Researchers at the Karolinska Institute measured carbon-14 in the fat cells of people of different ages and body weights, using the bomb-test signal the group had already used to date brain cells. They wanted to know whether a person's fat cells are fixed for life or steadily replaced.
About 10 percent of fat cells were replaced every year at every age and body mass index. Yet the total number of fat cells stayed essentially constant in each person, even after major weight loss, meaning new cells replaced dying ones in equal measure rather than adding to the total.
The finding reframed obesity as less about growing new fat cells in adulthood and more about a number set earlier in life, likely during childhood and adolescence, that the body defends afterward. Losing weight shrinks existing cells rather than eliminating them, which helps explain why keeping weight off is a different problem from losing it. Later work has focused on what locks that number in during development.
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The count stays; the cells don't.
Source
- Spalding, K. L., Arner, E., Westermark, P. O., Bernard, S., Buchholz, B. A., Bergmann, O., Blomqvist, L., Hoffstedt, J., Näslund, E., Britton, T., Concha, H., Hassan, M., Rydén, M., Frisén, J., & Arner, P. (2008). Dynamics of fat cell turnover in humans. Nature, 453(7196), 783–787. https://doi.org/10.1038/nature06902