Another myth about the human body busted…

Another myth about the human body busted…

For years we have been thinking that we are only 10% human, as our own cells were reportedly greatly outnumbered by bacteria. However, newer calculations suggest we are likely 50% human – and that actually “each defecation event may flip the ratio to favour human cells over bacteria”.

How comes we have been wrong about such an important thing for such a long time?

It is in fact quite difficult to establish reliable numbers for the bacterial load in our body.  A particular overestimate in the early works relates to the proportion of bacteria in our gut. The study on which most subsequent calculations were based estimated that our intestines contain around 1014 bacteria, by assuming that there were 1011 bacteria in a gram of faeces, and scaling that up by the one-litre volume of the alimentary tract. But among the sections of the alimentary tract only the colon contains high numbers of bacteria. The new reviewed data indicate that there are about 2.5 times less bacteria than stated in early work (and re-re-re-cited many times).

Another uncertainty factor is indeed also the number of human cells in the body. The updated calculations suggest that there are about 3 times more human cells than suggested by previous calculations. As cell types vary greatly in size and density, we cannot simply extrapolate from cells found in blood to the various other cell types found in the body. Most of our cells are red blood cells, while fat and muscle cells account for only 0.1% of cells but 75% of our body mass.

“Should we care about the absolute number of human cells in the body or the ratio of bacterial to human cells?”

Updating the ratio of bacteria to human cells from 10:1 to 1:1 does not take away from the biological importance of our microbiota. The new numbers will not revolutionize our way of thinking about the human body, or not tremendously bring forward our current research. Rather, we can agree with the authors of the study, saying that “In performing these kinds of calculations, we become intimately familiar with the limits of our current understanding”.
Reference: Sender, R., Fuchs, S. & Milo, R. (2015)

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