Are there more bacteria than human cells in your body?

You may have heard that you are only 10 percent human, because bacteria outnumber your own cells ten to one. It is a fantastic line. It appeared in textbooks, documentaries and countless articles. There is just one problem: when scientists finally checked the arithmetic, it fell apart.

The short answer

Bacteria and human cells are roughly even. In 2016, researchers Ron Sender and Ron Milo of the Weizmann Institute of Science in Israel, with Shai Fuchs of the Hospital for Sick Children in Toronto, estimated that a reference adult man carries about 39 trillion bacteria and about 30 trillion human cells. That is roughly 1.3 bacteria for every human cell.

The real story

The famous ten-to-one figure traces back to a rough estimate made in 1972 by the microbiologist Thomas Luckey. It was a back-of-the-envelope calculation that was probably never meant to be quoted for decades, but it took on a life of its own.

Sender, Fuchs and Milo went back to first principles. Their reference man was 70 kilograms, 20 to 30 years old and 1.7 metres tall. They noted that most of the body’s bacteria live in the colon, which holds only around 0.4 litres, rather than being spread through the whole digestive tract. Earlier estimates had effectively overcounted the bacteria by assuming far more space for them. Their paper, published in the journal Cell in January 2016, concluded that the ratio is much closer to one to one.

The human side of the count holds its own surprise. The vast majority of your cells by number are red blood cells, which are tiny and extremely numerous.

And then there is the twist that made headlines. Because so many of the bacteria live in the colon, the authors pointed out that a single trip to the toilet may shift the balance enough to flip the ratio in favour of human cells, at least for a while. In other words, depending on the time of day, you might be slightly more you, or slightly more them.

The researchers stressed that these are estimates with real uncertainty. Individual people could have roughly half or twice as many bacteria. But the headline stands: nobody is outnumbered ten to one.

Why people get it wrong

The ten-to-one claim is a perfect fact for repeating. It is simple, startling and makes a great opening line, so it travelled from one source to the next without anyone going back to check where it came from. That is how scientific folklore works: a rough estimate gets quoted, then quoted again, and eventually feels like settled fact. The real number is less dramatic, but it is still remarkable. You are carrying around tens of trillions of bacteria, and they are very nearly a match for you.

The Sole sums it up: “You’re not ten percent human. You’re about half. And one bathroom break can tip the vote.”