As discussed below, relatively modest quantitative differences apply for women due to changes in characteristic body mass, blood volume, and the genital microbiota. We find such estimates as effective sanity checks and a way to improve our quantitative understanding in biology.Ī major part of the available literature used in the derivation of human cell numbers was based on cohorts of exclusively or mostly men, and as we use these sources, our analysis starts with adult men. Doing so, we repeat and reflect on the assumptions in previous back-of-the-envelope calculations, also known as Fermi problems. Here, we provide an account of the methodologies employed hitherto for cell count and revise past estimates. This ratio was criticized recently in a letter to the journal Microbe, but an alternative detailed estimate that will give concrete values and estimate the uncertainty range is needed. By updating the cell counts in the body, we also revisit the 10:1 value that has been so thoroughly repeated as to achieve the status of an established common knowledge fact.
We give up-to-date detailed estimates where the calculation logic and sources are fully documented and uncertainty ranges are derived. The aim of this study is to critically revisit former estimates for the number of human and bacterial cells in the human body. Similarly, the ubiquitous statements regarding 10 14–10 15 bacteria residing in our body trace back to an old back-of-the-envelope calculation. 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 is discussed below).