterol in blood.
4. Nucleic Acid Absorption
o Nucleic acid digestion results in formation of pentose sugars, nitrogenous bases, phosphate ions are all transported by the active transport through the villi using carriers.
5. Water Absorption
o Small intestine receives around 8 to 9 liters of fluid out of which 90% of the water is absorbed by the small intestine.
o Water absorption is by gradient concentration hence, water moves from the higher concentration to lower as the water content is more in chyme than is cells.
6. Vitamin and Mineral Absorption
o Fat soluble and water-soluble vitamins are absorbed transported via simple diffusion but vitamin B 12 being an exception is a very large molecule and requires an intrinsic factor.
o Intrinsic factor binds to the vitamin in the terminal ileum and is transported by endocytosis.
o Ions that dissociate from the water are absorbed in the small intestine by active transport whereas the sodium ions are accumulated in the cells during the absorption through the co-transport mechanism.
o But the potassium ions are reduced in the cells. For the potassium and sodium ions to balance, sodium-potassium pumps are needed using ATP that sends sodium out and potassium in the cells.
o All the minerals are absorbed in the small intestine irrespective of the need.
o Iron and calcium remain exception and are only absorbed according to the requirement of the body.
o Hemoglobin production require iron that is transported by active transport from the mucosal cells.
o Iron binds to the ferritin protein forming complexes. These complexes are stored and when the requirement is accomplished the stored iron are shed off.
o During the acute or chronic blood loss the absorption of iron increases and so is the transportation in bloodstream.
o In women four times of iron is transported than in men due to menstruation.
o Calcium requirement is determined by ionic calcium in the blood and that results in the absorption of dietary calcium.
o When calcium level is low then the parathyroid hormone from the parathyroid gland accelerates the release of ions from the bones and reabsorption of calcium begins by the kidney.
o Vitamin D also helps in calcium reabsorption.
You may like to read;
Food Absorption Citations
- Digestion and absorption of food: usefulness and limitations of in vitro models. Can J Physiol Pharmacol . 1994 Apr;72(4):407-14.
- Intestinal absorption of vitamin D: a systematic review. Nutr Rev . 2018 Jan 1;76(1):60-76.
- Physiology of Intestinal Absorption and Secretion. Best Pract Res Clin Gastroenterol . 2016 Apr;30(2):145-59.