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So I used to have this shirt.
I know at least one of you was watching back
in 2013…
Hey Kyle.
For the rest of you are probably wondering
the same thing as me.
What’s up with the hair?
What this shirt means is that the microbes
in and on our bodies outnumber our own cells.
Most common figure is by 10 to 1.
Except… that’s not true.
It’s a scientific urban legend.
Yet this factoid continues to be shared and
recited as fact.
I’m guilty of it too, I mean, my old video
is called “You’re Mainly Microbe” and
it’s literally centered around this erroneous
factoid.
It turns out that urban legends like this
are surprisingly common, even in science,
and how they begin and the reasons why they
persist can teach us a lot about how science
works, and when it doesn’t.
At some point the 10-to-1 bacterial to human
cell ratio became “common knowledge”.
Common knowledge is information that the average,
educated person in some group–the general
public, scientists, whoever–accepts as reliable
without having to look it up, like how we
all know that water freezes at 0˚C.
We all know that, right?
Somewhere along the line, people stopped asking
where this “common knowledge” came from.
There are countless facts in science that
have become common knowledge.
I mean, if research papers cited an original
source for every single fact they presented,
it would be an absolute mess.
Say you wrote a paper about synthesizing some
new chemical?
Do you have to cite a paper that proves chemicals
are arrangements of different atoms?
Ok, then do you need to cite something to
prove that atoms exist?
Maybe Einstein’s 1905 paper on Brownian
motion?
Or do you have to go back to John Dalton in
the early 1800s?
You can see things get ridiculous pretty fast.
But!
Sometimes things that aren’t true become
common knowledge, or they’re corrected later,
but the new information fails to replace the
old idea.
Here’s an example: I wouldn’t be surprised
if at some point in your life, you probably
heard that spinach was a particularly excellent
source of iron.
I certainly remember being taught that, I
can’t even remember where.
And–you can probably guess where I’m going
with this–it’s not true.
In 1981, a biologist named Terry Hamblin studied
historical science papers and realized the
iron content in spinach was misreported, thanks
to a misplaced decimal point, way back in
the early 1900s.
Except he didn’t cite a source for the misplaced
decimal point story either.
And it turns out that THAT’s a myth too.
Turns out the earliest old-school measures of iron in spinach were waaaay too high,
and wrong, but because of contamination, not a misplaced decimal point.
It's science! Details matter!
Spinach actually does contain large amounts
of iron, as much as red meat in some cases,
but it also contains compounds that make the
iron it does have harder for us to absorb.
So it’s not an exceptionally great source
of iron.
Incidentally, it turns out "Popeye" creator E.C. Segar chose spinach as the sailor man's food
of choice for its high vitamin A content, not because of iron.
It’s another case where the correction never
seems to spread as wide as the lie, and it’s
a good reminder that a good story is not necessarily
a true story.
And I’m willing to bet that at some point
in your life, you’ve taken vitamin C to
help cure or prevent a cold.
Yeah, that’s not true either.
That myth traces to legendary scientist Linus
Pauling.
In 1966 Pauling was convinced by a random
dude named Irwin Stone that taking large doses
of Vitamin C would help him live longer, and
Pauling started taking doses equivalent to
1800 glasses of orange juice every day, and
wrote books and articles claiming that the
colds he had suffered from his whole life
“no longer occurred”.
Even though Linus Pauling won not one but
two solo Nobel Prizes in his life, dozens
of studies since have proven he was wrong,
about vitamin C. It doesn’t significantly
affect colds, and the only disease it definitively
prevents is scurvy.
Yet somehow the cold myth still continues
today.
Or maybe you’ve heard that you lose most
body heat through your head?
That urban legend goes back to one military
study in the 1950s where people were left
out in the cold with no hats on.
I mean, you’re gonna lose most of your body
heat through your head if that’s all that’s
exposed.
Today scientists know the amount of body heat
you lose depends on the total surface area
exposed, but parents everywhere are still
making sure you don’t leave home without
a hat.
You also don’t need to drink 8 glasses of
water a day.
That urban legend probably goes back to one
set of dietary recommendations for water intake
from 1945.
Except many people who cited that number ignored
the part where it said most people get a majority
of the water they need from food.
It’s important to stay hydrated, but 8 glasses…
I mean, like what size of glasses even?!
And one of the most famous is that sugar causes
hyperactivity in children.
This one seems totally logical, but more than
a dozen randomized controlled trials have
failed to detect different behavior between
kids given large doses of sugar and kids who
weren’t.
That’s right, the cake is actually a lie!
Turns out when parents even think their children
have been given a drink containing sugar (even
if it’s actually sugar-free), they tend
to think their kids are being hyperactive.
