This may be due to surgery to your stomach or small intestine (such as gastric bypass surgery), abnormal bacterial growth in your small intestine, or an intestinal disease, such as Crohn's disease or celiac disease, that interferes with absorption of the vitamin.

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In gastric bypass surgery, a portion of the stomach is stapled to create the pouch and the intestines are rerouted to the new, smaller stomach.

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Each is intended to deal with the often unsightly and sometimes unhealthy aftereffects of gastric bypass surgery, a procedure in which part of the stomach is tied off to reduce the amount that patients eat.

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The gold-standard bariatric surgery is called Roux-en-Y gastric bypass, in which surgeons both shrink the stomach and reroute the digestive tract to bypass a portion of the intestines.

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