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."