Composition of the human body

Not to be confused with Human anatomy.
The main elements that compose the human body are shown from most abundant (by mass, not by fraction of atoms) to least abundant.

Body composition may also be analyzed in terms of molecular type (e.g., water, protein, connective tissue, fats (or lipids) Hydroxylapatite (in bones), carbohydrates (such as glycogen and glucose) and DNA. In terms of tissue type, the body may be analyzed into water, fat, muscle, bone, etc. In terms of cell type, the body contains hundreds of different types of cells, but notably, the largest number of cells contained in a human body (though not the largest mass of cells) are not human cells, but consist of bacteria (bacterial cells) residing in the normal human gastrointestinal tract.

Major, minor and trace elements

Pie charts of typical human body composition by percent of mass, and by percent of atomic composition (atomic percent).

Almost 99% of the mass of the human body is made up of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Only about 0.85% is composed of another five elements: potassium, sulfur, sodium, chlorine, and magnesium. All 11 are necessary for life. The remaining elements are trace elements, of which more than a dozen are thought on the basis of good evidence to be necessary for life. All of the mass of the trace elements put together (less than 10 grams for a human body) do not add to the body mass of magnesium, the least common of the 11 non-trace elements.

Other elements and questionable human-required elements

Not all elements which are found in the human body in trace quantities play a role in life. Some of these elements are thought to be simple bystander contaminants without function (examples: caesium, titanium), while many others are thought to be active toxics, depending on amount (cadmium, mercury, radioactives). The possible utility and toxicity of a few elements at levels normally found in the body (aluminium) is debated. Functions have been proposed for trace amounts of cadmium and lead, although these are almost certainly toxic in amounts very much larger than normally found in the body. There is evidence that arsenic, an element normally considered a toxic in higher amounts, is essential in ultratrace quantities, even in mammals (rats, hamsters, goats).[1]

Some elements (arsenic, silicon, boron, nickel, vanadium) are probably needed by mammals also, but in far smaller doses. Bromine is used abundantly by some (though not all) lower organisms, and opportunistically in eosinophils in humans. One study has found bromine to be necessary to collagen IV synthesis in humans.[2] Fluorine is used by a number of plants to manufacture toxins (see that element) but in humans only functions as a local (topical) hardening agent in tooth enamel, and not in an essential biological role.

Elemental composition list

Main article: Dietary element

The average 70 kg (150 lb) adult human body contains approximately 7×1027 atoms and contains at least detectable traces of 60 chemical elements.[3] About 29 of these elements are thought to play an active positive role in life and health in humans.[4]

The relative amounts of each element vary by individual, mainly due to differences in the proportion of fat, muscle and bone in their body. Persons with more fat will have a higher proportion of carbon and a lower proportion of most other elements (the proportion of hydrogen will be about the same). The numbers in the table are averages of different numbers reported by different references.

The adult human body averages ~53% water. This varies substantially by age, sex, and adiposity. In a large sample of adults of all ages and both sexes, the figure for water fraction by weight was found to be 48 ±6% for females and 58 ±8% water for males.[5] Water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent, and these numbers along with the complementary % numbers for oxygen in water, are the largest contributors to overall mass and atomic composition figures. Because of water content, the human body contains more oxygen by mass than any other element, but more hydrogen by atom-fraction than any element.

The elements listed below as "Essential in humans" are those listed by the (US) Food and Drug Administration as essential nutritients,[6] as well as six additional elements: oxygen, carbon, hydrogen, and nitrogen (the fundamental building blocks of life on Earth), sulfur (essential to all cells) and cobalt (a necessary component of vitamin B12). Elements listed as "Possibly" or "Probably" essential are those cited by the National Research Council (United States) as beneficial to human health and possibly or probably essential.[7]

