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

Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS Biol 14(8): e1002533. doi:10.1371/journal.pbio.1002533

Published: August 19, 2016

Copyright: © 2016 Sender et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This work was funded by the European Research Council (Project NOVCARBFIX 646827, https://erc.europa.eu/funding-and-grants); Dana and Yossie Hollander; Helmsley Charitable Foundation; The Larson Charitable Foundation; The Estate of David Arthur Barton; The Anthony Stalbow Charitable Trust, and Stella Gelerman, Canada. RM is the Charles and Louise Gartner professional chair and an EMBO young investigator program member. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.

Abbreviations: CV, coefficient of variation; FISH, fluorescent in situ hybridization; SD, standard deviation; SA, surface area; SEM, Standard Error of the Mean.

object_of_studyphenomenonmechanisms↗claim_relationscontent_tagsReported 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(cid:1)1013. For human cells, we identify the dominant role of the hematopoietic lineage to the total count ((cid:3)90%) and revise past estimates to 3.0(cid:1)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.

Introduction

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). Similarly, the ubiquitous statements regarding 1014–1015 bacteria residing in our body trace back to an old back-of-the-envelope calculation [2–4].

contributionconcepts↗content_tags↗The aim of this study is to critically revisit former estimates for the number of human and bacterial cells in the human body. We give up-to-date detailed estimates where the calculation logic and sources are fully documented and uncertainty ranges are derived. research_questionclaim_typecontent_tagsBy 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 [4]. claim_relationscontent_tags↗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. Here, we provide an account of the methodologies employed hitherto for cell count and revise past estimates. method↗transfersconcepts↗concepts↗Doing so, we repeat and reflect on the assumptions in previous back-of-the-envelope calculations, also known as Fermi problems. We find such estimates as effective sanity checks and a way to improve our quantitative understanding in biology.

limitationsA 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

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with adult men. As discussed below, relatively modest quantitative differences apply for women due to changes in characteristic body mass, blood volume, and the genital microbiota. limitationsconcepts↗claim_relationscontent_tags↗content_tagsFor our analysis, we used the definition of the standard reference man as given in the literature [6] as: "Reference Man is defined as being between 20–30 years of age, weighing 70 kg, is 170 cm in height.” Our analysis revisits the estimates for the number of microbial cells, human cells, and their ratio in the body of such a standard man.

We begin our analysis by revisiting the number of bacteria through surveying earlier

sources, comparing counts in different body organs and finally focusing on the content of the colon. We then estimate the total number of human cells in the body, comparing calculations using a "representative" cell size to aggregation by cell type. We then contrast the cell number distribution by tissue type to the mass distribution. In closing, we revisit the ratio of bacterial to human cells and evaluate the effect of gender, age, and obesity.

Results

Origin of Prevalent Claims in the Literature on the Number of Bacterial Cells in Humans

Microbes are found throughout the human body, mainly on the external and internal surfaces, including the gastrointestinal tract, skin, saliva, oral mucosa, and conjunctiva. limitationscontent_tags↗Bacteria overwhelmingly outnumber eukaryotes and archaea in the human microbiome by 2–3 orders of magnitude [7,8]. We therefore sometimes operationally refer to the microbial cells in the human body as bacteria. The diversity in locations where microbes reside in the body makes estimating their overall number daunting. Yet, once their quantitative distribution shows the dominance of the colon as discussed below, the problem becomes much simpler. The vast majority of the bacteria reside in the colon, with previous estimates of about 1014 bacteria [2], followed by the skin, which is estimated to harbor ~1012 bacteria [9].

As we showed recently [4], all papers regarding the number of bacteria in the human gastrointestinal tract that gave reference to the value stated could be traced to a single back-of-theenvelope estimate [3]. That order of magnitude estimate was made by assuming 1011 bacteria per gram of gut content and multiplying it by 1 liter (or about 1 kg) of alimentary tract capacity. To get a revised estimate for the overall number of bacteria in the human body, we first discuss the quantitative distribution of bacteria in the human body. After showing the dominance of gut bacteria, we revisit estimates of the total number of bacteria in the human body.

