There is evidence that obese patients have an increased risk of renal stone formation, although this relationship could be less evident in some populations. The aim of this study was to evaluate the impact of overweight and obesity on the risk of renal stone formation in a population consuming a Mediterranean diet and to better elucidate the mechanisms underlying the increased risk of urolithiasis observed in obese subjects. We performed a retrospective review of 1698 stone forming patients (mean age 45.9 ± 14.6 years; 984/714 M/F), attending outpatient stone clinics in Milan and Florence, seen between January 1986 and June 2014. Records were reviewed and data collected pertaining to age, gender, weight, height, stone composition, association with diabetes type 2 or gout and metabolic profile of 24-h urine to perform a descriptive study. We estimated prevalence ratios for body mass index (BMI) categories (underweight: BMI <18.5, normal: BMI 18.5-24.9, overweight: BMI 25-29.9 and obese ≥30). Overweight and obesity were present in 40.7 and 8 % of the men and in 19.9 and 8.7 % of the women in the study population. The mean BMI of patients with urolithiasis was found to be 24.5 ± 7.5 kg/m. BMI values were positively correlated with age (p = 0.000) and mean BMI was higher in males than in females (25.5 ± 8.9 vs 23.2 ± 4.4 kg/m). In males, rates of overweight and obesity in renal stone formers were higher than the rates reported in the Italian general population in 2004 only for the age group 25-44 years, whereas males in all the other age groups and in females the rates of overweight and obesity in renal stone formers were similar to rates reported in the Italian general population. The rates of overweight and obesity were significantly different in patients with different chemical stone composition. In particular, patients with uric acid stones have rates of overweight and obesity higher than patients with calcium stones or other types of calculi. Also the rates of type 2 diabetes and gout were greater in patients with overweight and obesity. In overweight and obese patients, the urinary excretion of risk factors for stone formation, such as calcium, oxalate and urate, and also of inhibitory substances, such as citrate, were significantly higher than in patients with normal weight or underweight. The prevalence of overweight and obesity in patients with urinary calculi from a country consuming a Mediterranean diet is not higher than in the general population. It should be taken into account that not all the dietary patterns that are associated with obesity may involve a parallel increase in the risk of forming kidney stones and that epidemiological findings from one country could not be confirmed in other countries with different climatic, socioeconomic and cultural features.