Atomic number Element Fraction of mass[8][9][10][11][12][13]Mass (kg)[14]Atomic percentEssential in humans[15]!!Negative effects of excessGroup
8 Oxygen 0.65 43 24 Yes (e.g. water, electron acceptor)[16] Reactive Oxygen Species 16
6 Carbon 0.18 16 12 Yes[16] (organic compounds) 14
1 Hydrogen 0.10 7 62 Yes[16] (e.g. water) 1
7 Nitrogen 0.03 1.8 1.1 Yes[16] (e.g. DNA and amino acids) 15
20 Calcium 0.014 1.0 0.22 Yes[16][17][18] (e.g. Calmodulin and Hydroxylapatite in bones) 2
15 Phosphorus 0.011 0.78 0.22 Yes[16][17][18] (e.g. DNA and phosphorylation) white allotrope highly toxic 15
19 Potassium 2.5×10−3 0.14 0.033 Yes[16][17] (e.g. Na+/K+-ATPase) 1
16 Sulfur 2.5×10−3 0.14 0.038 Yes[16] (e.g.Cysteine, Methionine, Biotin, Thiamine) 16
11 Sodium 1.5×10−3 0.10 0.037 Yes[17] (e.g. Na+/K+-ATPase) 1
17 Chlorine 1.5×10−3 0.095 0.024 Yes[17][18] (e.g. Cl-transporting ATPase) 17
12 Magnesium 500×10−6 0.019 0.0070 Yes[17][18] (e.g. binding to ATP and other nucleotides) 2
26 Iron* 60×10−6 0.0042 0.00067 Yes[17][18] (e.g. Hemoglobin, Cytochromes) 8
9 Fluorine 37×10−6 0.0026 0.0012 Yes (AUS, NZ),[19] No (US, EU),[20][21] Maybe (WHO)[22] toxic in high amounts 17
30 Zinc 32×10−6 0.0023 0.00031 Yes[17][18] (e.g. Zinc finger proteins) 12
14 Silicon 20×10−6 0.0010 0.0058 Possibly[7] 14
37 Rubidium 4.6×10−6 0.00068 0.000033 No 1
38 Strontium 4.6×10−6 0.00032 0.000033 —— 2
35 Bromine 2.9×10−6 0.00026 0.000030 —— 17
82 Lead 1.7×10−6 0.00012 0.0000045 No toxic 14
29 Copper 1×10−6 0.000072 0.0000104 Yes[17][18] (e.g. copper proteins) 11
13 Aluminium 870×10−9 0.000060 0.000015 No 13
48 Cadmium 720×10−9 0.000050 0.0000045 No toxic 12
58 Cerium 570×10−9 0.000040 No
56 Barium 310×10−9 0.000022 0.0000012 No toxic in higher amounts 2
50 Tin 240×10−9 0.000020 6.0×10−7 No 14
53 Iodine 160×10−9 0.000020 7.5×10−7 Yes[17][18] (e.g. thyroxine, triiodothyronine) 17
22 Titanium 130×10−9 0.000020 No 4
5 Boron 690×10−9 0.000018 0.0000030 Probably[7][23] 13
34 Selenium 190×10−9 0.000015 4.5×10−8 Yes[17][18] toxic in higher amounts 16
28 Nickel 140×10−9 0.000015 0.0000015 Probably[7][23] toxic in higher amounts 10
24 Chromium 24×10−9 0.000014 8.9×10−8 Yes[17][18] 6
25 Manganese 170×10−9 0.000012 0.0000015 Yes[17][18] (e.g. Mn-SOD) 7
33 Arsenic 260×10−9 0.000007 8.9×10−8 Possibly[1][7] toxic in higher amounts 15
3 Lithium 31×10−9 0.000007 0.0000015 —— toxic in higher amounts 1
80 Mercury 190×10−9 0.000006 8.9×10−8 No toxic 12
55 Caesium 21×10−9 0.000006 1.0×10−7 No 1
42 Molybdenum 130×10−9 0.000005 4.5×10−8 Yes[17][18] (e.g. the molybdenum oxotransferases, Xanthine oxidase and Sulfite oxidase) 6
32 Germanium 5×10−6 No 14
27 Cobalt 21×10−9 0.000003 3.0×10−7 Yes (cobalamin, B12)[24][25] 9
51 Antimony 110×10−9 0.000002 No toxic 15
47 Silver 10×10−9 0.000002 No 11
41 Niobium 1600×10−9 0.0000015 No 5
40 Zirconium 6×10−6 0.000001 3.0×10−7 No 4
57 Lanthanum 1370×10−9 8×10−7 No
52 Tellurium 120×10−9 7×10−7 No 16
31 Gallium 7×10−7 No 13
39 Yttrium 6×10−7 No 3
83 Bismuth 5×10−7 No 15
81 Thallium 5×10−7 No highly toxic 13
49 Indium 4×10−7 No 13
79 Gold 3×10−9 2×10−7 3.0×10−7 No uncoated nanoparticles possibly genotoxic [26][27][28] 11
21 Scandium 2×10−7 No 3
73 Tantalum 2×10−7 No 5
23 Vanadium 260×10−9 1.1×10−7 1.2×10−8 Possibly[7] (suggested osteo-metabolism (bone) growth factor) 5
90 Thorium 1×10−7 No toxic, radioactive
92 Uranium 1×10−7 3.0×10−9 No toxic, radioactive
62 Samarium 5.0×10−8 No
74 Tungsten 2.0×10−8 No 6
4 Beryllium 3.6×10−8 4.5×10−8 No toxic in higher amounts 2
88 Radium 3×10−14 1×10−17 No toxic, radioactive 2

