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Bariatric surgery, vitamin C, and kidney stones

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To the Editor: I read with interest the excellent review by Dr. Kashyap and coauthors of bariatric surgery for patients with type 2 diabetes.1 I am writing to contribute an additional caveat to their otherwise detailed list of postoperative complications—the increased risk of nephrolithiasis.2,3 The majority of kidney stones that develop after bariatric surgery tend to be composed of calcium oxalate. Increased intestinal absorption of oxalate appears to promote hyperoxaluria.2,3

Vitamin C deficiency is not usual after bariatric surgery, and the dietary reference intake of vitamin C for adults is no more than 90 mg. Therefore, I was surprised to see Dr. Kashyap recommend supplementation with vitamin C 500 mg daily (in Table 4 of her article). In my practice I have avoided supplemental vitamin C, other than that in a multivitamin, because of the risk of increasing urinary oxalate and stone formation.4

Iron deficiency can be a challenge after bariatric surgery. Although they do not state it in the review, the authors may believe that additional vitamin C can improve iron absorption. However, there are no compelling data of which I am aware for this belief in patients who have undergone gastric bypass,5 and the benefit of taking vitamin C along with iron in otherwise normal people with iron deficiency remains controversial.6

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