Greens
Contain all the nutrients and proteins a human needs.
Since plants get their energy from sunlight with a minimal amount of nutrients from the Earth, plants are indeed far easier on the planet's resources than animals as a source of food for humans. But eating plants alone wouldn't be sustainable either, if you're talking about the planet's entire population. As I understand it, chemical fertilizers are indispensable today to keep the world from starving. If everyone on the planet ate only plants, we might very well be in a pickle just as we are by so many people eating animals. What I was trying to get at was that, as I see it, no realistic diet is truly sustainable. Sure, we could all eat bricks of algae or insects or something along those lines for maximum sustainability. I'm not even sure that could be sustainable forever. Absent some cataclysm, humans will strip the Earth of its resources. Maybe then our population will return to a balanced level.