*Iron = ~3 g in men, ~2.3 g in women

Most of the elements needed for life are relatively common in the Earth's crust. Aluminium, the third most common element in the Earth's crust (after oxygen and silicon), serves no function in living cells, but is harmful in large amounts.[29] Transferrins can bind aluminium.[30]

Other elements

Of the 94 naturally occurring chemical elements (which now include Neptunium and Plutonium) 60 are listed in the table above. Of the remaining 34, it is not known how many occur in the human body. For some of these elements, numbers for concentrations in various tissues or organs is available, typically from studies involving small population sample sizes.

Noble Gases

Concentration of noble gases in whole blood.[31]

Atomic number Element Fraction of volume Atomic percent Positive health role in mammals Negative effects of excessGroup
2 Helium 3.7×10−8 asphyxiant 18
10 Neon 1.38×10−7 asphyxiant 18
18 Argon 2.3×10−4 asphyxiant 18
36 Krypton 5.5×10−7 asphyxiant 18
54 Xenon 9.7×10−9 asphyxiant 18
86 Radon* 7×10−19 highly radioactive 18

* hypothetical value for radon based on 10 Bq/m3 and 0.4 blood/air partition coefficient.[32]

Lanthanides

Of the seventeen rare earth elements (REEs), fifteen belong to the lanthanide series. The other two, scandium and yttrium, are listed in the table above, as are three lanthanides: lanthanum, cerium, and samarium. Of the remaining twelve lanthanides, eleven are listed below. No information is available regarding the remaining lanthanide, promethium.

Concentration of rare earth elements / lanthanides in blood serum.[33]

Atomic number Element g/ml−1 blood serum Atomic percent Positive health role in mammals Negative effects of excess[34] Group
59 Praseodymium 11×10−12 low to moderate toxicity n/a
60 Neodymium 33.7×10−12 low to moderate toxicity n/a
63 Europium 82×10−12 comparable to other heavy metals n/a
64 Gadolinium 7.2×10−12 free ions highly toxic n/a
65 Terbium 1.3×10−12 low to moderate toxicity n/a
66 Dysprosium 9.6×10−12 mildly toxic when ingested n/a
67 Holmium 2.55×10−12 low acute toxicity n/a
68 Erbium 9.5×10−12 low to moderate toxicity n/a
69 Thulium 1.69×10−12 soluble salts slightly toxic in large amounts n/a
70 Ytterbium 13.2×10−12 all compounds highly toxic n/a
71 Lutetium 2.46×10−12 oxide powder is toxic n/a

Platinum group metals (PGMs)

Concentration of platinum group metals in the blood.

Atomic number Element g/ml−1 whole blood g/ml−1 blood serum Positive health role in mammals Negative effects of excessGroup
44 Ruthenium[35] 54×10−12 — n/a
45 Rhodium[35] 9×10−12 — n/a
46 Palladium[36][37] 50×10−12 — small particles dissolve in biological media (gastric juice, blood serum) - long-term effects unknown[38] n/a
76 Osmium[39][40] — 400×10−15–800×10−15 oxidizes to highly poisonous Osmium tetroxide[41][42] n/a
77 Iridium[36][37] 300×10−15 — n/a
78 Platinum[43] <800×10−15–6.9×10−12 — n/a

