Named in honor of Dr. Castle, an American doctor who discovered the cause of pernicious anemia, the “Castle” factor involves a component in stomach acid that, when combined with a separate element found in specific foods, produces an anti-anemic substance typically stored in the liver. This research led to the finding that liver extract or powdered hog’s stomach could provide the essential factors to treat pernicious anemia. Nowadays, vitamin B12 injections are usually administered to provide this anti-anemic effect, making the use of liver extract and hog’s stomach relatively rare.