Abstract
Reported values in the literature on the number of cells in the body differ by orders of magnitude and are very seldom supported by any measurements or calculations. Here, we integrate the most up-to-date information on the number of human and bacterial cells in the body. We estimate the total number of bacteria in the 70 kg "reference man" to be 3.8·1013. For human cells, we identify the dominant role of the hematopoietic lineage to the total count (≈90%) and revise past estimates to 3.0·1013 human cells. 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.
Highlights
How many cells are there in the human body? Beyond order of magnitude statements that give no primary reference or uncertainty estimates, very few detailed estimates have been performed
Beyond providing up-to-date estimates on the average values of the number of cells, we aimed to give representative uncertainty ranges and the variation across population segments. This is based on comparing independent studies and the variation observed within studies
The biggest knowledge gap we find is how realistic is the usage of the measured fecal bacteria density to represent the average bacteria density in the colon
Summary
How many cells are there in the human body? Beyond order of magnitude statements that give no primary reference or uncertainty estimates, very few detailed estimates have been performed (the one exception [1] is discussed below). By updating the cell counts in the body, we revisit the 10:1 value that has been so thoroughly repeated as to achieve the status of an established common knowledge fact [4]. This ratio was criticized recently in a letter to the journal Microbe [5], but an alternative detailed estimate that will give concrete values and estimate the uncertainty range is needed. We repeat and reflect on the assumptions in previous back-of-the-envelope calculations, known as Fermi problems We find such estimates as effective sanity checks and a way to improve our quantitative understanding in biology
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