Distribution of Bacteria in Different Human Organs

Table 1 shows typical order of magnitude estimates for the number of bacteria that reside in different organs in the human body. limitationscontrolsconcepts↗content_tags↗The estimates are based on multiplying measured concentrations of bacteria by the volume of each organ [9,10]. Values are rounded up to give an order of magnitude upper bound.

Although the bacterial concentrations in the saliva and dental plaque are high, because of their small volume the overall numbers of bacteria in the mouth are less than 1% of the colon bacteria number. The concentration of bacteria in the stomach and the upper 2/3 of the small intestine (duodenum and jejunum) is only 103–104 bacteria/mL, owing to the relatively low pH of the stomach and the fast flow of the content through the stomach and the small intestine [10]. mechanisms↗concepts↗concepts↗concepts↗concept_edgesconcept_edgescontent_tagsTable 1 reveals that the bacterial content of the colon exceeds all other organs by at least two orders of magnitude. Importantly, within the alimentary tract, the colon is the only substantial contributor to the total bacterial population, while the stomach and small intestine make negligible contributions.

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Table 1. Bounds for bacteria number in different organs, derived from bacterial concentrations and volume.

Location

Colon (large intestine)

Dental plaque

Ileum (lower small intestine)

Saliva

Skin

Stomach

Duodenum and Jejunum (upper small intestine)

Typical concentration of bacteria (1) (number/mL content) 1011 1011 108 109 <1011 per m2 (3) 103–104 103–104

Volume (mL)

400 (2) <10 400 (5) <100 1.8 m2 (4) 250 (5)–900 (6) 400 (5)

Order of magnitude bound for bacteria number 1014 1012 1011 1011 1011 107 107

(1) Except for skin, concentrations are according to [9]. For the skin, we used bacterial areal density and total skin surface to reach an upper bound. (2) See derivation in section below. (3) Skin surface bacteria density is taken from [11]. (4) Skin area calculated as inferred from standard formula by DuBois for the body surface area [12]. (5) Volume of the organs of the gastrointestinal tract is derived from weights taken from [13] by assuming content density of 1.04 g/mL [6]. (6) Higher value is given in [14].

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Revisiting the Original Back-of-the-Envelope Estimate for the Number of Bacteria in the Colon The primary source for the often cited value of ~1014 bacteria in the body dates back to the 1970s [3] and only consists of a sentence-long “derivation,” which assumes the volume of the alimentary tract to be 1 liter, and multiplies this volume by the number density of bacteria, known to be about 1011 bacteria per gram of wet content. Such estimates are often very illuminating, yet it is useful to revisit them as more empirical data accumulates. This pioneering estimate of 1014 bacteria in the intestine is based on assuming a constant bacterial density over the 1 liter of alimentary tract volume (converting from volume to mass via a density of 1 g/mL). Yet, the parts of the alimentary tract proximal to the colon contain a negligible number of bacteria in comparison to the colon content, as can be appreciated from Table 1. We thus conclude that the relevant volume for the high bacteria density of 1011 bacteria/g is only that of the colon. As discussed in Box 1, we integrated data sources on the volume of the colon to arrive at 0.4 L.

The Total Number of Bacteria in the Body

We are now able to repeat the original calculation for the number of bacteria in the colon [3]. Given 0.9(cid:1)1011 bacteria/g wet stool as derived in Box 2 and 0.4 L of colon, we find 3.8(cid:1)1013 bacteria in the colon with a standard error uncertainty of 25% and a variation of 52% SD over a population of 70 kg males. Considering that the contribution to the total number of bacteria from other organs is at most 1012, we use 3.8(cid:1)1013 as our estimate for the number of bacteria across the whole body of the "reference man."

We note that the uncertainty estimate value takes into account known variation in the colon

volume, bacteria density, etc., but cannot account for unquantified systematic biases. One prominent such bias is the knowledge gap on differences between the actual bacteria density in the colon, with all its spatial heterogeneity, and the measurements of concentration in feces, which serve as the proxy for estimating bacteria number.