Essential elements on the periodic table

Periodic table highlighting dietary elements

H  He
LiBe  B C N O FNe
NaMg  AlSiP S ClAr
K CaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKr
RbSrY ZrNbMoTcRuRhPdAgCdInSnSbTeI Xe
CsBa* LuHfTaW ReOsIrPtAuHgTlPbBiPoAtRn
FrRa** LrRfDbSgBhHsMtDsRgCnUutFlUupLvUusUuo
 
  * LaCePrNdPmSmEuGdTbDyHoErTmYb
  **AcThPaU NpPuAmCmBkCfEsFmMdNo
The four organic basic elements Quantity elements Essential trace elements Possible structural or functional role in mammals

Composition by molecule type

The composition can also be expressed in terms of chemicals, such as:

The composition of the human body can be viewed on an atomic and molecular scale as shown in this article.

The estimated gross molecular contents of a typical 20-micrometre human cell is as follows:[45]

Molecule Percent of Mass Mol.Weight (daltons) Molecules Percent of Molecules
Water 65* 18* 1.74×1014 98.73
Other Inorganics 1.5 N/A 1.31×1012 0.74
Lipids 12 N/A 8.4×1011 0.475
Other Organics 0.4 N/A 7.7×1010 0.044
Protein 20 N/A 1.9×1010 0.011
RNA 1.0 N/A 5×107 3×10−5
DNA 0.1 1×1011 46* 3×10−11

*The amount of water is highly dependent on body composition and amount of fat. In adults in developed countries it averages ~53% water. This varies substantially by age, sex, and adiposity. In a large sample of adults of all ages and both sexes, the figure for water fraction by weight was found to be 48 ±6% for females and 58 ±8% water for males.[5] DNA: A human cell also contains mitochondrial DNA. Sperm cells contain less mitochondrial DNA than other cells. A mammalian red blood cell normally contains no nucleus in adulthood, and thus no DNA. However, nucleated red blood cells, or NRBCs, are present in the fetal and neonatal circulations, and may appear in mature mammals when disease is present.

Materials and tissues

Body composition can also be expressed in terms of various types of material, such as:

Composition by cell type

There are many species of bacteria and other microorganisms that live on or inside the healthy human body. In fact, 90% of the cells in (or on) a human body are microbes, by number[46][47] (much less by mass or volume). Some of these symbionts are necessary for our health. Those that neither help nor harm humans are called commensal organisms.

