Revised Estimates for the Number of Human and Bacteria Cells in the Body.
Level 5 - mechanism / opinion, no new human data
Level 5 by design analogy: quantitative estimation and secondary mathematical modeling from existing literature, not direct human clinical trial or cohort data.
PubMed 27541692 · doi:10.1371/journal.pbio.1002533
What was done
The authors synthesized literature data and performed updated mathematical calculations to estimate the total number of human and bacterial cells, as well as total bacterial mass, in a standardized 70 kg "reference man."
What was found
The total count of bacteria in a 70 kg reference man was estimated at 3.8 × 10¹³, with a total bacterial mass of approximately 0.2 kg. Total human cells were calculated at 3.0 × 10¹³, with the hematopoietic lineage accounting for ≈90% of the total human cell count. These values update the widely cited 10:1 ratio to approximately 1:1, indicating bacterial and human cell counts are within the same order of magnitude.
Why it matters
It quantitatively refutes the pervasive 10:1 bacterial-to-human cell dogma and establishes a standardized, evidence-grounded baseline for microbiome and human physiology research.
Limits
The findings rely on secondary literature synthesis and mathematical estimation rather than direct whole-body empirical measurements. Estimates are based on a theoretical 70 kg adult male baseline, and the abstract does not report ranges across different sexes, ages, body compositions, or health states.
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