Only a small part of the mouse’s lower intestine — the very end of the small bowel — has the special cells that detect bile acids and send signals to the liver to control how much is made.

When mice can’t send bile acids out of their gut cells, the bile acids pile up inside the cells, tricking the body into thinking there’s too much bile acid overall — so the liver stops making more, even though the total amount in the body actually goes down.

When mice lose a specific protein (Ostα/Ostβ) in their lower intestine, bile acids get stuck inside the gut cells instead of going back to the liver, which messes up the whole recycling system and reduces the total amount of bile acids in the body.