We’re only 10% human. According to…who?

Reading an interesting post at Genomes Unzipped, “Human genetics is microbial genomics“, which states:

Only 10% of cells on your “human” body are human anyway, the rest are microbial.

Have you read a sentence like that before? So have I. So has a reader who left a comment:

I was wondering if you have a source for “Only 10% of cells on your “human” body are human anyway, the rest are microbial”

It’s a good question. Everyone quotes this figure, almost no-one provides a reference. Let’s go in search of one.

Wikipedia. Don’t make it your primary source. However, it’s often a good place to start. From the human microbiome article:

Bacterial cells are much smaller than human cells, and there are at least ten times as many bacteria as human cells in the body (approximately 1014 versus 1013).[7] [8]

Reference [8]: Berg, RD (1996). The indigenous gastrointestinal microflora. Trends Microbiol. 4 (11): 430–435. If you have access to the full text, you’ll find a PDF which seems to be a poor-quality scan of the printed article. From page 432:

In summary, there are ten viable indigenous bacteria in the GI tract for every cell in the human body: 1013 total GI bacteria compared with 1012 total cells making up the human body.

Note the order of magnitude differences compared with the Wikipedia article. I was unable to find a reference in this article to any paper where these numbers were measured or estimated. And so…

…reference [7]: Savage, DC (1977). Microbial Ecology of the Gastrointestinal Tract. Ann. Rev. Microbiol. 31: 107-133.

Another PDF so old that we cannot copy/paste from it. The opening sentences:

The adult human organism is said to be composed of approximately 1013 eukaryotic animal cells (27). That statement is only an expression of a particular point of view. The various body surfaces and the gastrointestinal canals of humans may be colonized by as many
as 1014 indigenous prokaryotic and eukaryotic microbial cells (70).

Note the order of magnitude discrepancies compared with the previous article.

Reference (27) is: Dobzhansky, T. (1971). Genetics of the Evolutionary Process, Vol. 1. New York: Columbia Univ. We’ll leave that one there.

Reference (70) is: Luckey, TD (1972). Am. J. Clin. Nutr. 25: 1292-95. Time to search PubMed:

luckey td[au] 1972[dp]

It’s the first entry, titled “Introduction to intestinal microecology” and it’s freely-available.

Hey look, another ancient PDF – and the last page is 1294, not 1295. First page, second paragraph:

The composition of this system is surprising. Adult man carries 1012 microbes associated with his epidermis and 1014 microbes in his alimentary tract (Fig. 1). The latter number is based upon 1011 microbes/g contents of an alimentary tract with a capacity of approximately 1 liter. The 1013 cells (2) in his body are a distinct numerical minority of the total being that we call man. If we abandon anthropomorphism for the microbic view, we must admire the efficiency of these microbes in using man as a vehicle to further their own cause.

This would seem to be the “definitive” reference, for now. Reference (2) in that quote is Dobzhansky, again.

No mention of adult woman. It’s OK, that’s just how people spoke and wrote in the 1970s.

10 thoughts on “We’re only 10% human. According to…who?

  1. Pingback: We're only 10% human. According to...who? | Too...

  2. Tabitha M. Powledge

    The 9/10 figure is my favorite statistic. My desultory efforts to track it down haven’t been fruitful (possibly because desultory), so I’m immensely grateful for your serious digging here. It wasn’t clear to me, though, how comfortable you are with relying on this source. Is it substantial enough for science journalists like me to keep declaring that we are nine-tenths microbe?

    1. nsaunders Post author

      Well, it’s interesting that even the 1972 Luckey paper does not include a reference to how the numbers were derived. However, he’s clearly referring to some experiment when he says “based upon 10^11 microbes/g contents of an alimentary tract with a capacity of approximately 1 liter.” So yes, you can keep declaring that 9/10.

  3. judah rosner

    I too share your scepticism over the popular quatation of numbers that few have tracked back to their sources. In fact, a wonderful article on the number of human cells is found in:
    Bianconi et al, 2013, Annals of Human Biology doi: 10.3109/03014460.807878
    They estimate 3.72 X 10^13 human cells per body (on average of course).
    The Luckey paper has to be a very low estimate since we now know that many bacteria are non-cultivable. The real number could easily be 10-fold greater.

  4. Liz

    Am I the only one who takes issue with the fact that these estimates are based on the GI tract and the skin? The bacterial diversity and sheer numbers there are not even close to being representative of the rest of the body. What about the brain, or even the bladder?

    1. nsaunders Post author

      It’s a good question. I’d guess that gut and skin account for the vast majority (in terms of total cells) of the microbiome, but would certainly be interesting to know what lives elsewhere.

  5. Pingback: Microbe Counts and Transgenerational Epigenetics | On Science Blogs

  6. Fred

    The number of cells is 10% human, fine, but what about the mass of cells? There may be an order of magnitude more bacterial (i.e. non-human) cells in/on the body, but I suspect the majority of the cell mass belongs to human cells, not bacterial. I can only scan Wikipedia with wide ranges of bacteria size (0.5-5 micrometers) compared to, say, a red blood cell (6-8 micrometers). Any ideas, thoughts, numbers on that?

    1. nsaunders Post author

      Good question. Clearly, human cells (or more accurately, the water inside them) are the larger proportion by mass or volume. I don’t know of a reference with those numbers, but there are sure to be estimates from various sources.

      You could start from this Wikipedia page; mass of average human cell = 1 ng, average E. coli cell = 1 pg. That would make the non-human cells (10^14 x 10^-15 = 0.1) / (10^13 x 10^-12 = 10) = 1% of the human cells, by mass.

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