Importance of Bile

In the process of elimination, waste compounds are conjugated by liver hepatocytes to make them more water soluble for excretion, these conjugated compounds are then added to bile. During the normal process of digestion, bile from the gall bladder and liver passes through the common bile duct and into the duodenum of the small intestine via the Sphincter of Oddi.

Waste products present in the bile pass through the small intestine and colon where they leave the body in the faeces. The main waste product is conjugated bilirubin, a waste product produced by the liver during the breakdown of old red blood cells. Bilirubin gives bile its yellowish-brown hue and contributes to the brown colour of stool. Changes in stool colour, such as white stool, suggests bile related issues such as a blocked bile duct.

Fats and fat soluble molecules are essential to life, but for absorption must pass from the aqueous environment of the digestive tract, across the phospholipid membranes of absorptive cells, and in to the aqueous tissues and fluids of the body.

Bile salts secreted in bile are crucial to this process and are responsible for the emulsification of dietary lipids and the formation of mixed micelles. Bile salts are amphipathic molecules with a hydrophobic side and a hydrophilic side.

In the digestive process, bile salts and phospholipids are important for the emulsification of large dietary lipid droplets. Their amphipathic nature allows them to coat the surface of these droplets and break them down into much smaller droplets. This vastly increases the surface area for pancreatic lipase digestion, thereby increasing lipid digestion. The products of digestion, such as monoacylglycerol, can be incorporated into mixed micelles with bile salts and phospholipids, allowing the dietary fats to make their way to the epithelium for absorption. These mixed micelles are crucial for the solubility and absorption of fat-soluble vitamins and drugs such as vitamin D. These molecules are incorporated into mixed micelles and which can access the epithelium for absorption.

The primary bile acids cholic acid and chenodeoxycholic acid are produced by the liver and conjugated with either taurine or glycine, greatly improving their water solubility.

Primary bile acids that reach the colon are dehydroxylated by the colonic microbiome into secondary bile acids deoxycholic acid and lithocholic acid. These are absorbed by the colon and sent back to the liver where they are conjugated and recirculated in bile.