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Can a human eat grass and digest it? Could it be possible to use it as food just like other plants such as wheat or beans?

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Note that many of our grains are grasses. We can't metabolize cellulose, but the seeds contain enough other materials -- especially after cooking or grinding to break down any cellulose barriers around them -- to be worth eating. That may not be what you intended to ask, though. –  keshlam Mar 7 at 19:46
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From a taxonomic point of view, wheat is a grass. So yes, we can eat it. Narrow down and clarify your question. –  fileunderwater Mar 10 at 11:54
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To elaborate on A random zoologist's answer, the problem is that the human digestive system does not contain any cellulase enzymes. Cellulases are a class of enzymes that break down cellulose, the chief structural component of plants. You might be able to obtain a small amount of nutrition from grass or other cellulose-rich materials, but as the plant cell walls are made of cellulose, most of the plant material will be indigestible.

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At the end of the linked article it is stated that cellulase is used to treat phytobezoars, which apparently are indigestible masses of cellulose stuck in the stomach. Could one take a supplement that renders the grass more digestible? –  Tobias Hagge Mar 7 at 19:58
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It would probably not be useful as a supplement, as the enzymes wouldn't last very long in the stomach. Fun fact: An alternative to cellulase for treating phytobezoars is coca-cola administred through the nose: medicine.virginia.edu/clinical/departments/medicine/divisions/… –  jarlemag Mar 7 at 20:02
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Could we cook the grass and break down the cell walls? –  Andrey Mar 7 at 22:07
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Not easily. To break the crystalline structure of cellulose by cooking requires a temperature of 320 C and 250 atmospheres of pressure: en.wikipedia.org/wiki/Cellulose#Structure_and_properties –  jarlemag Mar 7 at 22:11
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Cellulases are generally slow, which is why mammals that have microbes that can break down cellulose also have very long transit/residence times in their guts. In the extreme case, ruminants (cows for example) have developed huge, multi-chambered guts to ensure that food stays in their digestive system long enough for the cellulases to act on a useful percentage of the food they ingest. So a human gut with cellulases is still not going to get useful amounts of nutrition out of ingested cellulose. –  Chinmay Kanchi Mar 8 at 6:10
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Well, humans can eat grass, but it will not be digested. Cause to digest grass our body needs different kind of enzymes, which lack in humans so they can't digest grass directly although humans can eat grass, but not recommended.

Similarly our body has different enzymes to digest wheat and beans etc. Also remember rice, wheat and beans are fruit, not plant. Although we can digest some of the plant.

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Aren't fruits parts of plants? –  Xander Smeets Mar 7 at 21:00
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"Also remember rice, wheat and beans are fruit, not plant". what is meant by this ?? –  WYSIWYG Mar 8 at 16:02
    
I think this is a beginners mistake. I think A random zoologist thinks fruits aren't parts of plants. This isn't true, I think. –  Xander Smeets Mar 8 at 16:26
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I think he meant that the rice, wheat and beans that we eat are the fruits of the rice, wheat and bean plants, and thus have a different composition than the rest of the plant. The Wikipedia article on cereals says "A cereal is a grass, a member of the monocot family Poaceae, cultivated for the edible components of its grain (botanically, a type of fruit called a caryopsis)" –  jarlemag Mar 8 at 18:01
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Even in times of famine, when people are literally dying from hunger, they do not eat grass - it is not digestible. But they eat cooked artiplex, which commonly grows by roadsides.

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Wheat and other grasses are very significant sources for human nutrition. Grass seeds are digestible; seeds have to digest themselves to sprout, with very little biochemical machinery. Human civilization is founded on the ability to cultivate grass to eat.

It is due to pepsin that a cellulase supplement would not help to digest grass stems and leaves. Pepsin is the enzyme in the stomach that breaks peptide bonds and from its perspective, cellulase is just another peptide chain (pepsin is so effective, the stomach has to prevent itself from being digested while at the same time maintaining enough pepsin to digest food — because pepsin digests pepsin). Cellulose is a tough sugar polymer (as is chitin, which forms insect and fungus exoskeletons), so after all the cellulase was digested by pepsin only bacteria from the intestine could process the remaining cellulose. What the bacteria convert it to might make one ill.

Humans can derive significant nutrition from bamboo shoots, which have less robust cellulose than in mature plants, and bamboo is a grass. Pandas also cannot digest cellulose effectively, but that doesn't stop them from trying. They might eventually develop a cellulase gene by sheer willpower and tenacity.


@jarlemag: inhaling sugar water could quickly become a very bad idea. Encouraging microbes to colonize near the brain may result in them converting neurons into more microbes. The Naegleria amoebae can be very aggressive. In Pakistan and India it is somehow traditional to treat illness by putting water up one's nose; when it is not recognized how clean the water must be to do that safely, people die.

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The soda is not inhaled. It's transferred directly to the stomach through a tube inserted through the nose: en.wikipedia.org/wiki/Gastric_lavage Anyway, it is a medical treatment and I'm no doctor - I'm sure there are proper protocols for it but I don't know anything more about it. –  jarlemag Mar 7 at 23:32
    
Also, though I do not know, it is likely that the soda is being used for its acidic properties. If so, diet soda would work just as well, and so it is as likely to be aspartame water that is being administered via a nasogastric tube (not inhaled) as it is to be sugar water. –  David Conrad Mar 8 at 0:52
    
Yes. Apparently some protocols use regular soda and some use diet soda. –  jarlemag Mar 8 at 9:58
    
Good point about the bamboo shoots. –  keshlam Mar 8 at 18:21
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