For humans, what's the difference between an edible leaf like lettuce, and a non-edible (in the sense of null nutritional value) leaf like grass?


closed as too broad by John, theforestecologist Jun 11 at 19:36

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  • $\begingroup$ What would the answer to this question help you achieve? $\endgroup$ – Jan Jun 7 at 17:06
  • $\begingroup$ What context are you looking for, digestibility, why one is more palitible, what selective pressures make humans not think grass tastes good. Because as it stands the answer is "one tastes good and the other does not" Please flesh out your question, also show some attempt to answer the question yourself as "homework" questions are considered off topic without it. $\endgroup$ – John Jun 8 at 4:39

For grasses leaves tend to be inedible due do with the high quantity of silica deposited in the leaves to form structures known as phytoliths Wikipedia.

While this has been shown to inhibit digestion by insects (Hunt et. al., 2008, Massey et.al., 2006), I'm not aware of any studies on how this affects digestibility for humans. There are however anecdotes about people starving to death with grass stains around their mouths during the Irish Potato famine.

  • $\begingroup$ I'm not convinced the ability of humans to subsist on grass is due to inhibition of digestion; I think it's more likely that we wouldn't be able to digest the carbohydrates in grass anyway. It doesn't seem likely to me that high quantities of silica would be the major limitation of us digesting something. Grazing animals manage just fine on grass, albeit with adapted digestive systems. $\endgroup$ – Jam Jun 21 at 18:25
  • $\begingroup$ @Jam I'm not aware of a significant difference in the types of carbohydrate present in grass leaves vs. those of "edible" leaves like lettuce. However, there is some research showing that silica inhibits both palatability and digestibility for grazing mammals (Shewmaker et. al., 1989, Hartley and DeGabriel, 2016). $\endgroup$ – tyersome Jun 22 at 20:49

Nutrient content of lettuce and grass can be similar.

According to USDA.gov, 100 g of green leaf lettuce contains:

  • Protein = 1,4 g
  • Digestible carbohydrates = 1.5 g
  • Calcium = 36 mg
  • Potassium = 194 mg
  • Magnesium = 13 mg
  • Fiber (cellulose or similar) = 1.3 g

According to Feedipedia (see Nutritional Tables), 100 g of Bahia grass contains:

  • Protein = 3.5 g
  • Digestible carbohydrates = 0.6 mg
  • Calcium = 160 mg
  • Potassium = 400 mg
  • Magnesium = 27 mg
  • Fiber (cellulose) = 7.8 g

The "leaves" (shoots) of most grasses are considered inedible, because they:

  • are unpalatable
  • are hard to chew because of high cellulose content
  • can probably make you sick; at least it can make dogs vomit (PNAS)
  • contain silica that can wear your teeth; some types of grass are also toxic (eattheweeds.com)

Would it make sense to eat grass in a survival situation? No, because 100 g of grass contains only about 20 Kilocalories, mainly from protein and a little bit from cellulose fermentation by intestinal flora in the colon (gut.bmj.com). It's also quite possible you would vomit or get diarrhea after eating grass.

  • 2
    $\begingroup$ I don't know if all those things are true (i.e., I'd want to see a reference for having the same amount of micronutrients, differences in chewing difficulty) but in any case that comment I think is much closer to answering the original question. I'll clean up this comment thread. $\endgroup$ – Bryan Krause Jun 6 at 15:41
  • $\begingroup$ ndb.nal.usda.gov/ndb/foods/show/… $\endgroup$ – Alexander Gegg Jun 6 at 17:52
  • $\begingroup$ Good answer but the Daily Mail isn't a particularly academic source. $\endgroup$ – Jam Jun 21 at 18:31
  • $\begingroup$ @Jam, OK, it is known that dogs and cats eat grass to make them vomit when they are sick; there are literally no studies that would even mention eating grass by humans. $\endgroup$ – Jan Jun 22 at 8:40

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