I've always heard (from professionals as well) that each step in a food change has a roughly 10% efficiency rate. That is, you earn 10% of the energy the lower chain animal/plant earned when it ate/did whatever. Now my question is in regards to what you hear a lot of hippie-types say: Humans should eat off the land and eat grass. They use teh idea of the 10% efficiency as a way to convince you that it should be done. What i'm wondering though is how much grass would you need to eat in one day to fulfill the 2000kcal daily allowance people are adviced to keep? How many vegetables/fruits on average would you need to eat in a day to fulfill that allowance?