Kidney Health

Lose the Weight and Lose the Stones

If you have ever had the experience of passing a kidney stone, you understand why it is universally described as unbearable. And if you have had one stone, you would probably do anything to avoid another. One answer, research suggests, may be to maintain a healthy weight.

One study, for example, found that both obesity and substantial weight gain during adulthood increase the odds of developing kidney stones.

"Our study suggests that maintaining a healthy weight is important," says Dr. Eric Taylor, a clinical and research fellow in nephrology (the study of kidneys) at Brigham and Women's Hospital in Boston. "It also suggests to me that weight loss should be explored as a treatment for people with recurrent kidney stones."

In the study, published in the Journal of the American Medical Association, Taylor and his colleagues examined data from about 250,000 men and women who participated in three large studies and were followed for a combined total of 46 years. The researchers found that a relatively heavy person was at greater risk of developing kidney stones than a leaner person was. For example, men who weighed more than 220 pounds were more likely to develop kidney stones that men who were 150 pounds or lighter.

Weight gain after young adulthood also seems to be a factor, particularly for women. Women in the study who gained more than 35 pounds since the age of 18 increased their risk of kidney stones by 80 percent, while men who gained more than 35 pounds since age 21 had a 40 percent increased risk.

The study authors adjusted their findings to take participants' diet and fluid intake into consideration since a rich diet and/or a low fluid intake may raise kidney stone risk.

It is not well-understood how obesity increases the risk of kidney stones, but it's theorized that insulin resistance, a common condition in obese people, increases the amount of calcium in the urine. This, in turn, contributes to the formation of crystals that develop into kidney stones. After they are formed, the stones may become trapped in the ureters — the narrow tubes that carry urine from the kidneys to the bladder — causing terrible pain until they pass on their own or with the help of medication or medical procedures.

While it was already known that larger body size raises kidney stone risk, Taylor and his colleagues specifically looked at weight gain, body mass index and waist size to try to separate the effects of fat tissue versus lean muscle tissue. Body mass index, or BMI, is a measure of weight in relation to height, and waist size is a gauge of abdominal fat; both are used to define obesity.

According to Taylor, the researchers examined weight gain, for example, because "on average, people gain weight as they age and most of that gain isn't usually muscle." The study actually found that all three measurements — weight gain, BMI and waist size — were associated with a higher risk of kidney stones, suggesting that fat tissue was a culprit. However, Taylor notes, "that's not to say lean body mass in unimportant."

So should people with recurring kidney stones work on weight loss?

"Unfortunately, there weren't enough people in our study who lost weight for us to conclude that weight loss is an effective treatment," Taylor says. "But the major points are the importance of maintaining a healthy weight and the need for more research in this area to reduce the rate of stone formation in the United States."