Naturally Occurring Stable Calcium Isotope Ratios in Body Compartments Provide a Novel Biomarker of Bone Mineral Balance in Children and Young Adults

Abstract

Serum calcium (Ca), bone biomarkers, and radiological imaging do not allow accurate evaluation of bone mineral balance (BMB), a key determinant of bone mineral density (BMD) and fracture risk. We studied naturally occurring stable (non-radioactive) Ca isotopes in different body pools as a potential biomarker of BMB. 42 Ca and 44 Ca are absorbed from our diet and sequestered into different body compartments following kinetic principles of isotope fractionation; isotopically light 42 Ca is preferentially incorporated into bone, whereas heavier 44 Ca preferentially remains in blood and is excreted in urine and feces. Their ratio (δ 44/42 Ca) in serum and urine increases during bone formation and decreases with bone resorption. In 117 healthy participants, we measured Ca isotopes, biomarkers, and BMD by dual-energy X-ray absorptiometry (DXA) and tibial peripheral quantitative CT (pQCT). 44 Ca and 42 Ca were measured by multi-collector ionization-coupled plasma mass-spectrometry in serum, urine, and feces. The relationship between bone Ca gain and loss was calculated using a compartment model. δ 44/42 Ca serum and δ 44/42 Ca urine were higher in children (n = 66, median age 13 years) compared with adults (n = 51, median age 28 years; p < 0.0001 and p = 0.008, respectively). δ 44/42 Ca serum increased with height in boys (p < 0.001, R 2 = 0.65) and was greatest at Tanner stage 4. δ 44/42 Ca serum correlated positively with biomarkers of bone formation (25-hydroxyvitaminD [p < 0.0001, R 2 = 0.37] and alkaline phosphatase [p = 0.009, R 2 = 0.18]) and negatively with bone resorption marker parathyroid hormone (PTH; p = 0.03, R 2 = 0.13). δ 44/42 Ca serum strongly positively correlated with tibial cortical BMD Z-score (n = 62; p < 0.001, R 2 = 0.39) but not DXA. Independent predictors of tibial cortical BMD Z-score were δ 44/42 Ca serum (p = 0.004, β = 0.37), 25-hydroxyvitaminD (p = 0.04, β = 0.19) and PTH (p = 0.03, β = −0.13), together predicting 76% of variability. In conclusion, naturally occurring Ca isotope ratios in different body compartments may provide a novel, non-invasive method of assessing bone mineralization. Defining an accurate biomarker of BMB could form the basis of future studies investigating Ca dynamics in disease states and the impact of treatments that affect bone homeostasis.

Lactation and gestation controls on calcium isotopic compositions in a mammalian model

“…In pre-insemination conditions, blood is depleted in heavy Ca isotopes relative to urine (Δ 44/42 Ca urine-blood = +1.17 − 0.12 +0.11 %, n urine = 3, n blood = 3; figure 3a), which is conform to the value of the literature (Δ 44/42 Ca urine-blood ≈ +1.15 ‰ 3,17,21,23 ), an isotopic fractionation generated by the preferential reabsorption of light Ca isotopes from primary urines to blood in kidneys 17,[20][21][22][23][24]27 .…”

“…However, this observation calls for further investigations on other mammal species and, more essentially, on physiological factors at play to generate these sexually driven isotopic differences. Improving our knowledge about the mammalian Ca isotope cycle is motivated both by the development of biomedical innovations based on Ca isotope measurements 6,[20][21][22][23][24] and potential applications in paleoanthropology and paleontology 1,2,10,[13][14][15]18,19,25,26 . For example, new methods for bone balance and osteoporosis monitoring depend upon an accurate description of the Ca isotope cycle 6,[20][21][22][23][24] .…”

“…Improving our knowledge about the mammalian Ca isotope cycle is motivated both by the development of biomedical innovations based on Ca isotope measurements 6,[20][21][22][23][24] and potential applications in paleoanthropology and paleontology 1,2,10,[13][14][15]18,19,25,26 . For example, new methods for bone balance and osteoporosis monitoring depend upon an accurate description of the Ca isotope cycle 6,[20][21][22][23][24] . Besides, sexually driven Ca isotopic differences could help in detecting sex or past lactation events from teeth or bones, if their causing factors could be further constrained and quantified 14 .…”

“…For example, induced bone loss in Göttingen minipigs 17 is not associated with higher Ca urinary excretions or with a decrease of blood and urine δ 44/42 Ca values 17,42 . However, blood and urine δ 44/42 Ca values do decrease for humans with bone loss 20,24,28 , while there are clues that bone loss comes with higher Ca urinary excretions for them (e.g. higher Ca concentrations in urines: Eisenhauer et al, 2019;…”

Hemodiafiltration Is Associated With Reduced Inflammation and Increased Bone Formation Compared With Conventional Hemodialysis in Children: The HDF, Hearts and Heights (3H) Study

“…J o u r n a l P r e -p r o o f -4 - Immuno Biological Laboratories Co. Ltd., Tokyo, Japan), sclerostin (Teco, Sissach, Switzerland), and CTX (Immunodiagnostic Systems, Boldon, UK) were measured in the Research Unit of the Department of Pediatrics, Rostock University Medical Center as described previously 13,14,26 .…”