The Question: I’ve read various internet articles that say that human beings replace “every cell in your body” every X years (usually 7, but sometimes 10). What’s the real deal?



Submitted by: Rick, NY

The Short Answer: Recent research has confirmed that different tissues in the body replace cells at different rates, and some tissues never replace cells. So the statement that we replace every cell in the body every seven years or every ten years is wrong. Using a revolutionary new technique (described below), researchers have shown that:

Neurons in the cerebral cortex are never replaced. There are no neurons added to your cerebral cortex after birth. Any cerebral cortex neurons that die are not replaced. Fat cells are replaced at the rate of about 10% per year in adults. So you could say that on average, human beings replace all their fat cells about every ten years. Cardiomyocyte heart cells are replaced at a reducing rate as we age. At age 25, about 1% of cells are replaced every year. Replacement slows gradually to about 0.5% at age 70. Even in people who have lived a very long life, less than half of the cardiomyocyte cells have been replaced. Those that aren’t replaced have been there since birth.

Scientists are now studying other tissues to determine the turnover rate.

More Information: What’s a little confusing about the data given above is that obviously, our brains grow bigger after birth, and so do our hearts. So where is all the extra bulk coming from? In the brain, no cerebral cortex neurons are added, but research hasn’t been completed on other parts of the brain, and even if it were to turn out that no other neurons are added, lots of other kinds of cells are added. Glial cells, for example, may actually make up 90% of the cells in the brain. It used to be thought that glial cells were simply the scaffolding of the brain, with no real role in the processing of the brain. In recent years, however, it has become clear that glial cells play key roles in processing.

Cardiomyocytes are the true muscle cells of the heart, but the heart is also made up of connective tissue and other cell types that may turn out to have different growth and replacement rates. And while cardiomyocytes replace very slowly, and some are never replaced, the individual cells do grow in size.

The Interesting Science: The technique used to investigate the replacement of cells in humans ingeniously utilizes the unfortunate fact that during the Cold War the nuclear states conducted above ground nuclear tests that spread radioactive Carbon-14 all over the globe. Carbon-14 combines with oxygen in the atmosphere to form CO 2 . This results in a mixture in the atmosphere of CO 2 formed with normal, non-radioactive Carbon-12 or Carbon-13, and CO 2 formed with Carbon-14. This CO 2 is then used by plants such as wheat and eaten by animals such as cattle. When we eat crops or livestock, the mixture of Carbon-12, Carbon-13 and Carbon-14 becomes part of our cells, and most importantly, part of the DNA formed when a new cell is born. Since the DNA is not replaced over the life of a cell, the Carbon-14 in a cell’s DNA when the cell is born is pretty much the Carbon-14 it will always have. Since we know how much Carbon-14 was in the atmosphere before nuclear testing, we know how much was in the air during the testing years, and we know how it was eliminated from the atmosphere after the Nuclear Test Ban Treaty outlawed above ground testing in 1963, it’s possible to estimate the turnover of cells.

For example, if a person born just before nuclear testing shows no Carbon-14 from the fallout years in his cerebral cortex neurons, that suggests that no cerebral cortex neuron cells were added after birth. If any new cells had been formed, they would have incorporated Carbon-14 into their DNA. If, on the other hand, a person born right at the peak of the fallout years shows little or no fallout Carbon-14 in his cerebral cortex cells, that would suggest that all the cerebral cortex neuron cells had been replaced. They would have incorporated non-radioactive carbon into their new DNA relatively recently, after most of the Carbon-14 had been washed out of the atmosphere. Otherwise most of them would have some Carbon-14 still in the DNA from when the person was born during the height of the Cold War.

This is a very much simplified version of what a team lead by Dr. Jonas Frisén at the Department of Cell and Molecular Biology at Karolinska Institute in Sweden has been doing. It is their studies that produced the estimates for turnover of cerebral cortex neurons, fat cells, and cardiomyocytes given above.

By the way, Dr. Frisén is very interested in tracking down the origin of the “We replace every cell every 7 or years” myth. If any readers have information on where they heard or read this idea, leave a comment on this page by clicking below and I’ll forward your information to Dr. Frisén.



Update: An article about Dr. Frisén’s continuing research appeared in New Scientist on June 10, 2013: http://www.newscientist.com/article/dn23665-nuclear-bomb-tests-reveal-brain-regeneration-in-humans.html

Sources:

Evidence for Cardiomyocyte Renewal in Humans. Olaf Bergmann, Ratan D. Bhardwaj, Samuel Bernard, Sofia Zdunek, Fanie Barnabé-Heider, Stuart Walsh, Joel Zupicich, Kanar Alkass, Bruce A. Buchholz, Henrik Druid, Stefan Jovinge, and Jonas Frisén. (3 April 2009) Science 324 (5923), 98.

Dynamics of fat cell turnover in humans. Spalding KL, Arner E, Westermark PO, Bernard S, Buchholz BA, Bergmann O, Blomqvist L, Hoffstedt J, Näslund E, Britton T, Concha H, Hassan M, Rydén M, Frisén J, Arner P. Nature. 2008 Jun 5;453(7196):783-7.[cite]