system
Given the following post, respond as if you're commenting on Reddit:
human
Let's say I'm lost in the woods and am an expert at identifying plants.  Would it be better to eat "non-food" plants than nothing at all?

To rule out the "don't eat random plants because they could be poisonous", let's say I'm an expert at plant identification and can say with 100% certainty that any given leafy plant is non-toxic.  I'm lost in the woods, and there are no berries or whatnot in the area.  Would I gain any energy from eating leaves or grasses?

If not, then what makes plants like cabbage and lettuce nutritious, but  grass or other leafy plants not nutritious?
gpt
Even if a leaf isn't poisonous, it might not be good to eat.

Poisons in a plant are an example of what is called a qualitative defense.  It is a chemical that directly interferes with an herbivore's metabolism.  These are things like caffeine, cocaine, quinine, nicotine, etc.

There are also quantitative defenses.  These don't necessarily directly act upon an herbivore's metabolism, they just make the plant difficult to consume.  Tannins are a good example of this.  If you ate oak leaves, you wouldn't get any good nutrition from them because tannins block your body's ability to absorb nutrition.  The cells walls of plant cells are made of cellulose and hemicellulose, and you don't have the enzymes to break such large sugars down into calories, so they would be indigestible.  It wouldn't be worth eating them.

Leaves that we can eat are generally lacking in defenses.