Obesity and the associated metabolic syndrome are among the most common and detrimental metabolic diseases of the modern era, affecting over 50% of the adult population in the United States. Surgeries designed to promote weight loss, known as bariatric surgery, typically involve a gastric bypass procedure and have shown high success rates for treating morbid obesity. However, following gastric bypass surgery, many patients develop chronic anemia, most commonly due to iron deficiency. Deficiencies of vitamins B1, B12, folate, A, K, D, and E and copper have also been reported after surgery. Copper deficiency can cause hematological abnormalities with or without neurological complications. Despite oral supplementation and normal serum concentrations of iron, copper, folate, and vitamin B12, some patients present with persistent anemia after surgery. The evaluation of hematologic disorders after gastric bypass surgery must take into account issues unique to the postsurgery setting that influence the development of anemia and other cytopenias. In this paper, the clinical characteristics and differential diagnosis of the hematological disorders associated with gastric bypass surgery are reviewed, and the underlying molecular mechanisms are discussed.

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