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Green leaves make carbohydrates by photosynthesis. This means that green leaves are abundant source of energy. Can humans eat enough green leaves off the trees and get necessary daily energy intake?

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    $\begingroup$ Welcome to the site! You may know this already, but early-season leaves of many common trees are edible, including hawthorn and beech (apparently also linden, birch and spruce, but I haven't tried those). Later in the season they accumulate tannins and other secondary metabolites that can be toxic. Unfortunately, I can't find much about their calorie content (although I'm sure it's low), so I can't post a full answer, but I'll leave this here for anyone else who's planning to. My gut feeling is it would be very difficult; assuming similar calories to lettuce, you'd need to eat 13-14kg per day. $\endgroup$ – arboviral Oct 3 '18 at 10:45
  • $\begingroup$ Do you mean exclusively the leaves of trees? If not, there are a number of common foods that are leaves: lettuce, spinach, cabbage, green onions... Offhand I can't think of any tree leaves commonly used as food, but bay leaves are a common seasoning for soups and stews. And I like to nibble the new green leaves of spruce when out hiking... $\endgroup$ – jamesqf Oct 3 '18 at 18:00
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Something can be called a food when it is edible, and most tree leaves are not.

Carbohydrates or other nutrients can be a source of energy when they are digestible, but cellulose and (most) other carbs in leaves are not (Elmhurst.edu).

Finally, to get "necessary daily energy," you need to consume, for example, 2,000 Kcal. So, even if you find edible tree leaves with digestible carbs, they will still not likely provide enough calories. Think of lettuce: 360 grams of lettuce = 54 Kcal.

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