Between 1955 and 1963, open-air tests dumped radioactive material, including the isotope carbon-14, into the atmosphere for all of us to breathe. By its peak in 1965, the amount of this mildly radioactive isotope had approximately doubled over the constant level in the air during the previous 4,000 years. Plants take up carbon dioxide in the air (including the carbon-14-containing CO₂). We eat the plants, and ultimately the isotope is incorporated into the DNA of new cells formed in the body. By determining the proportion in fat cell DNA of carbon-14 (compared with the much more common stable form, carbon-12), it is possible to determine the year in which a new cell formed.

Fat cells that formed before the era of testing would contain only very low levels of the radioactive marker. Cells formed during the period of elevated atmospheric carbon-14 were marked by their incorporation of more of that carbon. Later, when the excess carbon-14 in the air dissipated, newly formed cells again had normal low levels of carbon-14.

This means that, if our fat cells remain stable and unchanged after childhood, then the people who became adults before the era of bomb testing ought to have low levels of the marker for carbon-14 in their adipocytes. If, on the other hand, these adults generated new fat cells, then the new cells would contain more carbon-14. With a little mathematical sleight of hand, the researchers could calculate the number of cells that died and the number of new cells that were produced by their sample. Which led them to this answer: New fat cells are continuously formed during adulthood, but the rate of formation is independent of weight gain and, instead, matches the annual rate of cell loss.