You may have come across the “fact” that the human body is, well, not really all that human. That’s because, up until 2016, most scientists believed that the bacteria and other microorganisms in our bodies outnumbered our own cells by a ratio of 10:1.
As recently as a decade ago, this statistic was widely accepted by the scientific community, appearing everywhere from rigorously peer-reviewed journals to the websites of highly reputable organizations like the National Institutes of Health (NIH).
However, more recent estimates suggest that the microbes-to-human-cells ratio is more like 1.3 to 1. That’s based on the human body containing 30 trillion cells, compared to around 39 trillion microbes (mostly bacteria). Another way to think about it is that around 43% percent of cells in the body are human cells – the others belong to our microbiome, largely concentrated in the large and small intestines.
The updated figures come from a groundbreaking 2016 study published in the journal PLOS Biology. In their paper, Weizmann Institute of Science researchers Ron Milo, Ron Sender, and Shai Fuchs explained their process for revising the longstanding estimates of human and bacteria cells and how they corrected the 10:1 ratio, which had become widely accepted. The statistic, originally proposed by microbiologist Dwayne Savage in the 1970s, had been cited so many times (and by so many usually-reliable sources) that generations of researchers ended up repeating what Professor Ole Bjørn Rekdal of Bergen University College describes as “an academic urban legend.”
As complex, multicellular organisms, our cells are specialized with unique structures, sizes, and shapes, from red blood cells to skin cells to fat cells to neurons. Although everyone has a different number of cells based on their size and other factors, the researchers used the example of an “average” man in his 20s, with a weight of 154 lbs (70 kg) and a height of 5 feet 7 inches (170 cm) to undertake the tedious task of estimating the number of each cell type, based on the volume and density of the body’s internal organs, ultimately arriving at 30 trillion human cells and 39 trillion bacteria. Interestingly, the researchers estimated that a woman weighing 139 lbs (63 kg) would have roughly 21 trillion cells, but 44 trillion bacteria.
Though we have microbiota throughout our bodies, especially on our skin and in our mouths, lungs, and genitalia, the gut microbiome is particularly important, influencing everything from the immune system to mood to metabolism. So where do these trillions of microbes come from? A huge amount is transferred to infants from their mothers at birth, not only from the birth canal but also from skin-to-skin contact and, in some cases, breastfeeding. Others lurk in every bite of food we eat and are present in the countless environments we encounter daily.
The ecosystem within:
- The Weizmann Institute of Science researchers' improved “guesstimate” of human cells and bacteria certainly hasn’t closed the door to reaching a more precise calculation. And it really is a guesstimate, for many reasons. Not only is the number of human cells constantly changing, as cells die and are replaced, but the population of bacteria in the digestive tract can fluctuate by several trillion when a bowel movement occurs.
- A different way to compare human cells vs. bacteria in the body is by considering the number of unique genes. Humans have around 20,000 genes, while estimates of the unique genes in the microbiome range from 30 times to 500 times that figure.
- Although bacterial cells slightly outnumber our own cells (and hugely outnumber us in terms of unique genes), it’s worth noting that these cells make up a tiny proportion of the body’s total weight.
- Red blood cells are (by far) the most abundant type of human cell. The average adult has around 25 trillion red blood cells (over 80% of his or her total cells).
