A pilot study on the use of natural calcium isotope (44Ca/40Ca) fractionation in urine as a proxy for the human body calcium balance

We explored the possibility of using natural calcium (Ca) isotope variations in the urine (δ44/40Caurine) as a proxy for the Ca balance in the human body. We chose two test persons extremely different in their health status, gender and age (4-year-old healthy boy and a 60-year-old woman known to suffer from osteoporosis). During a 5 day interval the Ca isotope composition of the individual diet (δ44/40Cadiet) was monitored for both test persons to be in general agreement to the Ca isotope composition of the normal western European diet (∼ − 1.02 ± 0.1‰). However, measurements showed that (1) δ44/40Caurine of both test persons are ∼ 1.37 and ∼ 2.49‰, respectively, heavier than δ44/40Cadiet and that (2) the δ44/40Caurine-boy is ∼ 1.1‰ heavier when compared to the value of the woman. The individual offset between diet and test persons is interpreted to reflect individual Ca reabsorption rates in the kidneys being the result of Rayleigh type Ca isotope fractionation related to the partitioning of Ca between the glomerular filtrate and filtered residue. The relative difference between δ44/40Caurine-boy and δ44/40Caurine-woman of ∼ 1.1‰ may reflect individual differences in the balance of bone mineralization and demineralization processes related to age, gender and health status. By arbitrarily defining an equilibrium value for Δ44/40Cadiet-urine of − 1.93‰ being the arithmetic mean of δ44/40Caurine for both test persons the measured δ44/40Caurine values may be applied to model the individual bone mineralization and demineralization processes in a qualitative way. Note, second order influences of intestinal Ca absorption during sequestration of Ca between intestine and blood have to be subject of further studies.