This is a novel, non-invasive method of assessing Ca balance by natural Ca isotope fractionation.

Detailed description and study protocol will be available to individuals on request.

I - For healthy children

Inclusion criteria

• All children <18 years
• Normal renal function (eGFR 90-120ml/min/1.73m2 [calculated by Schwartz formula] in children >1 year and serum creatinine <35μMol/L in children <1 year)
• Weight, height and BMI within 2 SD of normal using WHO growth charts In order to make this study as 'real-life' as possible, free-living UK children on their usual diet and dietary supplementation (including Ca and Vit D), if any, will be included, but analysis will account for medication doses and blood levels.

Exclusion criteria

• Pre-existing bone disease - inherited or acquired
• Fractures in the preceding 6 months
• Glucocorticoid therapy in the preceding year, or a lifetime cumulative steroid exposure ≥6 months
• Bisphosphonate therapy at any time in the past
• Any acute illness in the preceding 2 weeks (when the child is unable to maintain their usual diet or has bed rest)

II - For CKD and dialysis children

Inclusion criteria

• Children of all ages with an eGFR <60ml/min/1.73m2, including those on dialysis.

Exclusion criteria

• As listed above for healthy children.

All children with chronic kidney disease stage 3-5 and on dialysis and age matched controls

IPD sharing will be considered at the end of the study.