This particular urban legend traces its origin
back to California allergy doctor Benjamin
Feingold in 1973, who with little to no evidence,
recommended removing artificial colors and
flavors from the diets of hyperactive children,
and I guess people were like “why not sugar
too!”
I mean, kids are just kids, and they’re
gonna go nuts some times.
Let’s go back to that 10-to-1 mainly microbe
cell number from the beginning.
In 2010 a couple of researchers went on a
deep dive to find the original source, and
the paper cited most often was this one, from
1977.
It states the human body contains 100 trillion
microbial cells and 10 trillion of its own
cells.
Ten to one.
Scroll down to reference #70 and we find the
source of the 100 trillion microbial cell
number is this 1970 paper by Thomas Luckey,
which, when we read the paper, turns out was
just a back of the envelope estimate, and
wasn’t based on any actual experiments.
This has nothing to do with the rest
of the video, but I just have to mention Dr.
Luckey was literally an honorary samurai,
which is awesome
And going back to the original 1977 paper,
the human cell number comes from reference
#27, a 1970 textbook by biologist Theodosius
Dobzhansky.
I dug through the internet to find a copy
of it, and right there in chapter 1, with
absolutely zero supporting evidence, is the
claim that a human body contains ten trillion
cells.
And there you have it.
A back of the envelope estimate combined with
a totally unsupported approximation to create
the very wrong and very widely shared fact
that human cells are outnumbered by microbes
10-to-1.
Right about now you’re probably wondering
what the real numbers are.
First, the original estimate for microbes
living inside us was calculated using the
volume of the entire lower intestine.
But the vast majority of your body’s microbes
live in your colon, which is only a portion
of that volume.
And yes, that’s where your poop is made.
Using a more accurate volume of the average
colon–409 milliliters–and the number of
bacteria we typically find per volume of poop,
in 2016 researchers calculated that your inner
microbial population is… drumroll please…
39 trillion.
Not 100 trillion.
And as for the number of cells in the human
body?
This is a seemingly simple question that you
might assume we biologists have known for
a long time.
But the truth is, until very recently, no
one really knew.
Over the past couple centuries, estimates
have ranged from 5 billion to more than a
quadrillion cells in our body.
What makes it so difficult is that cells in
our body vary hugely in size and how tightly
packed they are, so the only way to get a
good count is to estimate each organ individually.
And that’s what a group of researchers did
in 2013.
Based on actual evidence, their new number
is… 37.2 trillion cells in the average human
body.
That makes the ratio of microbe to you more
like 1 to 1… pretty much even stevens.
Amazingly, although most of your mass comes
from muscle and bone cells, by sheer number,
red blood cells make up more than 80% of the
cells in your body.
bit more in favor of the microbes.
But remember how I said almost all of your
inner microbes live in your colon?
Well, you lose almost a third of them every
time you have a bowel movement, so every time
you poop, the ratio swings in your favor,
at least for a few hours until they get their
numbers back up.
Doesn’t make as catchy a shirt though…
Things we consider common knowledge can be
based on bad information, and despite the
amazing power of science to correct its own
mistakes and uncover better and better knowledge
over time… that good knowledge doesn’t
always spread out and replace the bad knowledge.
So how do these scientific urban legends continue
to persist?
More scientific journals exist today than
ever before, and we’re doing more science
today than ever before.
Most of that science is peer-reviewed, but
peer-reviewed doesn’t always mean something
is true.
If one false citation makes it into the system,
it can set up a domino effect as other people
cite that bad fact instead of verifying the
original.
The solution?
Well, for you out there in the “general
public” at least, wherever you can, even
if you think something is common knowledge,
try to learn where it came from.
You might be surprised by what you find.
But that’s easier said than done, because
most published science today isn’t freely
available, at least not legally.
Most scientific research today sits behind
paywalls, so even if you wanted to check a
source, you couldn’t.
Then what about this?
Now, it’s easy to dump on Wikipedia.
Anyone can edit it, and I mean, they have
en entire page titled “Wikipedia is not
a reliable source”.
It’s a paradooooox… wait, why don’t
I have a wikipedia page?
Come on Kyle.
But Wikipedia represents a collection of our
common knowledge.
It’s the most widely read and widely accessible
information source on Earth.
And at least one study has shown that Wikipedia
pages are more likely to cite scientific sources
that are freely available.
This isn’t an ad for Wikipedia, it just
seems like if you want to get good science
out to the broadest audience, making it freely
available is not a bad place to start.
The point, to me at least, is pretty clear.
If you want common knowledge to be true, you
have to let true knowledge be common.
Every one of us carries quite a few pieces
of incorrect knowledge in our heads.
That is nothing to feel bad about.
What matters is being comfortable enough with
the idea of not knowing everything that you’re
able to replace bad knowledge when you find
better knowledge.
Stay curious.
