Body’s Bacteria Outnumber Human Cells 10 to 1?  Not So Fast…

Body’s Bacteria Outnumber Human Cells 10 to 1? Not So Fast…

Posted by Lori Shemek; March 29, 2016

Although you might like to think of yourself as an individual person, you actually share your body with trillions of bacteria.  The bacteria in our body are critical to our health.  We need the healthy bacteria; without these microscopic critters, we would essentially, not be us. Seventy percent of our immune system resides within our gut, ninety percent of the feel-good neurotransmitter serotonin is not made in the brain..but in the lining of the gut –  directly influencing how the mind works, affecting mood and behavior. In fact, our gut is considered our ‘Second Brain’  and our weight is directly connected to specific strains and amount of healthy gut bacteria.

Most people equate bacteria to illness and disease and are often surprised that on and inside the body, we are dependent upon these microscopic organisms for our health such as: breaking down fiber (that we are not so great at), supplying us with needed vitamins like biotin and vitamin K (we extract more nutrition from our food because of bacteria) and protecting us from harmful pathogens.



It’s a long-held belief in the science and health community that the bacteria in our body (our microbiome), outnumber our cells 10 to 1.  This statistic is a famous and a staggering conception – one that has created a multitude of different health recommendations and generalizations. Imagine the power these bacteria wield in terms of every area of our health and well-being; there are more of them than of us.  This 10-1 ratio is a crude guesstimate from 1972 that crystallized itself, as a fact, through the decades with multiple citations and reports.

  “All disease begins in the gut.” -Hippocrates

This is the 10-1 data that health experts, including yours truly, have ascribed to for decades. You’ll find this statistic in prestigious scientific papers, articles, books, conferences, interviews, scientific symposiums and while it does a good job at capturing just how critical bacteria are to our optimal health, mental health and weight... it isn’t even remotely true.

Recently, research led by Dr. Ron Milo from the Weizmann Institute of Science, used a ‘reference man’  (as his model) and  found that for a man between 20 and 30 years old, with a weight of about 154 pounds (70 kg) and a height of about 5’7 (170 cm), there would be about 39 trillion bacterial cells living among 30 trillion human cells. Which gives us a ratio of about 1.3:1 – almost equal parts human cells to microbes.

Human Cells


Despite the downgrade in the ratio of bacterial cells to human is still quite impressive that there is as much of them as there is of us. 



Sender, R., Fuchs, S. & Milo, R. Preprint on bioRxiv (2015).