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From the AACE/TOS/ASMBS guidelines for nutritional support of bariatric patients: R54(113-116)-r. Baseline and postoperative evaluation for Vitamin B12 deficiency is recommended in all bariatric surgery and annually in those with procedures that exclude the lower part of the stomach (e.g., LSG, RYGB) (Grade B; BEL 2). Oral supplementation with crystalline vitamin B12 at a dosage of 1000 μg daily or more may be used to maintain normal vitamin B12 levels (Grade A; BEL 1). Intranasally administered vitamin B12, 500 μg weekly, may also be considered (Grade D). Parenteral (intramuscular or subcutaneous) B12 supplementation, 1000 μg/mo to 1000-3000 μg every 6 to 12 months, is indicated if B12 sufficiency cannot be maintained using oral or intranasal routes (Grade C; BEL 3). R56(119)-r. Nutritional anemias resulting from malabsorptive bariatric surgical procedures might also involve deficiencies in vitamin B12, folate, Protein, copper, selenium, and zinc and should be evaluated when routine screening for Iron deficiency anemia is negative (Grade C; BEL 3). Additionally, this study assesses whether B12 supplementation is required for VSG patients, which found 20% of patients developed a vitamin B12 deficiency. It describes the reason for this deficiency as follows: Vitamin B12 usually absorbed from the small bowel at the terminal ileum in the presence of the intrinsic factor which IS secreted by the parietal cells in the body of the stomach. Resection of a significant proportion of the gastric body during LSG may reduce the production of intrinsic factor. Vitamin B12 deficiency can be manifested as anaemia (megaloblastic/macrocytic anaemia) or it can be more serious with neurological sequelae including spinal cord degeneration, which if untreated may lead to permanent neurological deficit. TL;DR = Intrinsic factor is a glycoprotein produced by the parietal cells of the stomach which is necessary for the absorption of vitamin B12 later on in the small intestine. If part/most of the stomach is removed, there is the possibility of less production of intrinsic factor and subsequently less absorption of B12.