Over half of all the cells in our bodies are non-human. This amounts to about 0.3% by mass (roughly half a pound).
The human body needs a substantial amount of bacteria and other micro-organisms to aid in digestion, immunity, and other vital functions required for it to sustain itself. But how much is needed?
Put differently, how much of our self is not ourself?
Human cells make up only 43% of the body’s total cell count. The rest are microscopic colonists.
Does the statement above mean that 57% of the mass of a typical human body is non-human? No. Because not all cells are created equal.
Our microbiome consists of bacteria, viruses, fungi and archaea (bacteria-like organisms). These organisms are very small, but very abundant.
What about the mass?
In a study published in 2012, by the National Institute of Health in the US, there was an estimate that the mass of all non-human cells added up to 1 to 3% of the human body. This was extrapolated from their conclusion that there was only one human cell for every 10 non-human cell.
The average mass for a North American human is around 80kg, so that estimate would mean that the average person is made up of about 1- 2.5 kg (or 2 to 5 lb) of foreign organisms.
In 2016, the US National Library of Medicine published a revised estimate by the NIH that is considered more accurate than the earlier assumption. This is where the 43/57% number above comes from.
Our analysis also updates the widely-cited 10:1 ratio, showing that the number of bacteria in the body is actually of the same order as the number of human cells, and their total mass is about 0.2 kg.
So, 0.2 kg (about half a pound) seems to be the best estimate available for how much of the human body is made up of other organisms.
So next time you buy a half a pound of butter or 0.2 kg of potatoes, give a thought to those other organisms along for the ride, helping (for the most part) keep you alive.