I don’t like to be wrong, not many people do, yet I frequently am and so I need to be corrected. I welcome this, I really do, I’d hate to be responsible for spreading a misconception or something that is flat out wrong. Occasionally, though, it can be annoying.
For example, I am very much against smoking and I spent a long, long time telling people I don’t want their second hand smoke in my lungs. A few years ago, however, the definitive report on this issue was brought to my attention. It was conducted by the UN and showed, quite conclusively, that passive smoking simply didn’t exist, not in adults anyway. It can and does affect children but, as an adult, it just doesn’t.
I don’t mention passive smoking anymore and I don’t often admit the fact that it can’t do you any harm. I don’t think most people are aware of it and I’m happy to let them assume it’s another good reason for quitting. Perhaps this makes me an arse, you can be the judge.
The reason I bring this up is that today I learnt that there is another little tidbit I’m going to have to never use again. Luckily, this one is rather more frivolous although I’m still miffed at having to drop it because it was super cool. The factoid in question relates to the ratio of human cells in our bodies to that of bacteria. Whenever the opportunity arose I’d like to knowledgeably claim that you were 90% bacteria as you carry within you ten times as many bacterial cells as you do cells of your own.
A study recently published in the BiorXiv corrects what it refers to as the “‘common knowledge’ that bacteria outnumber human cells by a ratio of at least 10:1 in the human body.” Their new calculations reveal that the ratio is actually pretty even, we are equal parts bacteria and human which, I guess, is still pretty cool. The balance is so finely placed, they go on, that each ‘defecation event’ may shift that balance in favour of the host as we lose about a third of our bacteria each time we defecate, whether we make an event out of it or not.
One other interesting finding they came across was the proportion of the different cell types, by absolute number, as a proportion of the total. If you look at the graphic at the bottom of this post, taken from the original article, you’ll see that red blood cells make up the bulk by quite some distance. Throw in the platelets too and you could say that we’re 90% enucleated blood cells, which makes humanity sound rather less impressive than it is. Obviously muscle and fat make up most of our bulk, but these cells are very large by comparison and so in absolute numbers they don’t stack up well.
The authors made an effort to trace the origins of the 10:1 claim as, amongst microbiologists, it would appear that the veracity of the claim had been considered dubious for some time. They hunted it down to a calculation that was considered as rough even by the authors of the paper at the time. This was published in the American Journal of Clinical Nutrition in 1972; it was then cited in a review conducted by Savage in 1977. This 1977 review went on to be cited over 1000 times and so the myth was born.
Frustratingly, it can be far harder to correct a widespread misconception than it is to spread that misconception in the fast place. As Jonathan Swift said: Falsehood flies, and the Truth comes limping after it.