See also

References

  1. 1 2 Anke M. "Arsenic". In: Mertz W. ed., Trace elements in human and Animal Nutrition, 5th ed. Orlando, FL: Academic Press, 1986, 347-372; Uthus E. O., "Evidency for arsenical essentiality", Environmental Geochemistry and Health, 1992, 14:54-56; Uthus E.O., Arsenic essentiality and factors affecting its importance. In: Chappell W. R., Abernathy C. O., Cothern C. R. eds., Arsenic Exposure and Health. Northwood, UK: Science and Technology Letters, 1994, 199-208.
  2. ↑ McCall AS, Cummings CF, Bhave G, Vanacore R, Page-McCaw A, Hudson BG (2014). "Bromine Is an Essential Trace Element for Assembly of Collagen IV Scaffolds in Tissue Development and Architecture". Cell. 157 (6): 1380–92. doi:10.1016/j.cell.2014.05.009. PMID 24906154.
  3. ↑ How many atoms are in the human body?
  4. ↑ "Ultratrace minerals". Authors: Nielsen, Forrest H. USDA, ARS Source: Modern nutrition in health and disease / editors, Maurice E. Shils ... et al.. Baltimore : Williams & Wilkins, c. 1999, p. 283-303. Issue Date: 1999 URI:
  5. 1 2 See table 1. here
  6. ↑ "Guidance for Industry: A Food Labeling Guide 14. Appendix F"
  7. 1 2 3 4 5 6 Institute of Medicine (29 September 2006). Dietary Reference Intakes: The Essential Guide to Nutrient Requirements. National Academies Press. pp. 313–19, 415–22. ISBN 978-0-309-15742-1. Retrieved 21 June 2016.
  8. ↑ Thomas J. Glover, comp., Pocket Ref, 3rd ed. (Littleton: Sequoia, 2003), p. 324 (LCCN 2002-91021), which in
  9. ↑ turn cites Geigy Scientific Tables, Ciba-Geigy Limited, Basel, Switzerland, 1984.
  10. ↑ Chang, Raymond (2007). Chemistry, Ninth Edition. McGraw-Hill. p. 52. ISBN 0-07-110595-6.
  11. ↑ "Elemental Composition of the Human Body" by Ed Uthman, MD Retrieved 17 June 2016
  12. ↑ Frausto Da Silva, J. J. R; Williams, R. J. P (2001-08-16). "The Biological Chemistry of the Elements: The Inorganic Chemistry of Life". ISBN 9780198508489.
  13. ↑ Zumdahl, Steven S. and Susan A. (2000). Chemistry, Fifth Edition. Houghton Mifflin Company. p. 894. ISBN 0-395-98581-1.)
  14. ↑ Emsley, John (25 August 2011). Nature's Building Blocks: An A-Z Guide to the Elements. OUP Oxford. p. 83. ISBN 978-0-19-960563-7. Retrieved 17 June 2016.
  15. ↑ Neilsen, cited
  16. 1 2 3 4 5 6 7 8 Salm, Sarah; Allen, Deborah; Nester, Eugene; Anderson, Denise (9 January 2015). Nester's Microbiology: A Human Perspective. p. 21. ISBN 978-0-07-773093-2. Retrieved 19 June 2016.
  17. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Subcommittee on the Tenth Edition of the Recommended Dietary Allowances, Food and Nutrition Board; Commission on Life Sciences, National Research Council (1 February 1989). "9-10". Recommended Dietary Allowances: 10th Edition. National Academies Press. ISBN 978-0-309-04633-6. Retrieved 18 June 2016.
  18. 1 2 3 4 5 6 7 8 9 10 11 12 Code of Federal Regulations, Title 21: Food and Drugs, Ch 1, subchapter B, Part 101, Subpart A, §101.9(c)(8)(iv)
  19. ↑ Australian National Health and Medical Research Council (NHMRC) and New Zealand Ministry of Health (MoH)
  20. ↑ "Fluoride in Drinking Water: A Review of Fluoridation and Regulation Issues"
  21. ↑ "Scientific Opinion on Dietary Reference Values for fluoride". EFSA Journal. 11 (8): 3332. 2013. doi:10.2903/j.efsa.2013.3332. ISSN 1831-4732.
  22. ↑ WHO/SDE/WSH/03.04/96 "Fluoride in Drinking-water"
  23. 1 2 Safe Upper Levels for Vitamins and Mineral (2003), boron p. 164-71, nickel p. 225-31, EVM, Food Standards Agency, UK ISBN 1-904026-11-7
  24. ↑ Yamada, Kazuhiro (2013). "Cobalt: Its Role in Health and Disease". 13: 295–320. doi:10.1007/978-94-007-7500-8_9. ISSN 1559-0836.
  25. ↑ Banci, Lucia (18 April 2013). Metallomics and the Cell. Springer Science & Business Media. pp. 333–368. ISBN 978-94-007-5561-1. Retrieved 19 June 2016.
  26. ↑ Fratoddi, Ilaria; Venditti, Iole; Cametti, Cesare; Russo, Maria Vittoria (2015). "How toxic are gold nanoparticles? The state-of-the-art". Nano Research. 8 (6): 1771–1799. doi:10.1007/s12274-014-0697-3. ISSN 1998-0124.
  27. ↑ "Scientific Opinion on the re-evaluation of gold (E 175) as a food additive". EFSA Journal. 14 (1). 2016. doi:10.2903/j.efsa.2016.4362. ISSN 1831-4732.
