Revised Estimates for the Number of Human and Bacteria Cells in the Body

The study asks: How many human and bacterial cells are in the human body, and what is the true ratio between them? Prior claims (e.g., 10^14–10^15 bacteria and a 10:1 bacteria-to-human cell ratio) largely derive from a decades-old back-of-the-envelope estimate and lack transparent calculations or uncertainty bounds. The authors aim to critically revisit and update these estimates for a standard 70 kg adult man, documenting assumptions, methods, and uncertainties. They focus on recalculating bacterial counts by organ—highlighting the colon’s dominance—and systematically reassessing human cell counts by cell type, then reevaluating the bacteria-to-human (B/H) cell ratio and its variability across population segments (sex, age, obesity). The work emphasizes rigorous, transparent quantitative reasoning to replace widely repeated but weakly supported figures.

The paper traces the origin of prevalent claims about bacterial numbers to a single back-of-the-envelope calculation from the 1970s that assumed 10^11 bacteria per gram of gut content over a full liter of alimentary tract volume. Subsequent literature often cited values of 10^14–10^15 body bacteria and a 10:1 or 100:1 B/H ratio without robust derivations. The authors survey measured bacterial concentrations across body sites, showing the colon overwhelmingly dominates bacterial abundance, while other sites (skin, saliva, stomach, small intestine) contribute ≤10^12 bacteria. For human cells, prior broad estimates (10^12–10^14) and a detailed census (Bianconi et al., 2013; ~3.7×10^13 cells) are reviewed, with the current analysis revisiting key high-count cell types (RBCs, glia, endothelial cells, dermal fibroblasts) to correct overestimates and refine totals.

- Bacterial counts: The authors compile organ-specific bacterial concentrations and volumes to bound total bacterial numbers, showing the colon is the dominant reservoir. Colon content volume is estimated at ~0.4 L for a 70 kg man, integrating MRI-based measurements (height-standardized inner volume, gas correction, and stool fraction) and earlier physiological data, with SEM ~17% and CV ~25%. Bacterial concentration in the colon is estimated from stool sample literature (14 studies), converting dry-stool counts to wet-stool basis using reported or average dry matter fractions; the geometric mean is 0.92×10^11 bacteria/g wet stool (SEM ~19%, average CV ~46%). Total colon bacteria = concentration × mass of colon content (~0.4 kg), giving ~3.8×10^13 bacteria. Contributions from other organs are negligible by comparison (<10^12 overall). - Bacterial mass: Using colon content (~0.4 kg) and bacteria mass fraction (~50%), total bacterial mass ~0.2 kg (dry mass ~50–100 g), corroborated by average gut bacterium wet mass (~5 pg) times cell count. - Human cell counts: Two approaches are contrasted: (1) a naïve mass/representative-cell approach giving 10^13–10^14 cells, and (2) a census by cell type (building on Bianconi et al., 2013). The authors focus on six dominant contributors (~97% of cells): RBCs, glia, endothelial cells, dermal fibroblasts, platelets, and bone marrow cells, and revise key categories using updated physiological and histological data (details in Box 4, S1 Appendix). Revisions notably reduce glia, endothelial, and dermal fibroblast counts to ~0.9×10^12 (vs. prior ~7.5×10^12). Summing categories yields ~3.0×10^13 human cells, with RBCs and platelets comprising ~90% of total cells and non-blood nucleated cells ~3×10^12 (~10%). - Mass-centered sanity check: The authors verify cell-count-derived masses against body composition. For a 70 kg man, with ~25% extracellular fluid and ~7% extracellular solids, expected cellular mass is ~46 kg. Tissue mass distributions (Reference Man data) and intracellular fractions (inferred from total body potassium) show muscle and adipose dominate cellular mass (~75%) despite forming a tiny fraction of total cell count, while RBCs dominate cell numbers but contribute little to mass. The mass balance supports the plausibility of the revised counts. - B/H ratio estimation across populations: Using key parameters (colon volume, colon bacterial density, blood volume, hematocrit), the study computes B/H for other groups (adult women, infants, elderly, obese). Literature indicates similar colon bacterial concentrations across ages (after the first month) and no strong gender effects. Resulting B/H ratios vary ~1.3–2.3 across groups, driven mainly by blood and colon volume differences.

