I read this today on the Australian and Nz pbc website and found it very informative with info I never understood so am sharing here
Human bile is made up of a combination of five different bileacids, including ursodeoxycholic acid (UDCA). UDCA is "hydrophilic"(mixes with water) while the other four bile acids are "hydrophobic"(don't mix with water).These four hydrophobic bile acids are highly corrosive and very thick. UDCA is the least corrosive, and because it mixes with water, is thin and flows well. The liver recycles bile from the digestive tract. Recycled bile contains vitamins and minerals that are carried back to the liver to enter the bloodstream. The liver produces "fresh" bile to replace what was excreted as waste. Bilirubin is one of the waste products that bile helps eliminate. High bilirubin indicates bile is not eliminating waste the way it should. Essentially, the bile acid production-to-reabsorption cycle is a"feedback loop" that tells the liver how much bile to produce.
When PBC goes untreated, bile acids build up in the liver due to inflammation and/or damage to the biliary tree. This inhibits the normal secretion of bile into the digestive system. Consequently, the liver does not reabsorb bile acids because the acids aren't getting out to the digestive tract. To compensate, the liver makes more bile that can't get out, which increases the damage to the liver. It's a vicious cycle...
UDCA normally makes up about 5% of the mix of bile acids in the liver. The remaining 95% is made up of the other four, more corrosive bile acids. Taking Ursodiol (synthetic UDCA) reintroduces one missing bile acid into our digestive systems. Some of the ursodiol is eliminated as waste, and some is reabsorbed. The reabsorbed ursodiol fools the liver's feedback loop into producing less of all five bile acids. The aim is to take enough ursodiol to change the mix of bile acids so that UDCA makes up about 50% of our bile. This does two things:
1) Makes the mix of bile acids in the liver less corrosive, and (2) Thins the bile, improving the way it flows.
This results in slowing down the damage to the biliary tree. The change in the mix of bile acids also affects our ability to absorb vitamins and minerals. Fat-soluble vitamins are less likely to be absorbed while water-soluble vitamins are more likely to be absorbed.....