In general, the 1950s were a very dynamic times for radiochemists like de Hevesy, because a lot of new radioactive isotopes could be made in the frame of the commissioning of the first “huge” particle accelerators with energies in the GeV range. One of these elements, and a main subject of this talk, is calcium. Using the artificial, radioactive calcium isotope with the mass number 45, de Hevesy and his team studied the behaviour of calcium in the organism by means of a rat model. They came up with a range of interesting figures. One result, for example, was the insight that that we keep half of the calcium atoms, present in our body at birth, for our entire life. This means that only one half is exchanged for calcium obtained from food. Considering that the calcium in the body of a new born is obtained exclusively from the mother, this implies that a couple of calcium atoms remain in the same family for up to twelve generations.
Despite the obvious disadvantages of feeding or injecting radioactive material, the methods developed by de Hevesy are still used today for selected purposes. This is mainly due to the fact that they give rather reliable data for living organisms and that they allow the study of fluxes, i.e. metabolic turnover rates.
Still, there also have been new developments, which avoid the artificial contamination with radioactive material. In the frame of investigations done on workers involved in the incident at the nuclear power plant in Chernobyl in 1986, it was found, for example, that the potassium isotope with the mass number 40, naturally present in the human body, emits high energy gamma-radiation, which may be quantified with suitable detectors . This allows for the facile quantification of the total amount of potassium present in a living organism. Other elements, which do not show this natural radioactivity, may be activated by exposing the body to neutron radiation first. This so-called neutron activation analysis allows for the in vivo quantification of virtually all the major elements in the human body, such as hydrogen, oxygen, carbon, nitrogen, calcium, phosphorus, sodium, and chlorine . On the basis of results obtained with these new techniques, a couple of the figures mentioned by de Hevesy had to be corrected slightly. While he estimated, for example, that a human of 80 kg body weight contains 55 grams of sodium, today we know that the correct figure is approximately double of that.
 K. J. Ellis, Physiological Reviews 2000, Vol. 80, No. 2, p. 649.