Risk of kidney stones increases higher if overweight

Researchers at UT Southwestern Medical Center at Dallas have found that the more overweight a person is the more at risk he or she is for forming uric acid kidney stones.

Their study will be published in the April issue of Kidney International and is currently available online. These findings for the first time directly link excess body weight with uric acid kidney stones, found in about 5 percent of kidney-stone patients and in about 30 percent of diabetics with kidney stones.

"This is yet another price to pay for being overweight or obese," said Dr. Khashayar Sakhaee, program director of the General Clinical Research Center (GCRC) at UT Southwestern.

The more overweight a person is, the more likely he or she is to develop uric acid kidney stones, according to UT Southwestern researchers. Participating in the study and an earlier related study are (from left) Drs. Nicola Abate, assistant professor of internal medicine, Mary Ann Cameron, postdoctoral trainee in mineral metabolism, Khashayar Sakhaee, program director of the General Clinic Research Center, Naim Maalouf, postdoctoral trainee in mineral metabolism, Manisha Chandalia, assistant professor of internal medicine, and Orson Moe, director of the Charles & Jane Pak Center for Mineral Metabolism and Clinical Research.

An estimated 10 percent of people in the United States will have a kidney stone some time in their lives.

Kidney stones are solid deposits that form in the kidneys from substances excreted in urine. When waste materials in urine do not dissolve completely, microscopic particles begin to form and over time grow into kidney stones. These stones may remain in the kidney or can break loose and travel down the urinary tract. Small stones may pass out of the body naturally, but a larger stone can get stuck in a ureter, the bladder or the urethra, possibly blocking the flow of urine and often causing intense pain.

Uric acid kidney stones develop when the urine's acid level is too high, typically from the ingestion of too much dietary animal protein or when there are insufficient amounts of buffers to neutralize acid in the urine.

The latest study, which included researchers at the University of Chicago, tracked nearly 5,000 kidney-stone patients in Dallas and Chicago. Results did not vary between men and women, nor for patients who restricted the types of foods eaten.

"Larger people have very acidic urine even when they control their diets," said Dr. Sakhaee, professor of internal medicine at UT Southwestern. "Other studies we have done in the GCRC support this concept. For the first time, we are advising weight loss as part of our therapy. That connection had not been made in the past."

A study published by UT Southwestern in the February issue of Kidney International concluded that uric acid stones also are associated with insulin resistance and type 2 diabetes. Heavier people tend to have more insulin resistance, too, said Dr. Orson Moe, a co-author of the earlier report. Dr. Moe is the director of the Charles & Jane Pak Center for Mineral Metabolism and Clinical Research.

According to Dr. Charles Y.C. Pak, senior author of the current study and professor of internal medicine, the discovery of a link between body weight and uric acid kidney stone formation is significant.

"In 1986 we coined the term 'gouty diathesis' to describe uric acid stones forming in the absence of any discernible cause," Dr. Pak said. "We now know that one of the causes is obesity that leads to insulin resistance and diabetes. The challenge for our research team is to determine whether weight loss and/or sensitization to insulin would bring about a relief of stone formation."

Other UT Southwestern researchers contributing to the current study were Beverley Adams-Huet, mineral metabolism faculty associate, and Dr. Naim M. Maalouf, postdoctoral trainee in mineral metabolism. Dr. Frederic L. Coe and Joan H. Parks from the University of Chicago also contributed.

The research was supported by grants from the National Institutes of Health.