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You’ve probably seen cows enjoying a nice mouthful of grass, but why can't we do the same?

Hosted by: Michael Aranda
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Michael: Cows really do have it made. I mean, basically everywhere they look, there’s grass for them to eat! We humans don’t have it as easy. You could try eating grass, but it wouldn’t do much for you. So what’s the difference? If cows can digest grass, why can’t we?

Well, cows have the tools to digest the cellulose in grass, but we don’t. Our digestive systems just aren’t equipped for it. Cellulose is a complex carbohydrate that consists of long chains of glucose units. It makes up plant cell walls, which is why it’s found in basically all plants, like spinach, kale, and grass. Most of the plants we eat do also have some nutrients we can digest. But grass is basically all cellulose, and that cellulose is really hard to break down. Well, unless you’re an animal like a cow.

When a cow munches on some grass, it travels down its esophagus, and into its four-chambered stomach. And for every serving of grass, this actually happens more than once. After the grass gets digested a little, it passes into one of the stomach chambers called the reticulum where it forms chunks called cud. Then the chunks are regurgitated. They’re brought back up to the cow’s mouth so the cow can grind them up a bit more and break down the food even further. Eventually, the cow swallows the food again, which makes its way back to the stomach. Sounds delicious.

It’s not just cows that do this—other animals, like sheep and goats, regurgitate their food too. They’re called ruminants. The main area of the stomach is the largest chamber of the four: the rumen. It’s where the grass-digesting magic happens. See, it’s not actually the cow that’s digesting the cellulose in the grass, it’s the tiny microbes living inside the cow’s rumen. These guys do their job without oxygen, in a process known as anaerobic cellulose digestion. It involves two main steps: enzyme production and fermentation.

In enzyme production, the microbes in the rumen secrete certain enzymes, like cellulase, which helps break down the cellulose. One of the main ways that’s done is by hydrolyzing the cellulose, where a chemical reaction involving water breaks the cellulose up into smaller carbohydrates like glucose. But the enzymes are the real stars of the show, acting as catalysts that kickstart the reaction.

From there, the leftover, smaller carbohydrates are fermented, meaning that they’re metabolized and converted into fatty acids like acetic acid, the acid in vinegar; butyric acid, which is found in milk; and propionic acid, an acid that’s also often used as a food preservative. These later get absorbed as nutrients. After all that, the partially-digested grass eventually reaches the abomasum, which is the acidic part of the stomach that’s similar to ours.

Here, the food is digested even more and eventually enters the cow’s small and large intestines. So, the main players in grass digestion are the microbes. Humans can’t digest grass because we don’t have those microbes to produce the enzymes we’d need to break down cellulose.

We do have the enzymes to digest other carbohydrates, like starch and simple sugars — we’re just missing the ones that digest cellulose. But what if you just take the same microbes that are in a cow’s rumen, put them in your stomach, and let them do their thing? That probably wouldn’t work, because your stomach is way too acidic for the cellulose-digesting process to happen. The

pH of your stomach is normally around 1 to 3, which is very acidic. The pH of the rumen, where the grass-digesting microbes live in cows, is closer to a more neutral 6 or 7. The microbes stop breaking down cellulose at a pH of 5.5 or lower, so putting them in your stomach wouldn’t give you the ability to digest grass.

But, there are two other potential homes for these microbes: your small and large intestines. But neither is a good choice. The pH in your small intestine is much more neutral, but the microbes might try to compete with you for the nutrients in the digested food.

And your large intestine wouldn’t be able to absorb the nutrients from the grass, so putting microbes in it to break down cellulose wouldn’t make much of a difference. Another option might be to just swallow some cellulase — like how people who are lactose-intolerant can take a pill with the lactase enzyme, which allows them to have dairy. But researchers haven’t yet developed a practical method for extracting the enzymes that would allow you to digest grass, and even if they did, we don’t know what effects it would have on your health. So as convenient as it would be to graze on your front lawn for dinner, it’s probably best to leave the grass to the cows.

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