Why can cows and sheep survive by eating grass alone, but humans can’t?

Author: Li Zi

Reviewer: Tao Ning, Institute of Biophysics, Chinese Academy of Sciences

Since we were young, our parents have taught us not to be picky eaters and to eat a little bit of everything. We should not only eat rice, vegetables, but also meat, so that we can have rich nutrition. Especially now, everyone's living standards are much better than before, and many people emphasize the importance of protein intake. But then again, I wonder if you have ever thought about this question: Why can herbivores such as cows and sheep grow so strong even though they are extremely "picky" and only eat grass? If we humans do this, we may suffer from malnutrition.

To answer this question, we must first look at what cows and sheep actually eat when they eat grass.

In fact, when they are chewing, they not only swallow the leaves of grass, but also the seeds and roots of grass - pay attention, there are also some nutrients such as protein and fat in them. We humans mainly get protein and fat by eating meat, eggs and milk, but cattle and sheep are more convenient, they just swallow the grass whole. Moreover, many farms now feed cattle and sheep with feed made from soybean dregs, so that they get more nutrition.

But the key issue is not here. People cannot eat grass alone because most of the grass is composed of cellulose. Cellulose is one of the main components of plant cell walls and is also the most abundant polysaccharide in nature. When we eat vegetables every day, we swallow cellulose into our stomachs. Although we eat a lot, the cellulose molecules are too large for the human intestine. Except for a few soluble cellulose, most of the cellulose cannot be absorbed by the human body. Today we often say that some vegetables are "indigestible" after eating, and after eating, they are excreted intact. Among these excreted things, there is a lot of cellulose.

You may have heard that consuming fiber is good for the human body. Yes, a small amount of fiber can promote intestinal peristalsis. When many people with constipation see a doctor, they will receive advice to "eat more fiber-rich foods." However, imagine if we humans only eat grass and eat only fiber in our stomachs, we will not be able to digest and absorb it, and we will not be able to absorb nutrients. This is definitely not feasible.

However, herbivores such as cows and sheep have microorganisms in their stomachs that produce a special cellulase that can break down cellulose. Specifically, these cellulases act like a pair of scissors, breaking large cellulose molecules into small glucose molecules. Glucose can be directly absorbed by the intestines, so herbivores can get enough nutrition by eating grass.

In addition, herbivores usually have several stomachs and long intestines, which provide ample space to digest large amounts of cellulose. Therefore, they get enough nutrition from the grass, which is something that we humans cannot envy. To stay healthy, humans still need to take in a comprehensive nutrition, including water, protein, fat, carbohydrates, vitamins, minerals, and dietary fiber. None of these seven nutrients can be missing.