Why Can't We Eat Grass?

Some animals like cows can live on grass, so why can't humans? And why do dogs and cats eat it even though they can't live on it either? Learn about digesting grass in this episode of BrainStuff, based on these articles: https://animals.howstuffworks.com/pets/why-do-dogs-and-cats-eat-grass.htm; https://health.howstuffworks.com/human-body/systems/digestive/why-humans-dont-eat-grass.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio. Hey brain Stuff,
Lauren Volabon. Here. It's early April as I record this episode,
and spring is springing. My neighborhood's lawns are growing again,
which means we're getting close to the long, warm season
of one of my favorite scents, freshly cut grass. The

(00:26):
scent is amazing, sweet, bright green, but those clippings would
not be delicious on top of a salad or in
the middle of a blt. A grass is both leafy
and green, like spinach and lettuce, but assuming that you're
a human, it is not good for you to eat.

(00:47):
Generally speaking, you can eat grass. It's natural and not
toxic unless it's been sprayed with pesticides. You just can't
digest it. A grass has zero of the nutritional value
for humans that it does for cows, yacks, deer, sheep,
and other ruminants. Ruminants are mammals that have special digestive systems,

(01:08):
including a four chambered stomach, that allow them to get
at the nutrients in grass. But okay, let's back up
a little. What is grass. Grasses are a family of
plants that occur on every continent, yes, even Antarctica. There
are over eleven five hundred species in over seven hundred

(01:29):
and fifty genera. It's one of the largest families of
plants on Earth. The family does include lots of plants
that we humans have domesticated and specialized to use as food.
Grains like wheat, oats, rice, and corn are all the
seeds of different grasses. Sugar cane is a tall, sturdy
category of grass that we press to obtain sweet juice

(01:50):
that we then process into sugar, molasses, and rum, but
usually we don't eat the leafy part of the grass itself.
One exception is young and tender bamboo shoots, though even
those have to be boiled first. We generally don't eat
the grass part of grasses because we don't have the
stomachs for it or the teeth. Leaves of grass contain

(02:15):
a lot of silica, which is a tough mineral deposit
that helps give them structural support. A research has shown
that it also makes grasses more difficult for insect and
mammalian herbivores to chew and digest. If we tried to
eat grass over time, it would wear away the enamel
on our teeth and we only get the one set
as adults. Ruminants, however, have teeth that are constantly growing,

(02:40):
so as their teeth wear down, they grow back up again.
Ruminants also tend to chew in a side to side motion,
not up and down like we do. This helps shred
tough grass into tiny bits, making it easier to digest
when it gets to the stomach. Unlike humans, ruminants, like cattle,
don't have a single past their food before it enters

(03:02):
their digestive tract. They chew their cud. After swallowing a
mouthful of grass, it enters a cow's reticulum, which is
the first chamber in their four chambered stomach. The muscular
reticulum can push the grass back into the cow's mouth
for extra chewing. Pieces of grass can pass easily from
the reticulum into the second chamber, called the rumen after

(03:25):
which ruminants are named. This is the largest compartment of
the stomach. In fully grown cattle, it can be the
size of a fifty five gallon drum that's over two
hundred liters in the roomin lots of friendly bacteria help
break down tough parts of plants by eating away at
them and releasing nutrients that the cow's body can absorb.

(03:46):
A similar thing happens in human intestines with human safe foods.
The trick is that the cow's complex stomach keeps the
grass they are long enough for the process of rechewing
and the bacteria to really do the work and breaking
everything down and getting more out of the grass than
you or I could. There was a period in history

(04:08):
where a distant and hairer cousin of humans might have
been able to digest grass around three and a half
million years ago. But if a human ate grass today,
it might cause stomach upset, vomiting, or diarrhea as our
body attempted to deal with this, not food, or at
best it might just pass through undigested. But okay, the

(04:32):
digestive systems of dogs and cats are more like humans
than they are like ruminants, So if they can't digest it,
why do dogs and cats eat grass? The scientific answer
is that no one is sure. The common conception is
that dogs and cats do this because they're ill and
trying to make themselves vomit, or because they're missing something

(04:55):
in their diets, but research doesn't back that up. In
two thousand and eight, scientists the University of California Davis
tried to cut through the weeds and shed some light
on this mystery. They sent out surveys to twenty five
dog owning veterinary students. All reported that their canines ate grass.
A nun said that they observed any signs of illness

(05:17):
before their dogs chowed down, and only eight percent so
that their dogs vomited afterwards. The researchers also surveyed forty
seven dog owners who took their pets to the university's
teaching hospital for outpatient care. Seventy nine percent that they
saw their pets eating plants, mostly grass. Four dogs were
ill beforehand, and only six vomited afterward. The team then

(05:43):
opened the survey up to three thousand people who answered
a series of online questions, yielding useful data from about
one thousand, six hundred people. Of those, sixty eight percent
saw their dogs eating plants mostly grass on a daily
or weekly basis. Only eight percent showed signs of sickness beforehand,
and some twenty two percent vomited afterwards. In the same study,

(06:07):
the researchers found that grass eating is also common in
cats and also has nothing to do with upset stomachs
or other illnesses. Most cats, like dogs, do not vomit afterward.
They found that it's a common behavior in normal cats
and dogs, regardless of diet, and they specified that the
behavior of vomiting seems to be incidental to grass eating.

(06:30):
So maybe dogs and cats just like the taste or
texture of grass, or maybe it's just there and borum
snacking can strike us all. Today's episode is based on
the articles why do dogs and cats eat grass? And
Many animals eat grass, so why don't humans? On how

(06:51):
Stuffworks dot Com, both written by John Partano. Brain Stuff
is production by Heart Radio in partnership with how Stuffworks
dot Com and is produced by Tyler Klang. Four more
podcasts my heart Radio, visit the iHeartRadio app, Apple Podcasts,
or wherever you listen to your favorite shows.