Bet you've heard this amazing fact before: the human body replaces itself, at the cellular level, every seven years. How awesome are cool science facts! Unfortunately, cool science facts are often not really facts, which doesn't make them science, and that means they're definitely not cool. But why is this fun fact so wrong?
Like many fun 'facts' this one is based on reality, but it's oversimplified it to the point of it being a total myth that sounds about right, and that's why we keep repeating it to each other.
Remember that fun fact about our bodies containing 10 times the number of bacteria cells as human cells? Nope, it's wrong.
That myth has been circulating since the 1970s, and is based on a strange, dodgy fact that was thrown into a scientific paper, and scientists kept citing it in their own papers until very recently. Today's Sad Truth #1: Not everything you read in a peer-reviewed scientific paper is fact.
As Ed Yong reported over at The Atlantic earlier this year:
"More recent estimates ... put the total number of human cells at anywhere from 15 trillion to 724 trillion, and the number of gut microbes at anywhere between 30 trillion and 400 trillion. Which gives a ratio that can best be expressed as ¯\_(ツ)_/¯."
So back to that 'whole new body' factoid. As the Sci Show video above explains, it's based on actually reality, because our bodies do constantly experience bouts of cell die-off and regeneration. It's just that some organs work harder than others, so the rate of regeneration changes depending on the cells' function.
Here's a fun fact for you: the cells that line your stomach and intestines only get to do their job - i.e. getting doused in chicken wings and stomach acid - for a few days before they end up in your toilet bowl along with those chicken wings.
Your skin cells are being cycled through every few days, but are fully replaced every few weeks - maybe this time with no pimples? And speaking of gross infected things, white blood cells called neutrophils are some of the first responders of your immune system, and they only last a few hours before dying and ending up as puss on your wound. Nice.
Take good care of your bones though - you have to live with skeleton you've got for at least 10 years before you get a total regeneration.
Now here's Sad Truth #2: a bunch of cell types are constantly getting replaced, but there are also a handful that never get replaced. Yep, what you're born with is what you get. FOREVER. I'll let Hank from Sci Show explain which cells those are above, but let's just say, if you're not flossing, you'd better get on that.