What is the total mass of bacteria in the body? From the total colon content of about 0.4 kg

and a bacteria mass fraction of about one-half [21,24], we get a contribution of about 0.2 kg

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Box 1. The Volume of the Human Colon Content This is a critical parameter in our calculation. We used a value of 0.4 L based on the following studies (see also S1 Data, tab ColonContent). The volume of the colon content of the reference adult man was previously estimated as 340 mL (355 g at density of 1.04 g/ mL [6]), based on various indirect methods including flow measurements, barium meal X-ray measurements and postmortem examination [13]. A recent study [15] gives more detailed data about the volume of undisturbed colon that was gathered by MRI scans. The authors report a height-standardized colonic inner volume for males of 97 ± 24 mL/ m3 (where the best fit was found when dividing the colonic volume by the cube of the height). Taking a height of 1.70 m for the reference man [6], we arrive at a colon volume of 480 ± 120 mL (where unless noted otherwise ± refers to the standard deviation [SD]). This volume includes an unreported volume of gas and did not include the rectum. Most recently, studies analyzing MRI images of the colon provided the most detailed and complete data. The inner colon volume in that cohort was 760 mL in total [16,17]. This cohort was, however, significantly taller than the reference man. Normalizing for height, we arrive at 600 mL total volume for a standard man. In order to deduct the volume occupied by gas, stool fraction in this report was estimated at (cid:3)70% of colon volume leading to 430 mL of standardized wet colon content. Therefore, this most reliable analysis together with earlier studies support an average value of about 0.4 L.

We can sanity-check this volume estimate by looking at the volume of stool that flows through the colon. An adult human is reported to produce on average 100–200 grams of wet stool per day [18]. The colonic transit time is negatively correlated with the daily fecal output, and its normal values are about 25–40 hours [18,19]. By multiplying the daily output and the colon transit time, we thus get a volume estimate of 150–250 mL, which is somewhat lower than but consistent with the values above, given the uncertainties and very crude estimate that did not account for water in the colon that is absorbed before defecation. To summarize, the volume of colon content as evaluated by recent analyses of MRI images is in keeping with previous estimates and fecal transit dynamics. Values for a reference adult man averaged 0.4 L (standard error of the mean [SEM] 17%, coefficient of variation [CV] 25%), which will be used in calculations below. Following a typical meal, the volume changes by about 10% [15], while each defecation event reduces the content by a quarter to a third [18].

(wet weight) from bacteria to the overall mass of the colon content. Given the dominance of bacteria in the colon over all other microbiota populations in the body, we conclude that there is about 0.2 kg of bacteria in the body overall. Given the water content of bacteria, the total dry weight of bacteria in the body is about 50–100g. This value is consistent with a parallel alternative estimate for the total mass of bacteria that multiplies the average mass of a gut bacterium of about 5 pg (wet weight, corresponding to a dry weight of 1–2 pg, see S1 Appendix) with the updated total number of bacteria. We note that this empirically observed average gut bacterium is several times bigger than the conveniently chosen “standard” 1 μm3 volume and 1 pg wet mass bacterium often referred to in textbooks. The total bacteria mass we find represents about 0.3% of the overall body weight, significantly updating previous statements that 1%–3% of the body mass is composed of bacteria or that a normal human hosts 1–3 kg of bacteria [25].

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Box 2. Concentration of Bacteria in the Colon The most widely used approach for measuring the bacterial cell density in the colon is by examining bacteria content in stool samples. This assumes that stool samples give adequate representation of colon content. We return to this assumption in the discussion. The first such experiments date back to the 1960s and 1970s [20,21]. In those early studies, counting was based on direct microscopic clump counts from diluted stool samples. Later experiments [22,23] used DAPI nucleic acid staining and fluorescent in situ hybridization [FISH] to bacterial 16S RNA. Values are usually reported as bacteria per gram of dry stool. For our calculation, we are interested in the bacteria content for the wet rather than dry content of the colon. To move from bacteria/g dry stool to bacteria/g wet stool we use the fraction of dry matter as reported in each article. Table 2 reports the values we extracted from 14 studies in the literature and translated them to a common basis enabling comparison.

The Number of Human Cells in a “Standard” Adult Male Many literature sources make general statements on the number of cells in the human body ranging between 1012 to 1014 cells [26,27]. An order of magnitude back-of-the-envelope argument behind such values is shown in Box 3.

Table 2. Values of bacteria density in stool as reported in several past articles.

Article

Author

Houte & Gibbons

Moore & Holdeman

Holdeman, Good & Moore

Stephen & Cummings

Langendijk et al.

