Bile acids (stool)

Bile acids in stool give indications of how bile is produced and recovered in the small intestine and how actively the gut bacteria convert it. Primary bile acids are converted by bacteria into secondary ones.

What bile acids do in the body

The liver makes the primary bile acids cholic acid and chenodeoxycholic acid from cholesterol, links them to glycine or taurine and releases them with the bile; the gallbladder stores them until the next meal. In the small intestine, bile acids coat dietary fats and form tiny droplets (micelles), so that lipase can act and fats as well as vitamins A, D, E and K are absorbed. Most bile acids are reabsorbed in the last section of the small intestine and return to the liver via the blood (enterohepatic circulation). Whatever reaches the large intestine is converted by bacteria into secondary bile acids such as deoxycholic acid and lithocholic acid. Bile acids are also messengers: via the receptors FXR and TGR5 they control their own production, sugar and fat metabolism and the release of the gut hormone GLP-1.

Low bile acids – what happens in the body?

Low bile acids in stool can mean that they are recovered well in the small intestine – or that less bile reaches the gut, for example when bile flow from the liver or bile ducts is impaired. In the second case, dietary fats are packed into micelles less well: fat and the fat-soluble vitamins A, D, E and K are absorbed less, and the stool can become pale and greasy. A low share of secondary bile acids also shows that the gut bacteria are converting less – this has been observed, for example, after antibiotic treatment.

Raised bile acids – what happens in the body?

When many bile acids reach the large intestine, they draw water and salts into the gut there and stimulate gut movement – watery diarrhoea may result, known as bile acid diarrhoea. The liver makes up for the losses by producing more bile acids from cholesterol. Raised values arise when the last section of the small intestine recovers bile acids less well, for example with Crohn's disease or after surgery on this section, after removal of the gallbladder, with bacterial overgrowth of the small intestine, with accelerated gut transit or with an inherited tendency to excessive bile acid production. The ratio of primary to secondary bile acids additionally shows how actively the gut bacteria convert them.

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