Photo Credit Jupiterimages/Polka Dot/Getty Images. Diarrhea is the result of your body eliminating stool before the large intestine can properly absorb water back into your body, which causes the stool to be runny and watery. Diarrhea is a symptom of several conditions that relate to a deficiency of vitamin B-12. Often though the condition that causes the deficiency also causes the diarrhea, but occasionally the diarrhea is a direct result of a vitamin B-12 deficiency. Vitamin B-12 is present in foods such as cheese, milk and eggs.