A larger body size has been shown to be associated with increased excretion of urinary lithogenic solutes, and an increased risk of
nephrolithiasis has been reported in
overweight patients. However, the type of stones produced in these subjects has not been ascertained. Based on a large series of
calculi, we examined the relationship between body size and the composition of stones, in order to assess which type of stone is predominantly favoured by
overweight. Among 18,845 consecutive
calculi referred to our laboratory, 2,100 came from adults with recorded body height and weight. Excluding
calculi from patients with
diabetes mellitus, as well as
struvite and
cystine stones, the study material consisted of 1,931
calcium or
uric acid calculi. All
calculi were analysed by infrared spectroscopy and categorized according to their main component. Body mass index (BMI) values were stratified as normal BMI (< 25 kg/m2),
overweight (BMI 25-29.9) or obese (BMI > or = 30). Overall, 27.1% of male and 19.6% of female stone formers were
overweight, and 8.4 and 13.5% were obese, respectively. In males, the proportion of
calcium stones was lower in
overweight and obese groups than in normal BMI group, whereas the proportion of
uric acid stones gradually increased with BMI, from 7.1% in normal BMI to 28.7% in obese subjects (P<0.0001). The same was true in females, with a proportion of
uric acid stones rising from 6.1% in normal BMI to 17.1% in obese patients (P=0.003). In addition, the proportion of
uric acid stones markedly rose with age in both genders (P<0.0001). The average BMI value was significantly higher in
uric acid stone formers aged < 60 years than in all other groups, whereas it did not differ from other groups in those aged > or = 60 years. Stepwise regression analysis identified BMI and age as significant, independent covariates associated with the risk of
uric acid stones. Our data provide evidence that
overweight is associated with a high proportion of
uric acid stones in patients less than 60 years of age, whereas beyond this limit, advancing age is the main risk factor.