# Vitamin D and Primary Ciliary Dyskinesia: A Topic to Be Further Explored
## 4 Vitamin D Role in PCD  
There is a positive association between lung function and body mass index in PCD [92; 93], but the nutritional status of children affected with PCD may be relatively low. [PERSON] et al. found that the healthy phenotype of a cohort of children with PCD may be quite altered [94]. Although weak, there is an association between lung function and nutritional status and the bioelectrical impedance spectroscopy (BIS) phase-angle measures. BIS has been explicitly utilized to determine body composition in different settings. It has been shown to identify differences in nutritional state and clinical outcome. [PERSON] et al. emphasized that the use of BIS phase-angle may allow for early identification of at-risk children and may, therefore, be of benefit for nutritional assessments in the appropriate clinical setting [94]. A low BIS phase angle is associated with more inferior dietary status, which may precede anthropometric changes and impact clinical outcomes. [PERSON] et al. found that in children with PCD, there were some associations between BIS phase angle, nutritional status, and clinical outcomes. They advocate for the routine use of BIS phase angle to add sensitivity and specificity in identifying children with nutritional risk earlier than using anthropometry alone. [PERSON] et al. 's study [94] is paramount for identifying PCD patients at nutritional risk and looking for biomarkers of chronic inflammation in PCD. Their study involved 43 children, of which 51% were male. The average age at diagnosis was 2.7 \(\pm\) 3.8 years, and at the time of the survey was 7.0 \(\pm\) 3.6 years. Although none of the children studied had a low energy intake, 63% had excessive intake and 6% of children consumed inadequate amounts of protein, and 72% had a consumption of protein 200% of the reference nutrient intake (RNI) [95; 96]. The authors found no associations between energy and protein intake concerning BMI, height for age, or BIS phase angle. Although there were no statistically significant variations between the groups, the FEV1 z score was higher in those with a protein intake of 200% of RNI than those with an adequate protein intake. Remarkably, vitamin D insufficiency (<50 nmol/L) was detected in 54% of the patients in the cohort where the plasma was available. [PERSON] et al. 's study data are critical because children in their cohort had insufficient vitamin D levels, but all other micronutrient levels were within the normal range. The observation that lower nutritional status is associated with more impaired lung function in PCD patients, although weak, is similar to that seen in patients with cystic fibrosis (CF), with a relationship described between FEV1 and BMI. Although CF's main nutritional issues are probably due to intestinal malabsorption secondary to pancreatic insufficiency, PCD patients' data may be puzzling. This aspect may suggest that chronic suppurative lung disease, independently from pancreatic involvement, may negatively impact nutritional status [97]. This data may indicate that an abnormal ciliary function is associated with malabsorption but need to be explored further in future extensive studies. [PERSON] et al. 's research findings are utmost. Still, limitations need to be considered, including dietary intake methods and a three-day semiquantitative food diary different from the seven-day diary. Apart from the suggested benefit of a multicenter design, the potentiality to evaluate a duodenal biopsy may be expected for complete identification of the normal intestinal anatomy [91]. Another study involved the measurement of vitamin D levels in 22 patients with PCD [98]. [PERSON] et al. [98] studied 15 males and seven females, 10.5 years (range, 2-34 years). These authors found that 72% of PCD patients were vitamin-D deficient-to-insufficient and 28% only were sufficient. In addition, 79% of PCD individuals had limitations in performing vigorous activities, and 53% completed less than 3 h of physical activity per week. [PERSON] et al.