The claim travels widely: your body replaces every cell every seven years, meaning you are, in a material sense, an entirely different person from a decade ago. The grain of truth in this is that most of your cells do turn over. The seven-year number, and the “every cell” part, are both wrong in ways that matter.
The science behind cellular turnover became much more precise after a clever technique was developed in the 1950s and refined in the 2000s. Nuclear weapons tests from the 1950s to the early 1960s deposited elevated levels of carbon-14 in the atmosphere. Carbon-14 gets incorporated into DNA when cells divide. By measuring carbon-14 levels in the DNA of specific tissues, researchers can date when those cells were last replaced, the same way archaeologists use carbon-14 to date organic material. The technique was pioneered by Swedish researcher Jonas Frisen and colleagues at the Karolinska Institute.
What they found is that different cells have wildly different lifespans.
Red blood cells live about 120 days. They are replaced constantly. The lining of your gut turns over every three to five days. Liver cells last roughly 300 to 500 days. Fat cells are replaced at a rate of about 8 percent per year, meaning a given fat cell in your body is on average about 8 years old. The cells lining your lungs last about a week if they are in the upper airways, or over a year if they are in the deeper tissue.
Bone is largely rebuilt over about 10 years, though the process is continuous rather than synchronized.
Then there are cells that do not turn over much at all.
Neurons in the cerebral cortex, the parts of your brain responsible for thinking, memory, and consciousness, are largely post-mitotic: they do not divide after early development and are not replaced when they die. The carbon-14 dating technique confirmed this: cortical neurons in adult humans were found to have the same carbon-14 signature as the individual themselves, meaning they were formed around the time of birth. Your cortical neurons are your age. They have been with you since you were an infant.
There are exceptions within the brain. The olfactory bulb (smell processing) and the hippocampus (memory and spatial navigation) do show some ongoing neurogenesis in adults, though the extent and functional significance in humans is contested.
Cardiac muscle cells, your heart muscle, also persist for a long time, with most cells replaced at a rate of less than 1 percent per year. A 70-year-old’s heart muscle cells have an average age of about 40 years.
So the accurate version of the popular claim is something like: most of you, by number of cells, is replaced relatively frequently, but some of the most important cells in your body (your neurons, many of your heart cells) have been there since childhood, and nothing replaces them when they go.
The ancient neurons have been processing everything you have ever experienced. They are not replaceable. Take care of them accordingly.
