Healthy weight reduces risk of kidney stones

Measures of large body size were associated with a significantly increased risk of stone formation in the three cohorts. Weight gain of >35 pounds since adulthood was also an independent predictor of increased stone risk in both gender groups, with increased relative risks of 70% among older women, 82% among younger women, and 39% among men. Weight loss was not found to decrease risk.

246,000 participants The study, published in JAMA (2005; 293:455-62), included data from nearly 46,000 men participating in the Health Professionals Follow-up Study and almost 200,000 women enrolled in the Nurses' Health Study I and Nurses' Health Study II (NHS I and NHS II). Ages at entry were 40 to 75 years for the Health Professionals subjects, 34 to 59 years for women enrolled in NHS I, and 27 to 44 years for NHS II subjects. Durations of follow-up for the three cohorts were 16, 20, and 10 years, respectively. A total of 4,827 symptomatic kidney stones were recorded by subjects who had no history of stones at study entry.

After adjustment for potential confounders (including age, dietary factors, fluid intake, and thiazide use), every measure of larger body size investigated-weight, body mass index, and waist circumference-was associated with a statistically significant increased risk of kidney stone formation in all three cohorts. The relative risk of stones was increased by 71% to 109% among younger and older women in the highest weight, BMI, and waist circumference categories when compared against their reference groups, and by 33% to 48% among the men.

"The findings of this study provide another reason for health care professionals to encourage people to maintain a healthy body weight," said Eric N. Taylor, MD, lead author and clinical and research fellow in nephrology, Brigham and Women's Hospital. "We did not find a benefit for weight loss, but few people in our cohorts lost weight as they aged. Further study to see if weight loss reduces the risk of stone recurrence is needed."

Expanding previous analyses Analyses to determine potential associations between obesity and kidney stone risk were also conducted several years ago using data from the Health Professionals Follow-up Study and NHS I. That study showed an association in the older women of NHS I, but no statistically significant relationship was found among the men.

"For the present analyses we had an additional 8 years of follow-up for the Health Professionals cohort, and during that time the number of incident stones recorded nearly doubled. Therefore, we now had greater statistical power to identify a significant association. With the addition of data from NHS II in this study, we also had information on younger women, as well as waist circumference data for the older women, and for the first time, we had information to allow us to investigate the effect of weight gain on risk," Dr. Taylor said.

The large sizes of the cohorts and reliability of the participants are two important strengths of this study. However, as Dr. Taylor pointed out, nearly 98% of all subjects in each cohort were Caucasian.

"There is no scientific basis to believe the findings should be any different among African Americans or other ethnicities," he said. "However, it is definitely true that the relationships we found were among Caucasians."

Underlying mechanism? The study design did not lend itself to addressing the underlying mechanism for the increased body size-kidney stone association. However, emerging evidence suggests a role for insulin resistance.

"Any hypotheses are purely speculative, but there is some pretty exciting new research to show that insulin resistance can dramatically affect urine composition. Further study in that area is warranted," Dr. Taylor said.