- Total bacteria in a 70 kg adult man: ~3.8×10^13 cells (SEM ~25%, population SD ~52%). - Total human cells: ~3.0×10^13 cells, with ~90% from hematopoietic lineage; RBCs alone account for ~84% of human cells; platelets add ~5%. - Non-blood nucleated human cells: ~3×10^12 (~10% of total cell count). - Bacteria-to-human cell ratio (B/H): ~1.3:1 for a 70 kg adult man, replacing the widely cited 10:1 figure; when comparing bacteria to nucleated human cells only (~0.3×10^13), the ratio is ~10:1 by definition choice. - Colon content volume: ~0.4 L for a reference man (supported by MRI-based estimates and fecal transit data). - Colon bacterial concentration: geometric mean ~0.92×10^11 bacteria/g wet stool (SEM ~19%, average CV ~46%). - Total bacterial mass: ~0.2 kg wet (~0.3% of body mass), revising previous claims of 1–3 kg (1–3%). - Cell number vs mass discordance: RBCs dominate by count but contribute little to mass; muscle and adipocytes constitute ~75% of cellular mass but only ~0.2% of cell number. - B/H ratio across populations: varies ~1.3–2.3 (reference woman ~2.2, infants ~1.7–2.3, elderly ~1.8, obese ~1.4).

The study resolves long-standing, poorly supported claims by providing transparent, data-driven estimates of human and bacterial cell counts. By focusing on colon-dominant bacterial loads and revising key human cell-type counts, the authors show that bacteria and human cells are of similar order in number, overturning the commonly quoted 10:1 ratio. The updated B/H ratio (~1.3) reflects improved estimates of both bacterial counts (dominated by colon content volume and concentration) and human cell counts (dominated by RBCs and platelets). The mass-centered validation supports the internal consistency of counts. These results refine quantitative baselines in human biology and microbiome studies, emphasizing that accurate absolute numbers are important for rigorous interpretation in fields such as microbiome research, cancer risk modeling by tissue stem cell divisions, and developmental dynamics. The work also highlights critical knowledge gaps (e.g., mapping in situ colon bacterial densities and accurate colon content volumes across conditions), guiding future measurement priorities.

The paper provides revised, well-documented estimates: ~3.8×10^13 bacterial cells and ~3.0×10^13 human cells in a 70 kg adult man, yielding a B/H ratio of ~1.3 and a total bacterial mass of ~0.2 kg. It corrects the widespread 10:1 narrative and demonstrates the dominance of hematopoietic cells in total human cell count, contrasted with muscle and adipose dominance in cellular mass. The approach integrates organ-specific concentrations, updated anatomical measurements, and a mass-balance sanity check. Future research should prioritize direct measurements of bacterial densities along the colon (not just in stool), improved characterization of colon content volume variability across individuals and conditions, and extensions to diverse population segments (e.g., during antibiotics, bowel prep, infections, or GI diseases).

- Reliance on stool measurements as a proxy for colon bacterial density may introduce systematic biases due to spatial gradients, water absorption, bacterial growth during transit, and mucosal shedding; fecal-based estimates might be upper bounds for average colonic density. - Limited precision on colon content volume across individuals and states; MRI-based estimates carry variability and may not capture all conditions. - Potential unquantified systematic errors beyond reported uncertainty ranges (SEM, CV). - Scope excludes many conditions that could alter counts (e.g., antibiotics, bowel prep, infections, chronic GI diseases). - Some definitional ambiguities (e.g., inclusion of enucleated cells like RBCs and platelets) affect the interpretation of the B/H ratio when comparing to nucleated-only counts.