The popular claim that your gut flora outnumbers your own cells 10 to 1 traces back to a rough, one-line estimate in a 1972 paper. A 2016 recount puts the ratio closer to 1.3 to 1, with about 38 trillion gut bacteria to 30 trillion human cells. That's so close that a single trip to the loo, which clears out up to a third of your colon's contents, can briefly tip the balance in favour of human cells.
Source: one
