Here’s how many of your body’s cells aren’t even human

If you’ve ever read about the colonies of bacteria that live on and in you, you’ve probably come across the “fact” that there are about 10 times as many microbial cells as human cells in your body.

It’s written about in scientific papers, magazine articles, TED talks, and popular science books. It’s not true, but it does a good job of showing how important bacteria are to humans.

In 2016, a review of more than 40 years of research into the human microbiome found that this often-cited fact is not supported by any scientific evidence.

Instead, it looks like the ratio is about 1.3 to 1, and the average person is home to around 100 trillion microbes, give or take. But that’s not the whole story, either.

To find out the real number, a group of biologists from the Weizmann Institute of Science led by Ron Milo set out to read everything written about the populations of microbes that live inside us.

They found that in a man between 20 and 30 years old, with a weight of about 70 kg (154 pounds) and a height of 170 cm (about 5’7″), there would be about 39 trillion bacterial cells living among 30 trillion human cells. They call this man the “reference man.”

This means that there are about 1.3 times as many microbes as people.

So where did the 10:1 ratio come from, and why did Milo and his team have to be so specific about a “reference man” instead of just coming up with a ratio for the average person?

The 10:1 ratio comes from a paper written by American microbiologist Thomas D. Luckey in 1970. In the paper, Luckey wrote that he thought there were 100 billion microbes in a gram of human intestinal fluid or feces.

He said that because the average adult has about 1,000 grams of these things, that adds up to 100 trillion microbes. (Keep in mind that none of these numbers are backed up by any scientific evidence, as Ed Yong of The Atlantic has pointed out.)

Seven years later, a well-known microbiologist named Dwayne Savage took this rough estimate and combined it with the fact that the average human body has about 10 trillion cells to come up with the 10:1 ratio.

Everyone, from scientists to the general public, ran with that fact, and it wasn’t until 2014 that someone tried to check if it was true.

In a letter to Microbe Magazine, Judah L. Rosner, a molecular biologist and geneticist from the National Institutes of Health, said that more recent estimates of the number of human cells were nowhere near 10 trillion.

Ed Yong explains that it would be almost impossible to get a cell count for the average person:

He pointed out that more recent estimates put the number of human cells anywhere from 15 trillion to 724 trillion, and the number of microbes in the gut anywhere from 30 trillion to 400 trillion. Which gives a ratio that is best written as ¯_(ツ)_/¯.

This is where the reference man comes in. Narrowing down the gender, age, weight, and height of a hypothetical person made it much easier for the reviewers to figure out what the average number of human cells would be.

The researchers think that the number is about 30 trillion for their reference man.

Milo and his team also found that the number of microbe cells in the colon, which is where Luckey got his original number, has often been overestimated in the scientific literature.

Lindsey Kratochwill of Popular Science said, “When previous studies made their estimates, they used the number of bacteria per gram of “wet content” of the colon times the size of the entire digestive tract.”

But, these researchers say, the number of bacteria in the colon is much higher than in the rest of the digestive tract. Assuming that all of the digestive tract is as full of bacteria as the colon is too much.

Taking into account this and the fact that we have a much higher concentration of bacteria in our guts than in other organs and body parts (which means you can’t take a sample from the colon and say it’s representative of the whole body like Luckey), Milo’s team came up with an updated and more scientifically accurate estimate of 39 trillion microbial cells.

But Ed Yong says that even the 1.3:1 ratio of 39 trillion microbial cells to 30 trillion human cells isn’t something that should be used in textbooks or scientific papers in the future.

“These new estimates may be the best we have right now, but the studies and numbers that Milo put together have their own biases and unknowns,” he says.

I’d rather you didn’t talk about ratios at all; you don’t need to to show how important the microbiome is.