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08-05-2012 | Surgery | Article

Low-oxalate diet may reduce kidney stone risk after bariatric surgery


Free abstract

MedWire News: A diet formulated to reduce kidney stone risk in patients who have undergone bariatric surgery decreases urinary calcium oxalate (CaOx) supersaturation, report US researchers.

However, the diet, which is low in oxalate and fat, moderate in protein, and normal in calcium, does not seem to decrease urinary oxalate levels, they say.

"It has recently been observed that certain patients develop enteric hyperoxaluria and kidney stone disease after modern bariatric surgical procedures, even though many were stone free preoperatively," report John Lieske (Mayo Clinic, Rochester, Minnesota) and colleagues. "The optimal treatment of this complication remains undefined, although a low-fat, low-oxalate diet has often been recommended."

The team therefore recruited nine patients who had developed CaOx nephrolithiasis after undergoing bariatric surgery. The surgery had been performed a mean of 10.9 years previously and patients had experienced a median of two stone events since.

Patients were instructed to swap their current free-choice diet for a diet that was normal in calcium (1000 mg/day), moderate in protein (15-19% of total daily calories), low in oxalate (70-80 mg/day), and low in fat (<25% of total daily calories).

Two baseline 24-hour urine samples were collected while participants were on their current free-choice diet, and two further samples were collected after 4 days of the controlled diet. The samples were then assessed for differences in concentrations of oxalate, calcium, citrate excretion, and determinants of supersaturation.

As reported in Urology, the overall mean CaOx supersaturation decreased significantly from 1.97 delta Gibbs with the free-choice diet to 1.13 delta Gibbs with the controlled diet.

However, this reduction occurred in the absence of any significant change in mean urinary oxalate levels, which only fell from 0.69 mmoL/d with the free-choice diet to 0.66 mmoL/d with the controlled diet.

"Presumably, the percentage of absorption of oxalate was very high in these patients with fat malabsorption," remark the authors.

The team says the decrease in supersaturation seemed to result from favorable mean changes in multiple urinary components, including significantly increased potassium, significantly decreased urine osmolarity, and slightly increased urinary volume, pH, and citrate excretion.

However, "if hyperoxaluria itself, absent an increase in overall CaOx supersaturation, is a risk factor for kidney stone or renal damage, other measures are required, because dietary restriction alone does not appear to suffice in this patient group," write Lieske et al.

"Additional strategies could be necessary, such as the use of oral calcium supplements as oxalate binders and a low fat diet," they conclude.

By Sally Robertson

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