Magnitude and timescales of Ca isotope variability in human urine: implications for bone mass balance monitoring

Calcium (Ca) isotopes in blood/urine are emerging biomarkers of bone mineral balance (BMB) in the human body. While multiple studies have investigated Ca isotopes in patients suffering from diseases affecting BMB, comparatively little effort has been devoted to understanding the homeostasis of Ca isotopes in healthy individuals. Here, we report on a longitudinal study of the urine Ca isotope composition (δ^44/42Ca_Urine) from 22 healthy participants (age 19–60) over timescales ranging from days to months. Data from a single participant collected over a 30-day period show that morning urine is an excellent proxy for 24-h pooled urine fractions. Data from all participants reveal large inter-individual variability in δ^44/42Ca_Urine (up to 2.2‰), which is partly due to anthropometric differences, as shown by a correlation between the participants’ body mass index (BMI) and δ^44/42Ca_Urine values. In contrast, intra-individual data reveal encouraging stability (within ∼±0.2–0.3‰) over timescales >160 days, indicating that self-referencing approaches for BMB monitoring hold greater promise than cross-sectional ones. Our data confirm that intra-individual δ^44/42Ca_Urine variations are mainly a function of Ca reabsorption in the kidney, but also reveal the impact of other (and at times equally important) drivers, such as diet, alcohol consumption, physical exercise, or fasting. We also find that a magnetic resonance imaging contrast agent (gadolinium) can lead to artifacts during Ca isotope analysis. Based on our results, a series of practical considerations for the use of Ca isotopes in urine as tracers of BMB are presented.