We explored the possibility of using natural calcium (Ca) isotope variations in the urine (δ 44/40Ca urine) 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 (δ 44/40Ca diet) 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) δ 44/40Ca urine of both test persons are ∼ 1.37 and ∼ 2.49‰, respectively, heavier than δ 44/40Ca diet and that (2) the δ 44/40Ca urine-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 δ 44/40Ca urine-boy and δ 44/40Ca urine-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 Δ 44/40Ca diet-urine of − 1.93‰ being the arithmetic mean of δ 44/40Ca urine for both test persons the measured δ 44/40Ca urine 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.