Bariatric surgery, vitamin C, and kidney stones

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

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

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