Franks et al.

Simmering & Kleessen

Tannock et al.

Harmsen, Raangs, He, Degener & Welling

Zoetendal et al

Zhong et al.

Thiel & Blaut

He et al.

Uyeno, Sekiguchi & Kamagata

Mean

Year

1966

1974

1976

1980

1995

1998

1999

2000

2002

2002

2004

2005

2008

2008

bac. #/g dry stool (x1011)

dry matter as % of stool

bac. #/g wet stool (x1011)

CV(%)

-

5

4.1

4

-

2.9

4.8

-

2.1

2.9

1.5

3.5

1.5

-

-

-

22%

31% 29%(1) -

-

-

- 30%

-

23% 25%

-

- 27% ± 2%

3.2

1.1 1.3 1.2 2.7 0.74(2) 1.3(2) 0.95

0.62 0.77(2) 0.35 0.87 0.39(2) 0.44 0.92 ± 19%

53%

78%

66%

25%

26%

39%

44%

40%

38%

24%

73%

53%

43%

34%

46%

Full references are provided in Table A in S1 Appendix. Mean bacteria number is calculated using the geometric mean to give robustness towards outlier values. Values quoted directly from the articles are written in bold, values derived by us are written in italic. Values reported with more than two significant digits are rounded to two significant digits as the uncertainty makes such overspecification nonsensible. ± standard error of the mean. (1) Value for [21] derived from their Table 1. (2) From derivation, assuming the averaged dry matter fraction of 27%. From the measurements collected in Table 2, we calculated the representative bacteria concentration in the colon by two methods, yielding very close values: the geometric mean is 0.92(cid:1)1011 (SEM 19%) bacteria per gram of wet stool, while the median of the values is 0.91(cid:1)1011 (SEM 19% by bootstrapping, see methods in S1 Appendix). The variation across the population, given by the average CV, is 46%.

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Box 3. Order of Magnitude, Naïve Estimate for the Number of Human Cells in the Body Assume a 102 kg man, consisting of “representative” mammalian cells. Each mammalian cell, using a cell volume of 1,000–10,000 μm3 , and a cell density similar to that of water, −11 kg. We thus arrive at 1013–1014 human cells in total in the body, will weigh 10 as shown in Fig 1. For these kind of estimates, where cell mass is estimated to within an order of magnitude, factors contributing to less than 2-fold difference are neglected. Thus, we use 100 kg as the mass of a reference man instead of 70 kg and similarly ignore the contribution of extracellular mass to the total mass. These simplifications are useful in making the estimate concise and transparent.

−12–10

An alternative method that does not require considering a representative "average" cell systematically counts cells by type. Such an approach was taken in a recent detailed analysis [1]. The number of human cells in the body of each different category (by either cell type or organ system) was estimated. For each category, the cell count was obtained from a literature reference or by a calculation based on direct counts in histological cross sections. Summing over a total of 56 cell categories [1] resulted in an overall estimate of 3.7(cid:1)1013 human cells in the body (SD 0.8(cid:1)1013, i.e., CV of 22%).

Updated Inventory of Human Cells in the Body

In our effort to revisit the measurements cited, we employed an approach that tries to combine the detailed, census approach with the benefits of a heuristic calculation used as a sanity check. We focused on the six cell types that were recently identified [1] to comprise 97% of the human cell count: red blood cells (accounting for 70%), glial cells (8%), endothelial cells (7%), dermal fibroblasts (5%), platelets (4%), and bone marrow cells (2%). The other 50 cell types account for the remaining 3%. In four cases (red blood cells, glial cells, endothelial cells, and dermal fibroblasts), we arrived at revised calculations as detailed in Box 4.

Our revised calculations of the number of glial cells, endothelial cells, and dermal fibroblast

yield only 0.9(cid:1)1012 cells, in contrast to 7.5(cid:1)1012 cells in the previous estimate. This leaves us

Fig 1. Back of the envelope estimate of the number of cells in an adult human body based on a characteristic volume and mass.

