Why don't we eat the leaves off the trees?

August 17, 2009 7:45 AM

Why do we eat leaves from plants that grow on the ground, but not from trees?

Help me give my three-year-old authoritative answers to his difficult questions. He wanted to know why we don't eat leaves off the trees. And I have no idea! Other animals eat tree leaves, and people eat all kinds of green leaves from lettuces, cabbages, herbs, etc. Are tree leaves non-nutritive for humans? Would they make us sick? Or do they just taste bad? Please help me be Dad Who Knows Everything.

posted by escabeche to science & nature (27 answers total)

Think of it from an evolutionary perspective. While there are both wild forbs and trees with edible leaves, forbs generally complete their reproductive cycle quicker, and so would be quicker to breed for palatability/nutritiousness. Also, there are just more species of forbs than trees, so more starting options in the agricultural domestication lottery.
posted by Tsuga at 7:53 AM on August 17, 2009

Too much cellulose, I would imagine. Also, I think it's more a question of terminology; what we eat are referred to as "leaf vegetables." Tree leaves tend to have tannins and other disagreeable tasting things evolved to prevent their being eaten. We do eat "leaves" that aren't on the ground as such - in my native country, grape leaves are eaten with far more regularity than most leaf vegetables, and these are harvested from supported vines.
posted by Dee Xtrovert at 7:55 AM on August 17, 2009 [1 favorite]

Aside from the tannins and whatnot, leaves provide very little energy relative to the amount of effort it takes to extract it; that's why people traditionally eat lettuce when they diet. Mammals which eat a diet of leaves from trees (e.g. sloths) generally spend their entire day grazing and don't move about more than they need to. The human digestive system has evolved to work with foods with a higher energy value (roots, fruits, meat). We now lack the necessary enzymes to digest the cellulose in fibrous plant materials.
posted by le morte de bea arthur at 8:01 AM on August 17, 2009 [1 favorite]

Bay leaves are from what is more or less a tree.
posted by Chocolate Pickle at 8:12 AM on August 17, 2009

Actually, the answer is, "Because tree leaves don't taste good."
posted by Chocolate Pickle at 8:12 AM on August 17, 2009 [4 favorites]

Like the leaves of the Bay Laurel mentioned above, the leaves of the Curry Tree and Kaffir lime are used for seasoning. And people certainly do drink a lot of tea. I don't believe that the leaves of any of these plants are customarily chewed, swallowed, and digested (probably for the physiological reasons listed above), but it's hard to make the case that all tree leaves "taste bad".
posted by Johnny Assay at 8:29 AM on August 17, 2009

Bay leaves are from what is more or less a tree.

Not "more or less," Laurus nobilis is a tree, period. But you don't eat the leaves, you steep them in a simmering sauce for flavor. There are several other highly aromatic leaves that are also used as herbs, including curry leaves and kaffir lime leaves.

The leaf veggies we grow can be cultivated into a reliable, digestible source of food quickly and the lettuces and cooking greens are largely "cut and come again" growers. Trees send out leaves less aggressively, the leaves are tougher, and don't taste like much.
posted by desuetude at 8:32 AM on August 17, 2009 [1 favorite]

uhh... for the record, pine needles are one of the "big 4" survival foods, along with cattails, grasses, and oaks. According to my survival hero Tom Brown
posted by Redhush at 8:33 AM on August 17, 2009

On, should'a previewed, Johnny Assay and I are on the same page. And tea leaves are indeed eaten in salad in southeast Asian cuisines, but I don't think that tea plants are considered trees. (And I think I know what I want for lunch now!)
posted by desuetude at 8:34 AM on August 17, 2009

The Wikipedia List of plants with edible leaves includes several trees. For some of them, such as the Moringa, the leaves are consumed directly as a leaf vegetable.
posted by jedicus at 8:36 AM on August 17, 2009

There are tons of ground plants whose leaves we don't eat (poison ivy, hostas, grass, pretty much anything else growing in your flower garden or local woods), but luckily for us we found a handful of tasty things growing on the ground that we managed to breed into a wide variety of vegetables. For instance, cruciferous vegetables all come from the Brassica family of closely-related plant species. Even more impressively, kale, cabbage, collard greens, brussels sprouts, etc. are all actually bred from the exact same species, Brassica oleracea.

