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Hank: Maybe you’ve read it on some online list of “Amazing Facts About Your Body.” Or you might’ve heard Tim from the IT department drop it at an office party when he was trying to sound all smart. The claim? The human body replaces itself, at the cellular level, every seven years. It’s kind of an appealing thought, you could have a fresh start, every seven years, right down to your cells. But it is true? Nah.

I mean, yes, many cells do replace themselves over time. But this “brand new body” thing is just one of those internet myths. Before we get into why it’s not really scientifically accurate, let’s do a quick anatomy review.

Your cells are your body’s most basic building material, and you’ve got trillions of them. You’ve got about 200 different types of cells, each specialized for different purposes. Groups of similar cells come together to form different kinds of tissues, which combine to form organs. All of these cells work together to keep you alive, but some duties are harder on individual cells than others. So cells are dying and being replaced all the time, but just how quickly can vary a lot, depending on where they are in your body, and what they do.

Take the cells that line your stomach and intestines. Their job is not easy, getting constantly battered by chicken wings and doused in stomach acid, they typically only last for a couple of days before they end up in the toilet. Your skin cells have it pretty rough and tumble job too, acting as your body’s first line of defense against pathogens. So they’re pretty much constantly being shed, but are fully replaced every few weeks.

Red blood cells are incapable of healing themselves because they don’t have DNA, so they last about four months or so, before all that oxygen-carrying and constant circulation in the bloodstream finally wears them out. Some white blood cells can also live for only a couple of months. While others, like your neutrophils, are some of the first responders to fight infections and only last hours. Their dead bodies form that pus on your skinned knee. And your bone cells are constantly being replaced, but your skeleton, which is not made of cells, but is only built by your cells, takes about ten years to get a full remodel.

But other cells stick around for a while, like most of your muscle cells. In fact, your cardiomyocytes, or heart muscle cells, replace themselves incredibly slowly, like 1% per year after you’re 20, and even more slowly as you get older. By the end of a long life, you’ll probably still have more than half of the cells you had when you were born.

Then, there are the cells that are never replaced, what you initially grow is all you will ever get. For example, biosex females are born with all the oocytes, or egg cells, that they will ever have. And male or female, and you can’t make more tooth enamel, either. Certain neurons, like those in the cerebral cortex, are also irreplaceable, which was one of the reasons why we can’t reverse diseases like Alzheimer’s and Parkinson’s.

So how do we even know all of this? It seems like it might be hard to pin down the life expectancy of any specific cell type. Well, it turns out, we know because of war. Nuclear weapon detonations during the Cold War released a whole bunch of radioactive material into the atmosphere, in a form of carbon called Carbon-14. This carbon-14 combines with oxygen to make carbon dioxide, which gets taken up into all sorts of living tissues, including your own cells and DNA. And since a cell doesn’t replace its DNA at all during its lifetime, its Carbon-14 levels remain the same from creation to death.

So, this allowed researchers in Sweden to discover that we can actually determine the age of a certain cell by measuring its Carbon-14 concentration. After all, we know how much Carbon-14 was in the atmosphere before these nuclear detonations, and we know levels have been declining since then.

So for example, if your grandma was born before the nuclear testing began, DNA in her cerebral cortex neurons don’t have any Carbon-14, we know those cells are old and haven’t been replaced. On the other hand, her bone, skin, and blood cells will all show different levels of Carbon-14, which can give a good estimate as to how often those particular cells are replenished. So... I guess... like thanks, nuclear testing? At least, it has helped us debunk another myth about the human body.

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