Why Can't Humans Eat Grass?

Cows live on a diet of mainly grass, but humans would quickly waste away if they tried to live solely on their lawns.

But why is that?

In principle, people can eat grass; it is non-toxic and edible, and in fact our ancient ancestors dined on grass millions of years ago, according to a study published in the Proceedings of the National Academy of Sciences. As a practical food source, however, your lawn leaves a lot to be desired.

There are two main problems with a grass diet. 

Grass is mainly composed of water and lignin, a woody protein that is common in the cell walls of plants, according to ChemistryisLife.com. Lignin can be hard to break down by human stomachs. Not only that, but the lignin in food can actively inhibit the access nutritional benefits of other types of digestible fiber, such as cellulos, according to a 2001 study in the Journal of Range Management.

Animals such as cows, on the other hand, have a specialized stomach with four chambers to aid in the digestion of grass (a process called rumination). This helps them access the starch and cellulose in grass, even when the lignin takes much longer to break down, according to "Tropical Dairy Farming: Feeding Management for Small-Holder Dairy Farmers n the Humid Tropics" (Landlinks Press, 2005).

Aside from the digestion issues, a second problem with grass as a food source is the mastication. Your dentist would not be pleased; grass contains a lot of silica, an abrasive which quickly wears down teeth. Grazing animals have teeth that are adapted to continually grow, replacing the worn tooth surfaces quickly, according to the Burke Museum.

Originally published on Live Science.