On the other hand, I don't think there are any trees with particularly nutritious or tasty leaves that are worth breeding into lots of veggie varieties. Without a good starting plant, and taking into account that trees take a lot longer to grow than ground plants (decades vs. one season!), there's no real incentive to breed trees for food. Plus, climbing into trees to harvest leaves is a lot more work than picking stuff off the ground, so it would have to be worth it - meaning that the leaves would have to be even more nutritious than what we could get without climbing. (Note that fruit manages to be worth the cultivation effort, by being delicious and full of sugar.)
posted by vytae at 8:42 AM on August 17, 2009

...people eat all kinds of green leaves from lettuces, cabbages, herbs, etc. Are tree leaves non-nutritive for humans?

Lettuce, cabbages and herbs are all basicaly non-nutritive too. The only counterexample I can think of is spinach, but IIRC that was oversold (thanks Popeye!). Some mineral or vitamin content, in some cases, but between zero and no calories.

That's your answer. "Leaves aren't good food for people. Except some leaves make stuff taste a little extra yummy."
posted by DU at 9:36 AM on August 17, 2009

We do eat hearts of palm.
posted by musofire at 9:38 AM on August 17, 2009

Other animals eat tree leaves, and people eat all kinds of green leaves from lettuces, cabbages, herbs, etc. Are tree leaves non-nutritive for humans? Would they make us sick? Or do they just taste bad?

A little of everything, depending on the plant in question. We don't eat leaves off of every plant that grows in the ground either, after all.

With anything edible/non-edible, it's a combination of non-nutritive in some cases, tastes-bad in others, and poisonous in still others. For a three-year-old, I think you can get away with saying "some kinds of leaves are people food, and other kinds are animal food. People food isn't good for animals, just like animal food isn't good for people." If you're especially worried he may nibble on an oak leaf just out of curiosity, you can maybe add "and a lot of times it also just tastes yukky".
posted by EmpressCallipygos at 9:41 AM on August 17, 2009

We don't eat them because the leaves of woody plants are fibrous. The leaves we most often eat are from annuals that do not build up the lignin and cellulose that makes eating leaves from tree or shrubs unpleasant. Humans can't digest cellulose, and our digestive system is not really set up to handle it in a way that makes it worthwhile. So (generally speaking) we use leaves of trees and woody shrubs for their flavor (tannins and other alkaloids that protect plants from pests) by steeping or grinding, or only use the tender green shoots.
posted by oneirodynia at 10:19 AM on August 17, 2009

Lettuce, cabbages and herbs are all basicaly non-nutritive too. The only counterexample I can think of is spinach, but IIRC that was oversold (thanks Popeye!). Some mineral or vitamin content, in some cases, but between zero and no calories.

I'm confused by this. How is a plant rich in vitamin C considered non-nutritive?
posted by desuetude at 10:38 AM on August 17, 2009 [1 favorite]

Because you'd need three more stomachs to digest tree leaves.
posted by electroboy at 11:09 AM on August 17, 2009

Lettuce, cabbages and herbs are all basicaly non-nutritive too. The only counterexample I can think of is spinach, but IIRC that was oversold (thanks Popeye!). Some mineral or vitamin content, in some cases, but between zero and no calories.

I'm confused by this. How is a plant rich in vitamin C considered non-nutritive?

Meaning, of negligible caloric value net of eating.
posted by grobstein at 11:50 AM on August 17, 2009

...there's no real incentive to breed trees for food.

Um, oranges, apples, peaches, pears. apricots, walnuts, filberts, almonds...

Orchards are big business in some areas.
posted by Chocolate Pickle at 11:55 AM on August 17, 2009 [1 favorite]

le morte de bea arthur's excellent answer translated for a 3-year old:

Aside from the tannins and whatnotTree leaves don't taste good because they have special chemicals that the trees need, leaves provide very little energy relative to the amount of effort it takes to extract itplus they're tough so you have to chew them too much.that's why people traditionally eat lettuce when they diet. Lettuce is easy to eat, plus it makes you skinny AND healthy so grown ups really like it. Mammals which eat a diet of leaves from trees (e.g. sloths) generally spend their entire day grazing and don't move about more than they need to.Some animals don't eat anything else, plus they're kind of lazy, so they can sit around eating all day, instead of playing and reading and shopping like you and I do. The human digestive system has evolved to work with foods with a higher energy value (roots, fruits, meat).We like our food to taste good, like peaches or hamburgers (or choose your toddler's favorite food) We now lack the necessary enzymes to digest the cellulose in fibrous plant materials.Plus, tree leaves would give you a tummy ache.
posted by nax at 12:12 PM on August 17, 2009 [5 favorites]

