What are the functions of bile salt in human body?

Bile is a complex aqueous secretion that originates from hepatocytes and is modified distally by absorptive and secretory transport systems in the bile duct epithelium.

The liver produces about 500 to 600 mL of bile each day. Bile salts are synthesized in the hepatocytes from cholesterol. Bile salts are excreted into the bile and transported, to the intestine, via the intra- and extrahepatic bile ducts.

Bile consists of ∼95% water in which are dissolved a number of endogenous solid constituents including bile salts, bilirubin phospholipid, cholesterol, amino acids, steroids, enzymes, porphyrins, vitamins, and heavy metals, as well as exogenous drugs, xenobiotics and environmental toxins.

Bile salts are the major organic solutes in bile and normally function to emulsify dietary fats and facilitate their intestinal absorption. Bile salts function as essential surfactants used to solubilize dietary fats in the hydrophilic milieu of gut.

These functions, and the complex metabolism undergone by bile salts, all take place in the context of the enterohepatic circulation and depend on its proper functioning. The enterohepatic circulation may be thought of as an endless flow of detergent through the liver, biliary tract, small and to some extent large, intestine, and back to the liver again.

Bile salts represent potent signaling molecules in liver and intestine: in the small intestine, they strengthen the defense against microbes by farnesoid X receptor (FXR)-dependent mechanisms and modulate hepatobiliary bile formation by FXR-controlled ileal release of the peptide hormone fibroblast growth factor 19.

Bile salts at low concentrations stimulate pancreatic triacylglycerol lipase (PTL) activity, but higher concentrations inhibit PTL activity. Pancreatic triacylglycerol lipase activity is regulated by colipase that interacts with bile salts and PTL and can release bile salt mediated PTL inhibition. Without colipase, PTL is unable to hydrolyze fatty acids from dietary triacylglycerols, resulting in fat malabsorption with severe consequences on bioavailability of dietary lipids and fat-soluble vitamins.

Bile salts are efficiently recycled via the portal system back to the liver in the so-called enterohepatic circulation.

Bile salts are 24 carbon water soluble products of cholesterol metabolism. Two primary bile salts are synthesized in mammalian liver: cholic acid, a trihydroxylated bile salt, and chenodeoxycholic acid (CDCA), a dihydroxy bile salt. Each can be conjugated at the side chain with either taurine or glycine.
What are the functions of bile salt in human body?

Intestinal bacteria produce vitamin K2

There are two sources of this essential vitamin, including vitamin K1, or phylloquinone which is primarily found in green leafy vegetables. Phylloquinone is absorbed in the jejunum and ileum and is primary stored in the liver. The second one is vitamin K2 or menaquinone which is synthesized by certain intestinal bacteria. Menaquinones are absorbed from the distal bowel and stored in the liver.

Specifically, Bacteriodes as well as a few other genera, produce this vitamin. In certain individuals bacteria supply up to 50% of the body’s total vitamin K requirements.

A decrease in dietary of vitamin K2 and/or a reduction in vitamin K2 production by gut bacteria can lower vitamin K levels. Antibiotics can destroy not only harmful digestive tract bacteria, but also the beneficial intestinal bacteria that is needed to create vitamin K.

The primary role of vitamin K in the body is in blood clotting. Vitamin K is also important to bone health. It assists in the mineralization of bone with calcium, thus keeping bones dense and strong.
Intestinal bacteria produce vitamin K2

Nutritional Processes: Gastrointestinal Tract

Nutritional Processes: Gastrointestinal Tract
The gastrointestinal tract (GI) is bordered by a layer of epithelial cells (with glands) sitting on a lamina propria (or basement membrane), comprising the mucosa and adjacent to the submucosa, which is penetrated by blood capillaries, lymphatics and nerves. Beneath the mucosa and submucosa are two layers of smooth muscle, lying in longitudinal and transverse directions, to allow contractions and peristalsis, Within the stomach , but particularly in the small intestine, the surface area of the mucosa is greatly increased. The mucosal and submucosal layer is folded into microscopic villi on the surface of larger folds or ridges. At the bases of the villi are the “crypts” where new epithelial cells are formed that migrate upward to the villi. These cells are sloughed off at a fairly rapid rate; the lifespan of villus cells in the small intestine is as little as 2 - 3 days (in man), that of colonic cells 3 – 8 days). Cells in the crypts include those with glandular and mucous-secreting functions, where as those in the villi are largely absorptive. Glandular cells are important in signaling the initiation and coordination of digestive processes, involving a large number of hormones neurotransmitters and paracrine factors. Mucous provided by “goblet” cells promotes lubrication within the lumen of the GI tract. In the small intestine, crypt cells are also the source of some digestive juices.

The epithelia cells of the mucosa have an apical (lumen –oriented) surface that is often additionally invaginated to form microvilli (or a brush border). In the small intestine the brush border contains transporter and some digestive enzymes. It is also more rigid than other parts of the cells membrane, a fact now attributed to high concentrations of sphingolipid in the outer half of the lipid bilayer. Surface cells are held together by tight junctions near the apical (top) parts of the cells. At the opposites (serosal) end, the cells membrane has a different (less rigid) structure (high in phosphatidyl-choline) and also serves different functions. Nutrients entering the blood or lymph for distribution to body tissues must first cross the brush border and ultimately the serosal surface of these cells to enter the intestinal fluid. Transport across either or both of these surfaces may be independently and/or differentially controlled, depending upon the nutrient. For there, capillaries and lymphatics take nutrients to the rest of the body. Nutrients not making it across the serosal membranes will remain with the mucosal cells until they are sloughed off, from whence they may be released by digestion and resorbed or lost with cell debris and bacteria in feces.
Nutritional Processes: Gastrointestinal Tract