If animal studies apply to humans, people taking the anti-obesity drug Xenical might have an increased likelihood of developing kidney stones, especially if they have a high intake of oxalate-containing foods.

Researchers from the Federal University of Sao Paulo, Brazil, and the Karolinska Institute, Sweden, tested the effect of Xenical (known generically as orlistat) in 39 adult rats that were given a diet rich in oxalate alone or combined with fat (soy oil).

With Xenical, oxalate levels in the urine were four to eight times higher than the baseline value, “elevating the risk of stone formation,” the team reports in the August issue of Kidney International.

It has been suggested previously that some people can develop increased levels of urinary oxalate following treatment with Xenical, but this is the first study relating this effect to the type of diet.

“Before prescribing orlistat, doctors should ask their patients if they have ever (had) a kidney stone,” Dr. Ita Pfeferman Heiberg, who led the study, told Reuters Health. “Patients should also be advised not to eat fat- and oxalate-rich foods while taking the drug.”

Foods that contain high levels of oxalate include spinach, rhubarb, chocolate, tea, parsley, beetroot, strawberries, wheat flour, pepper and some nuts.

Xenical blocks the absorption of fats from the intestines. According to the investigators, unabsorbed bile acids and fatty acids react with calcium forming “soaps” in the intestines, which limit the amount of free calcium binding with oxalate. Hence, more oxalate is available to be absorbed and eventually excreted with urine.

A long-term clinical trial with a controlled diet might be needed to confirm that Xenical increases the occurrence of kidney stones in people, Heiberg commented.

SOURCE: Kidney International, August 2004.