 The study will follow closely the protocol that was used previously by the PI at ETH Zurich to study calcium metabolism in healthy postmenopausal women. Calcium metabolism will be studied after a single oral administration of a calcium tracer Ca-41 and by following the excretion of the tracer in urine over 6 months. Obtained data will be used to assess parameters of calcium metabolism flux rates and sizes of body calcium compartments by kinetic modelling. A strontium tracer Sr-84 will be administered in parallel and its urinary excretion pattern compared to Ca-41 excretion to determine if strontium tracers can be used to assess calcium metabolism qualitatively. Subjects will be supplemented with Vit D throughout the study. Vit D supplementation will commence 1 month before isotope administration. 