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Box 4. Revised Estimates for the Number of Red Blood Cells, Glial Cells, Endothelial Cells, and Dermal Fibroblasts The largest contributor to the overall number of human cells are red blood cells. Calculation of the number of red blood cells was made by taking an average blood volume of 4.9 L (SEM 1.6%, CV 9%) multiplied by a mean RBC count of 5.0(cid:1)1012 cells/L (SEM 1.2%, CV 7%) (see Table 3 and S1 Data). The latter could be verified by looking at your routine complete blood count, normal values range from 4.6–6.1(cid:1)1012 cells/L for males and 4.2– 5.4(cid:1)1012 cells/L for females. This led to a total of 2.5(cid:1)1013 red blood cells (SEM 2%, CV 12%). This is similar to the earlier report of 2.6(cid:1)1013 cells [1].

The number of glial cells was previously reported as 3(cid:1)1012 [1]. This estimate is based on a 10:1 ratio between glial cells and neurons in the brain. This ratio of glia:neurons was held as a broadly accepted convention across the literature. However, a recent analysis [28] revisits this value and, after analyzing the variation across brain regions, concludes that the ratio is close to 1:1. The study concludes that there are 8.5(cid:1)1010 glial cells (CV 11%) in the brain and a similar number of neurons and so we use these updated values here.

The number of endothelial cells in the body was earlier estimated at 2.5(cid:1)1012 cells (CV

40%), based on the mean surface area of one endothelial cell [1] and the total surface area of the blood vessels, based on a total capillary length of 8(cid:1)109 cm. We could not find a primary source for the total length of the capillary bed and thus decided to revisit this estimate. We used data regarding the percentage of the blood volume in each type of blood vessels [29]. Using mean diameters for different blood vessels [30], we were able to derive (S1 Data) the total length of each type of vessel (arteries, veins, capillaries, etc.) and its corresponding surface area. Dividing by the mean surface area of one endothelial cell [31], we derive a reduced total estimate of 6(cid:1)1011 cells.

The number of dermal fibroblasts was previously estimated to be 1.85(cid:1)1012 [1], based on multiplying the total surface area (SA) of the human body (1.85 m2 [32]) by the areal density of dermal fibroblasts [33]. We wished to incorporate the dermal thickness (d) into the calculation. Dermal thickness was directly measured at 17 locations throughout the body [34], with the mean of these measurements yielding 0.11±0.04 cm. The dermis is composed of two main layers: papillary dermis (about 10% of the dermis thickness) and reticular dermis (the other 90%) [35]. The fibroblast density is greater in the papilpap. of 106 cells/cm2 (with 100 μm thickness lary dermis, with a reported areal density, σ of papillary, giving 108 cells/cm3) [33]. The fibroblast density in the middle of the dermis was reported to be about 3(cid:1)106 cells/cm3 [36], giving an areal density of σ ret. = 3(cid:1)105 cells/ ret.) = 1.85(cid:1)104 cm2 (106 + 3(cid:1)105) cm2. Combining these we find: Nder.fib. = SA(cid:1)(σ cells/cm2 = 2.6(cid:1)1010 cells. After this 100-fold decrease in number, dermal fibroblasts are estimated to account for only (cid:3)0.05% of the human cell count.

pap. + σ

with 3.0(cid:1)1013 human cells in the 70 kg “reference man,” with a calculated 2% uncertainty and 14% CV. We note that the uncertainty and CV estimates might be too optimistically low, as they are dominated by the reported high confidence of studies dealing with red blood cells but may underestimate systematic errors, omissions of some cell types, and similar factors that are hard to quantify. In Fig 2, we summarize the revised results for the contribution of the different cell types to the total number of human cells. Categories contributing >0.4% in cell count are

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Fig 2. The distribution of the number of human cells by cell type. Representation as a Voronoi tree map where polygon area is proportional to the number of cells. Visualization performed using the online tool at http://bionic-vis.biologie.unigreifswald.de/.

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presented. All the other categories sum up to about 2% together. We find that the body includes only 3(cid:1)1012 non-blood human cells, merely 10% of the total updated human cell count. The visualization in Fig 2 highlights that almost 90% of the cells are estimated to be enucleated cells (26(cid:1)1012 cells), mostly red blood cells and platelets, while the other (cid:3)10% consist of (cid:3)3(cid:1)1012 nucleated cells. concepts↗The striking dominance of the hematopoietic lineage in the cell count (90% of the total) is counterintuitive given the composition of the body by mass. This is the subject of the following analysis.

