Obesity not behind kidney stones in kids: study

The finding seems "counterintuitive," say researchers who have noticed an increasing number of both adult and kid patients with kidney stones as obesity levels have been on the rise.

"I think everyone would agree that there's an increasing incidence of stone disease," said Dr. Brian Matlaga, a urologist at the Johns Hopkins University School of Medicine in Baltimore who was not involved in the new study.

Whether overweight and obesity trends are behind that or not, "I think it's something in the modern American diet, a high-sodium diet that may be associated with this increased risk," he told Reuters Health.

Too much calcium in the urine is known to cause kidney stones, as is dehydration -- both conditions influenced by sodium. Studies of adults have found that those who are obese, or who have high blood pressure or diabetes, are at an increased risk of getting stones.

Nationwide numbers are generally lacking, but the rise in kidney stones among kids can be glimpsed in records from individual hospitals, medical practices and databases. One recent study using a pediatrics database as a sample identified 125 kids admitted to hospitals for kidney stone treatment in 1999, compared to 1,389 kids admitted in 2008. Adjusted for other factors, the change represented a 10 percent annual increase in cases.

But it's been unclear if weight is also driving kidney stone rates in kids. To get at that question, researchers led by Dr. Steve Kim from Children's Hospital Los Angeles consulted electronic health records from kids treated at Philadelphia-area doctors' offices.

Between 2002 and 2009, they found 110 cases of kidney stones in kids and adolescents 21 and younger. For each of those cases, the researchers picked out four kids of the same age and treated at the same practice who didn't have a kidney stone.

According to their health records, two percent of kids with kidney stones were overweight, and about four percent were obese. That compared to four percent of the non-kidney-stone group that was overweight, and close to five percent that was obese.

Girls and boys were equally likely to get kidney stones, whereas white kids were more likely than black kids to have a stone, according to the findings published in The Journal of Urology.

Experts suggest that the diet of adults who are overweight and the metabolic changes in the body caused by extra weight affect the contents and chemistry of the urine -- possibly leading to a higher risk of kidney stones.

Because "children are not little adults," weight might affect them differently, Matlaga said. But diet could still be behind the increase in childhood stones, he added.

Kim agreed. "We know that sodium is associated with calcium metabolism in the urine," he said. "Certainly a higher-salt diet would likely contribute to more kidney stones." But, he cautioned, "we don't have any good proof of that yet."

Kids are also getting fewer dairy products in their diet, he said, and that could be related to urine chemistry and kidney stone risk. Or, dehydration may be to blame. "In general, a lot of kids drink less water than they should, or less fluid in general than they should. They don't stay as well-hydrated," he told Reuters Health.

Clearly, Kim said, more studies are needed to nail down what is behind the "dramatic shift" in kidney stone cases that pediatricians have noticed in the past few years.

Once doctors have the answer to that question, they'll be more prepared to counsel kids at risk and their families about kidney stones and other conditions that might go hand in hand with stones, Matlaga added.