  28. ↑ Hillyer, Julián F.; Albrecht, Ralph M. (2001). "Gastrointestinal persorption and tissue distribution of differently sized colloidal gold nanoparticles". Journal of Pharmaceutical Sciences. 90 (12): 1927–1936. doi:10.1002/jps.1143. ISSN 0022-3549.
  29. ↑ Aluminum Toxicity
  30. ↑ Mizutani, K.; Mikami, B.; Aibara, S.; Hirose, M. (2005). "Structure of aluminium-bound ovotransferrin at 2.15 Å resolution". Acta Crystallographica Section D. 61 (12): 1636. doi:10.1107/S090744490503266X.
  31. ↑ Tomonaga, Yama; Brennwald, Matthias S.; Livingstone, David M.; Tomonaga, Geneviève; Kipfer, Rolf (2014). "Determination of Natural In Vivo Noble-Gas Concentrations in Human Blood". PLoS ONE. 9 (5): e96972. doi:10.1371/journal.pone.0096972. ISSN 1932-6203.
  32. ↑ Keith S, Doyle JR, Harper C, et al. "Toxicological Profile for Radon", section 3.4.1.1, page 51. Atlanta (GA): Agency for Toxic Substances and Disease Registry (US); 2012 May. Available from: http://www.ncbi.nlm.nih.gov/books/NBK158784/
  33. ↑ Inagaki, Kazumi; Haraguchi, Hiroki (2000). "Determination of rare earth elements in human blood serum by inductively coupled plasma mass spectrometry after chelating resin preconcentration". The Analyst. 125 (1): 191–196. doi:10.1039/a907781b. ISSN 0003-2654.
  34. ↑ Rim, Kyung Taek; Koo, Kwon Ho; Park, Jung Sun (2013). "Toxicological Evaluations of Rare Earths and Their Health Impacts to Workers: A Literature Review". Safety and Health at Work. 4 (1): 12–26. doi:10.5491/SHAW.2013.4.1.12. ISSN 2093-7911.
  35. 1 2 Rodushkin, I.; Ödman, Fredrik; Branth, Stefan (1999). "Multielement analysis of whole blood by high resolution inductively coupled plasma mass spectrometry". Fresenius' Journal of Analytical Chemistry. 364 (4): 338–346. doi:10.1007/s002160051346. ISSN 0937-0633.
  36. 1 2 Ravindra, Khaiwal; Bencs, László; Van Grieken, René (2004). "Platinum group elements in the environment and their health risk". Science of The Total Environment. 318 (1-3): 1–43. doi:10.1016/S0048-9697(03)00372-3. ISSN 0048-9697.
  37. 1 2 Begerow, Jutta; Turfeld, Martina; Dunemann, Lothar (1997). "Determination of Physiological Palladium, Platinum, Iridium and Gold Levels in Human Blood Using Double Focusing Magnetic Sector Field Inductively Coupled Plasma Mass Spectrometry". Journal of Analytical Atomic Spectrometry. 12 (9): 1095–1098. doi:10.1039/a701094j. ISSN 0267-9477.
  38. ↑ "Environmental Health Criteria" 226 ISBN 92-4-157226-4, ISSN 0250-863X
  39. ↑ Rodushkin, Ilia; Engstrom, Emma; Stenberg, Anna; Baxter, Douglas C. (2004). "Determination of low-abundance elements at ultra-trace levels in urine and serum by inductively coupled plasma—sector field mass spectrometry". Analytical and Bioanalytical Chemistry. 380 (2): 247–257. doi:10.1007/s00216-004-2742-7. ISSN 1618-2642.
  40. ↑ ALS Scandinavia, Reference data, Biomonitoring, "Trace elements in human biological material"
  41. ↑ Krebs, Robert E. (2006). The History and Use of Our Earth's Chemical Elements: A Reference Guide. Greenwood Publishing Group. p. 158. ISBN 978-0-313-33438-2. Retrieved 4 June 2016.
  42. ↑ Russell, James A.; Wirtz, James J. (4 December 2009). Globalization and WMD Proliferation: Terrorism, Transnational Networks and International Security. Routledge. p. 123. ISBN 978-1-134-07969-8. Retrieved 4 June 2016.
  43. ↑ Messerschmidt, J.; Alt, F.; Tolg, G.; Angerer, J.; Schaller, K. H. (1992). "Adsorptive voltammetric procedure for the determination of platinum baseline levels in human body fluids". Fresenius' Journal of Analytical Chemistry. 343 (4): 391–394. doi:10.1007/BF00322878. ISSN 0937-0633.
  44. ↑ Douglas Fox, "The speed of life", New Scientist, No 2419, 1 November 2003.
  45. ↑ Freitas Jr., Robert A. (1999). Nanomedicine,. Landes Bioscience. Tables 3–1 & 3–2. ISBN 1-57059-680-8.
  46. ↑ Glausiusz, Josie. "Your Body Is a Planet". Retrieved 2007-09-16.
  47. ↑ Wenner, Melinda. "Humans Carry More Bacterial Cells than Human Ones". Retrieved 2010-10-09.
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