Ruining fake facts: No, you do not have nine non-human cells for every human cell in your body!

A “funfact” that seem to circulate the internet pretty often is the myth that you have 9 non-human cells for every human cell, which would put you at 90 % foreign cells in your body. This myth has been around for a long time, and I even saw it on a lecture at my school once, but this has turned out to be completely false.

When we are talking about non-humans cells, we are talking about bacteria and fungi cells that are living in your body. These are mostly located in your guts and intestines, but are found in various numbers all over your body. However, they are for sure not outnumbering your own humans cells but a ratio of 9:1.

Where did the myth come from?

If you search for something along the lines of “non-human biomass in the human body”, you will get thousands of websites that tell you about this fact. However, after doing some digging into it, it seems that this myth originally came along in a paper from 1972. The number came from a short mathematical calculation at the back of an envelope where estimations were used. The estimations were extremely uncertain, and pretty much just an educated guess at best. This is not very scientific at all, but both scientists and regular people started to use this number after it became “common knowledge”.

The myth is false

A group of scientists from the Weizmann Institute of Science in Rehovot, Israel, and the Hospital for Sick Children in Toronto decided to test this myth, and the complete results can be viewed from the source page down below. Their findings showed that a 70kg male would have on average 30 trillion humans cells, and about 40 trillion bacterial cells. So we are still outnumbered by the foreign cells, but not to the degree most people think.

A photo of Escherichia coli, a bacterial cell you got lots of in your body. Image by Eric Erbe, posted as Public Domain.

This research has an error margin of 25 %, so it is much more accurate than the estimates of the original claim that really didn’t have any good sources to back up their numbers. This leaves us with a bacterial to human ratio of 1.3:1 on average.

These average numbers could of course still be off by a pretty long shot, due to the fact that people in different parts of the world have a huge difference in their bacterial cells. The number of human cells also varies heavily, and the researchers speculate that some people might even have more human cells than bacterial cells.

Except for the original research team, no one else has tried to calculate the experiment themselves to check if these numbers are more accurate, but microbiologists generally agree that this research is a lot better than the old 1:10 “fact” that used to be common knowledge.

Thanks for reading & sources

Thanks for reading this short post where I tried to debunk a “common knowledge” myth that has been sticking around for many years now. The original research paper I used for this post can be read by clicking here. It is actually pretty short, so I suggest to have a look at it if you got 20 minutes to spare.

About the author

Hi, I’m @valth! I live in Norway with my girlfriend, our newborn son, and our two dogs, one of which is seen wearing a bow tie in the profile picture!

I am very passionate about nature and biology, and have been studying ecology for a few years now. My passions are mostly within conservation biology, mycology (the studies of mushrooms), animal behavior and general microbiology. I really enjoy both the theoretical aspect, as well as the more practical aspect of biology, and I spend about as much time in front of biology textbooks as I do spend on finding and identifying plant, mushroom and animal species in the forests.

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