system
Given the following post, respond as if you're commenting on Reddit:
human
Can a human gain calories from eating common leaves?

I assume it wouldn't taste good, but if I was in a survival scenario would eating leaves from trees help? Could I get calories from them in any meaningful way? If not, is there something special about spinach and other leaf vegetables that makes them particularly healthy?
gpt
The calories that are available to humans even in the leafy greens we eat like spinach aren't much. It would take like 10-15 pounds of spinach per day for you to get enough calories to survive and at that point the amount oxalate you'd be getting would be a serious problem.

What makes leafy greens healthy to eat is not the calories, but the vitamins and minerals they contain. Particularly vitamins A, C, and K, potassium, calcium, and magnesium. All leaves will have these since they're indispensable parts of photosynthesis and cellular function for the plant.

The greens we eat have been bred for generations to have thinner cell walls, which means a higher nutrient-to-weight ratio, and also less of things like oxalates, tannins, and other metabolites that make them unpalatable, toxic, or even lock up the nutrients so that we can't digest them.

This would be the main problem, particularly with trees, which often have thickened leaves and a lot of secondary metabolites. They produce these things precisely to prevent animals from eating them. You'd be better off eating the leaves of herbaceous (non-woody) plants. In fact many like dandelion, lamb's quarters, lactuca (wild lettuce), plantago, etc. are perfectly edible and lots of people collect and eat them even outside of "survival scenarios".

tl;dr : You can get nutrition from any leaf that doesn't make you sick, but it might not be much. Definitely not much calories. You should look for the most tender, green leaves without nasty flavors.