P.S. I am in love with your kid. What a great question.
posted by nax at 12:13 PM on August 17, 2009 [2 favorites]

I think this is covered somewhat in "Guns, Germs, and Steel" by Jared Diamond in a somewhat opposite way you are looking at it. Instead of asking "what is wrong with these plants" he assumes that things were wrong with the plants (maybe he does offer evidence. It's been a few years since I read it) and that's why they aren't used today.

When you get down to it, people have tried to eat everything on the face of the earth they could get their hands on. The things that were better were essentially bred into the current varieties we enjoy and devour today or already tasted good. Likewise with animals. We've domesticated all of the ones we could and gave up on the rest.

Also, I didn't see this argument: If you're going to breed trees why not breed them for their fruit which tastes better than the leaves? I bet if we tried to breed trees with tasty leaves we'd eventually get something out of it. But why bother spending centuries/millenia on that when you can breed apple trees in only a few generations?
posted by Green With You at 3:59 PM on August 17, 2009

Bamboo shoots (though properly speaking, bamboo is a very large species of grass, not a tree).
posted by bad grammar at 6:10 PM on August 17, 2009

We now lack the necessary enzymes to digest the cellulose in fibrous plant materials.
le morte de bea arthur, I agreed with your entire post up until this technical glitch.

Ruminants rely on symbiotic intestinal bacteria & other microorganisms to extract nourishment from plant matter (much as we do). Enzymes also play a role, but without the bacteria, cows would wither on grass.

I'm not certain the cellulose is digested. Nutrients are absorbed; IIRC cellulose ends up in the poo (but I could be quite mistaken).


grobstein:
Lettuce, cabbages and herbs are all basicaly non-nutritive too. The only counterexample I can think of is spinach, but IIRC that was oversold (thanks Popeye!). Some mineral or vitamin content, in some cases, but between zero and no calories.

I'm confused by this. How is a plant rich in vitamin C considered non-nutritive?

Meaning, of negligible caloric value net of eating.

Under your theoretical definition of "nutritive", a sugar packet is, and the salad bar isn't (until dressing is added). That's not what that word means, according to every definition I found in a quick search.

Curiously, you contradicted yourself with your example of spinach, which no one eats for the calory-load (almost zero); it is the vitamin & mineral content (or nutrients, for short) that make it nutritive.
posted by IAmBroom at 8:40 AM on August 18, 2009

Meaning, of negligible caloric value net of eating.

Under your theoretical definition of "nutritive", a sugar packet is, and the salad bar isn't (until dressing is added). That's not what that word means, according to every definition I found in a quick search.

I've never heard this definition of "nutritive" for food either. Sweeteners are classified as nutritive or non-nutritive based on caloric content, but I have never seen this applied to food. In fact, you can search foods based on their nutrients here. Vitamins and minerals are certainly included.
posted by desuetude at 11:01 AM on August 18, 2009

OK, I mistook your defense of DU's post with the post itself. Mea culpa.
posted by IAmBroom at 11:12 AM on August 18, 2009

I love oneirodynia's answer and think it's correct:

We don't eat them because the leaves of woody plants are fibrous.

But I'd like to try to push it back one level of causation because there I think your child's marvelous perception reappears and takes on a central importance.

The leaves of woody plants have to be fibrous, tough, and dry because they are up off the ground and exposed to the wind. If they weren't tough and fibrous they'd be shredded by the wind, and if they weren't dry the tree would lose a lot more moisture, and the weight of the leaves would make it much harder for them to be held out so as to catch the light most efficiently.

Plants with leaves that grow straight out of the ground, where the wind is much less and water doesn't have to be pumped very far (and issues of evaporation are less) don't have these problems and can be much tenderer.

So in a very real sense, we eat the leaves of plants that grow on the ground because they grow on the ground.
posted by jamjam at 2:18 PM on February 6, 2010