Dear reader, I regret to inform you that you are, at most,
only 10 percent human. This may come as a surprise to you. Understandably,
most of us operate under the assumption that we are what we see in the mirror:
a collection of a trillion or so human cells coordinating, organizing, and
generally doing human cell things in order to keep us alive.
This is not true. Well, it’s not entirely true. When one of us advanced human entities looks in the
mirror, we are seeing trillions of
evolutionarily sophisticated cells acting in harmony to create the most complex
species on the planet. But that mirror is also reflecting trillions of smaller,
less complex organisms whose evolutionary history reaches back a staggering 3.5
billion years. To put that number into perspective, mammals evolved only 250 million years ago.
This ancient organismal class pervades every inch of your
body open to the outside air, and for every one skin cell, hair cell, tongue
cell, or any other kind of cell you see, your body is populated by 10 of their cells. They are bacteria. Though we mostly hear about the miniscule percentage
of harmful ones—see: E. coli or Staphylococcus—the vast majority of
bacteria living in our mouth, digestive tract, and skin actually aid in our
You may have encountered these symbiotic, or mutually
beneficial, bacteria in the context of your gut. These are the “good bacteria”
promoted by various yogurt and cottage cheese vendors claiming to have
“pro-biotic” products. But what exactly do the “good bacteria” do? Though the picture is still blurry,
a growing body of research is slowly providing an answer.
Likely the most studied link between the bacteria saturating
your body—known as the microbiota—and human health is the connection between
intestinal bacteria and weight. Enter Martin Blaser, a physician-scientist at
New York University who studies a particular gastrointestinal bacterium known as Helicobacter pylori. H. pylori generally gets a bad rap; it has been linked to peptic ulcers and stomach cancer. But
Blaser’s lab is set on reversing this reputation.
In 2007, Blaser showed that eradicating H. pylori in adult males was correlated with a 76-percent increase in the
amount of the protein ghrelin in the blood after a meal. Ghrelin is the hormone
thought to be responsible for telling your brain it’s hungry. Empty stomachs
produce high levels of ghrelin. But in Blaser’s study, even full stomachs produced ghrelin when H. pylori was wiped out.
Then, in 2008, Blaser studied individuals whose gut either
did or did not contain the bacterium. The blood of subjects with H. pylori contained about
half the concentration of the protein leptin as H. pylori-negative individuals. Leptin is essentially the
anti-ghrelin protein; it tells your brain, “Stop eating! You’re full already!”
Further studies have linked gut bacteria to weight gain. A
2013 study at Washington University in St. Louis took the gut bacteria from
identical twins, one overweight and one of normal weight, and transplanted them
into mice. Mice that received the overweight twin’s bacteria gained fat tissue
and problems linked to diabetes, even while on a low-fat diet. When those mice
then received the thinner twin’s bacteria, they returned to normal weights, but
only if they continued eating low-fat mouse fare.
In addition to the relationship between weight gain and
bacteria, changes in our microbiota have been connected to an array of health
problems like diarrhea, bad breath, irritable bowel syndrome, increased
allergies, celiac disease, and diabetes. And it seems that creating a healthy
internal environment is not a matter of kicking out the bad bacteria but
instead one of promoting the well-being of the good bacteria that I have already mentioned.
Unfortunately, many Americans have a bug about bacterial
bugs. We don’t like them. We fear them. And a host of modern practices is
changing their composition inside of us.
Perhaps the most troubling example of this is our rampant
use of antibiotics, which are powerful bacteria-killing drugs. We are
prescribed antibiotics for hundreds of different health problems—some serious such as Lyme’s
disease, and some not so serious such as sinus infections. These drugs wipe out illnesses,
but they also kill beneficial bacteria, leaving space for dangerous bacteria
to recolonize in the vacuum left by the antibiotics.
Other practices common in the U.S. further affect the types of bacteria
living within us. Fetuses are sterile in
utero, but their first few moments outside the womb literally fill them
with bacteria. C-sections deliver dramatically different types of bacteria than
do vaginal births, and the effects of this difference are still unknown.
Similarly, breast-feeding provides both prebiotics (food for good bacteria) and
probiotics (actual strains of live bacteria), but for years baby formula, which many families use for their infants, has not
been manufactured with either.
These cultural lifestyle changes may have serious consequences for health, or they may not. Some researchers blame them
for 21st-century epidemics like obesity and diabetes, but the
science does not entirely support these claims. Either way, it will be beneficial
for all of us to remember that no matter how complex we are as humans, we exist
within a sea of bacteria that, depending on our actions, can help us become
happy, healthy, or neither of the two.