Mass-Centered Approach as Sanity Check for Cell Count

transferscontrolsIt is prudent in making such estimates to approach the analysis from different angles. In that spirit, we now ask does the cumulative mass of the cells counted fall within the expected range for a reference adult? To properly tackle that question, we first need to state what the anticipated result is, i.e., total body cell mass. For a reference man mass of 70 kg, 25% is extracellular fluid [37], another 7% is extracellular solids [37], thus we need to account for (cid:3)46 kg of cell mass (including fat).

A comprehensive systematic source for the composition of total cell mass (rather than total cell

count) is the Report of the Task Group on Reference Man [6], which gives values for the mass of the main tissues of the human body. This mass per tissue analysis includes both intra- and extracellular components. To distinguish between intra- and extracellular portions of each tissue, we can leverage total body potassium measurements [38] as detailed in S1 Appendix. Fig 3 compares the main tissues that contribute to the human body, in terms of cell number and masses.

content_tagscontent_tags↗A striking outcome of this juxtaposition is the evident discordance between contributors to total cell mass and to cell number. The cell count is dominated by red blood cells (84%), among

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Fig 3. Distribution of cell number and mass for different cell types in the human body (for a 70 kg adult man). The upper bar displays the number of cells, while the lower bar displays the contribution from each of the main cell types comprising the overall cellular body mass (not including extracellular mass that adds another (cid:3)24 kg). For comparison, the contribution of bacteria is shown on the right, amounting to only 0.2 kg, which is about 0.3% of the body weight.

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the smallest cell types in the human body with a volume of about 100 μm3. concepts↗concepts↗concepts↗concepts↗concepts↗concept_edgesconcept_edgesconcept_edgescontent_tagscontent_tags↗In contrast, 75% of total cell mass is composed of two cell types, fat cells (adipocytes) and muscle cells (myocytes), both large cells (usually >10,000 μm3 by volume) that constitute only a minute fraction ((cid:3)0.2%) of total cell number. At the other extreme, bacteria have a minor contribution in terms of mass, but a cell count comparable to all human cells combined, as discussed above. The mass balance accounts well for all expected body mass, giving support to our analysis. The option of overlooking a collection of very small cells numerous enough to alter the total cell count is further discussed in the S1 Appendix.

The Ratio of Bacteria to Human Cells in the Adult Body With the revised estimates for the number of human (3.0(cid:1)1013) and bacterial cells (3.8(cid:1)1013) in the body (the numerator and denominator of the B/H ratio), we can give an updated estimate of B/ H = 1.3, with an uncertainty of 25% and a variation of 53% over the population of standard 70 kg males. concepts↗sensesclaim_relationscontent_tagscontent_tagscontent_tagsThis B/H value of about 1:1 (with the associated uncertainty range) should replace the 10:1 or 100:1 values that are stated in the literature until more accurate measurements become available.

We note that if one chooses to compare the number of bacteria in the human body

(3.8(cid:1)1013) to the number of nucleated human cells ((cid:3)0.3(cid:1)1013), the ratio will be about 10:1. mechanisms↗controlssensesThis is because the dominant population of non-nucleated red blood cells is not included in the calculation. We note that this ratio is the result of both the number of bacteria and the number of nucleated human cells in the body being several times lower than in the original 1970s estimate (that did not restrict the analysis to nucleated cells). limitationscontent_tagsThe issue of whether cells without a nucleus should be included or discarded in the calculation of the number of human cells, and thus the B/ H ratio, seems to be a question of definition. concepts↗We view red blood cells as bona fide cells, as their name suggests. But it is also plausible to choose not to include them as some may think of them as “bags full of hemoglobin.” concepts↗Inclusion of platelets in the count, which corresponds to their inclusion in previous counts, is also potentially disputable but has only a minor quantitative effect. content_tags↗Indeed, this opens an interesting tangential discussion on what should be defined as a cell.

Variations in the Ratio of Bacteria to Human Cells across Population Segments After reviewing the B/H ratio for the “reference man,” we now generalize our results by addressing other segments of the population. Looking at our estimate, we identify four main parameters that dominate the calculation:

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Table 3. B/H ratio for different population. See Table B in S1 Appendix for full