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D-fending the Colon: Bile component triggers vitamin D receptor

The protein that enables cells to respond to vitamin D also helps the gastrointestinal tract protect itself from an especially dangerous acid in bile, a new study suggests.

This unexpected finding about the protein–the vitamin D receptor–may help explain animal experiments and human-population studies suggesting that diets rich in vitamin D reduce the incidence of colon cancer.

"The vitamin D receptor probably evolved as a bile acid sensor of some kind," says study coauthor David J. Mangelsdorf of the Howard Hughes Medical Institute at the University of Texas Southwestern Medical Center in Dallas.

Bile, which the liver produces, contains detergent-like acids that start the digestion of dietary fats in the intestines. Most of these acids are reabsorbed by the small or large intestines, but one, lithocholic acid, isn't readily taken up there.

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