Obesity and overweight are closely associated with urinary stones. The incidence of urinary stones in overweight and obese population was higher than that in normal-weight population [5]. And weight gain would also increase the risk of stones [6]. Epidemiological data in the United States showed that the incidence of urinary stones in obese population was 11.2%, while in normal weight population was 6.1% [7]. Several mechanisms may be responsible for the association between obesity and urinary stones, including insulin resistance, dietary habits and more lithogenic substances in urine [8]. Metabolic profiles of urine in obese patients was different from that in normal-weight patients. For instance, the excretion of oxalic acid, uric acid, sodium, phosphorus, calcium, and citric acid were higher in obese patients, meanwhile the urine pH was lower than that of normal-weight patients [9–11]. It was reported that increased visceral fat was associated with changes in urine metabolic profiles, which was manifested by increased creatinine, potassium and urine output in men, and increased urinary phosphorus in women [12].
Childhood obesity is becoming a worldwide public health problem. In this study, overweight/obese children accounted for 22.5% of the whole sample. The proportion is slightly higher than epidemiological data of China, but lower than that of United States [13]. In most cases, overweight or obesity in childhood would last to adulthood, leading to increasing risk of various diseases including urinary stones in adulthood. Therefore, prevention and control of overweight and obesity in childhood is still an important issue [2].
However, unlike in adults, the correlation between obesity and urinary stones is not clear in children. On the one hand, several studies showed that obesity is a risk factor for stones in children. A cohort study in children indicated that the ORR of obesity and hypertension in stone patients were higher than those in the control group [14]. And in children with idiopathic hypercalciuria, the prevalence of urinary stones risen with the increase of body fat [15]. On the other hand, it was reported that there was no significant association between obesity and pediatric stones in some studies [16–18]. Thus The relationship between obesity and pediatric stones still needs further research.
The results of our study showed that calcium oxalate was the most common stone composition in children, accounting for 69.04%. The proportion was lower than that of the United States (83%) [19], and was comparable to the reported data of China, Middle East and North Africa [20–22]. Tasian et al. [23] reviewed the articles about pediatric stone composition, found that 75%~80% of the stones were composed of calcium oxalate, 5%~10% were calcium phosphate, 10%~20% were magnesium ammonium phosphate, and 5% were pure uric acid.
Among adult stone patients, the proportion of uric acid stones in obese patients was higher than that in normal weight patients [24, 25]. Recently, a multicenter study involving 46 hospitals in China found that obese patients were more likely to develop urate stones, while the proportion of carbonated apatite and dicalcium phosphate dihydrate stones were less likely to be seen [26]. The relationship between obesity and stone composition was rarely reported in children. One study showed that there was no difference in stone composition between normal weight and overweight/obese children, but the sample size was too small, only 38 children were included [27].
This study showed that among obese or overweight children, the proportion of calcium oxalate stone was lower, while the proportion of ammonium urate stone was higher than that of the control group. The result was in accord with epidemiological data of adult stone patients in China [26]. However, we found no difference of stone composition between two groups after statistical analysis. Two potential reasons may account for this result. Firstly, the sample size was relevant small, and second, either overweight of obesity hadn’t caused evident metabolic changes in childhood.
Overweight or obesity could cause changes of urine metabolic profiles in children. It’s reported that overweight and obese children had lower urine citrate, urine phosphorus and urine magnesium, as well as higher proportion of hypercalcinuria compared to normal weight children. But the urine pH was not statistically significant between the two groups [28]. Another study indicated that except lower urine citrate and urine sodium, lower urine pH could also be seen in obese children [27]. Our study revealed that comparing to control group, the serum uric acid was significant higher and the urine pH was significant lower in overweight and obese pediatric stone patients, which was consistent with the reported data from adults and children [27, 29].
There are several limitations to the present study. First of all, the sample size is relevant small, thus it’s necessary to collect relevant data in order to expand the sample size in the follow-up research. Second, this is a single-center study, children included in the study were mainly from northern China. Due to the fact that stone composition may be different among different areas, a multi-center study is needed to incorporate the stone composition data of children from different regions into the study. Last but not least, urine metabolic data is absent in this study, so it is impossible to deeply investigate the correlation between obesity and stone composition.