## 3 Vitamin D and CKD in Pediatric Patients
### Dental caries in CKD pediatric patients  
The contrasting results, reported in the literature regarding caries pathology and CKD, could also be related to different risks and protective factors of dental caries that can vary according to the study sample examined. In particular, the risk factors of dental caries in CKD patients that could justify the high prevalence are poor oral hygiene, dental calculus accumulation, xerostomia, high carbohydrate intake, DDEs, uremic gastritis and vitamin D deficiency. The risk factors are detailed below:  
* Oral hygiene. Oral hygiene in CKD pediatric patients is poor compared to the control group of healthy subjects [61; 66; 84]. Some authors underline how there are differences in oral hygiene between CKD patients in the different stages of the disease. Indeed, the highest mean of the simplified oral hygiene index (OHI-S) score was observed in stage 4-5 children (2.10 \(\pm\) 1.08), followed by kidney-transplanted children (1.46 \(\pm\) 1.19), by stage 1-3 (1.27 \(\pm\) 0.61) and by healthy children (0.45 \(\pm\) 0.44), respectively [62]. Poor oral hygiene is directly related to the increased risk of dental caries formation. * Dental calculus. Dental calculus in CKD patients is more frequent than in healthy subjects [62] and can also occur in 57.5% of CKD pediatric patients [67], especially in dialysis patients [105]. Dental calculus accumulation can cause difficulties in home oral hygiene, represent a retentive factor for dental plaque, and thus, can increase the risk of dental caries and periodontal disease. Dental calculus formation is determined by the precipitation of salivary calcium-phosphorus and calcium oxalate due to the high salivary pH and the changes in mineral balance with high levels of salivary urea and phosphate [105]. It is crucial to emphasize that CKD patients should be included and monitored in a preventive oral hygiene program. Frequent dental check-ups and non-surgical peri  
Figure 3: Risk and protective factors of dental caries in CKD pediatric patients. Abbreviations: CKD, chronic kidney disease; DDEs, development defects of enamel. odontal therapy sessions are important not only for adult CKD patients but also for pediatric ones. Indeed, the pediatric CKD patients may also exhibit alterations related to the periodontal attachment apparatus, such as gingival inflammation, gingival overgrowth, gingival recessions and, rarely, periodontal attachment loss [106]. Although these diseases are beyond the aim of this review, they must be diagnosed and treated early in order to ensure oral health. The oral prophylaxis intervention is related to the baseline degree of inflammation, assessed by periodontal indices (like gingival index, papilla bleeding index and periodontal screening index) and includes professional mechanical plaque removal, the local application of chlorhexidine gel, mouth rinses, oral hygiene instructions/motivation and periodic follow-ups [107]. Moreover, the oral prophylaxis intervention for CKD children/adolescents with an accelerated rate of dental calculus formation should include the professional mechanical calculus removal (scaling). The scaling must be performed at least every three months [108]. * Xerostomia. In CKD patients, xerostomia (decreased salivary flow) can be caused by a restricted fluid intake and by a side effect of drug therapy [109]. Several studies also state that salivary flow tends to decrease in CKD and in hemodialysis patients [49; 110]. The decreased salivary flow could cause important alterations of its function [111]. In fact, saliva exerts various protective functions against caries development through the dilution and the clearing of the oral cavity, the buffering capacity, the antimicrobial and immune activity and the ion exchange capacity on tooth surfaces (re-mineralization) [112]. * Composition of saliva. The composition of saliva is altered due to the fewer calcium ions in it [59; 113]. Furthermore, lower salivary calcium levels could also alter the re-mineralization mechanism (as described above). * Diet.