The 10:1 ratio was wrong. What that changes.

For forty years a number circulated through conferences, popular articles and textbooks: you carry ten times more bacterial cells than human cells. You are, at ninety per cent, somebody else. It was wrong. And the way it became wrong is worth telling.

Where the number came from

The 10:1 ratio goes back to an estimate published in the 1970s. It rested on reasoning that was sensible for its time: an approximate gut volume, a plausible bacterial density, a multiplication. The author himself presented it as an order of magnitude, not as a measurement.

Then the number was cited. Then the citation was cited. Three decades later it appeared in the introductions of peer-reviewed papers without anyone having gone back to the source to check what it actually said. This is an ordinary phenomenon in science — it is sometimes called citation by hearsay — and it implies no bad faith. Simply, nobody had redone the arithmetic.

What the 2016 revision showed

In 2016 Ron Sender, Shai Fuchs and Ron Milo redid the calculation. Properly this time: reconstructing the volumes of the different digestive compartments from imaging data, using measured rather than assumed bacterial densities, and estimating human cell numbers type by type — with a discovery along the way, which is that red blood cells alone account for the overwhelming majority of your human cells.

Their conclusion: the ratio is about 1.3 to 1. In other words, the same order of magnitude. For a reference 70 kg man, roughly 3.8 × 10¹³ bacterial cells to 3.0 × 10¹³ human ones. And the uncertainty is such that a single visit to the toilet meaningfully changes the ratio.

What it does not change

Here one has to resist the opposite temptation. The fact that you are not “ninety per cent bacterial” does not make the microbiota less important. Two things remain entirely true.

• The genetic weight is overwhelming. Your gut microbiota carries between one hundred and one thousand times more genes than your human genome. In metabolic capacity — which molecules can be made, broken down, transformed — you are very much the minority shareholder in your own body.
• Function is not counted in cells. Intestinal bacteria produce short-chain fatty acids that feed your colonocytes, modulate your immune system, and participate in signalling pathways that reach the central nervous system. A partner does not need to be in the majority to be structural.

Why the story interests me more than the number

I spent seventeen years in regulatory science, where the entire discipline consists of never accepting a causal claim without tracing its mechanism. It is tedious. It is also the only thing that protects you from building a whole line of reasoning on a 1972 estimate that nobody reopened.

The wellness field is saturated with claims of exactly this type — repeated, amplified, never traced back. When I tell you something about your biology, you are entitled to ask me where it comes from. That is the right question, and it is the one I would ask.

• Sender R, Fuchs S, Milo R — Revised estimates for the number of human and bacteria cells in the body. PLoS Biology 14:e1002533, 2016.