Renal stones.

There is an increased prevalence of kidney stones following
gastric bypass procedures. Frequent kidney stones are linked with worsening
of renal function and may contribute to development of CKD in patients with
otherwise normal renal function prior to surgery.
Kidney stones are associated with higher levels of oxalate in urine. It
appears that provision of oral calcium and oral citrate supplements can
decrease risk for stone formation in weight loss surgery patients. Although
provision of calcium citrate is common in many patients post weight loss
surgery, calcium citrate is not given to and is not safe for patients with
chronic renal failure due to increased serum levels of aluminum that can
accompany this particular supplementation.

Restrictive procedures (e.g., banding and gastric sleeve) do not appear to
hold this same risk for development of kidney stones. In a recent study,
patients who had a restrictive procedure for weight loss had urinary oxalate
levels similar to levels found in nonstone forming and stone-forming
controls. Levels of urinary oxalate were significantly lower than in
patients who underwent RYGB.