I've read that vegetarian food (with the right combination of veg. items) can supply all the nutrients that humans need, except for vitamin B12. For B12 you need dairy or meat products, IIRC.

About amino acids and a vegetarian diet: I had read long ago in a book about nutrition, that diets like the staple Indian diet of cereals and pulses (and vegetables too, of course) can supply all the needed amino acids, if the ratio is around 3:1 for cereal:pulse (where pulse == legume (seeds), like dals / beans / lentils / chickpeas / etc.). And that ratio is roughly what people use when eating rice with dal, or rotis/chapatis with dal, for example. This works because the cereals and pulses are both deficient in certain amino acids that humans need, but different ones, so combining them works to give enough of all of them, in the right proportions.

Coincidentally, just today I was reading up on Chinese cuisine, and one link led to another and I looked up Legume in Google and then in Wikipedia:

http://en.wikipedia.org/wiki/Legume

See under the section Nutrition Facts on that page. Interestingly, the article also says that that theory of protein combining - http://en.wikipedia.org/wiki/Protein_combining - is now out of favor, but still says that that ratio works, and gives examples of people in India and other countries eating foods that give that combo - e.g. "Mexican beans with corn tortillas, and the Middle Eastern Hummus (Chickpea spread commonly served with Pita bread) and Mujaddara (A dish consisting mainly of rice and lentils).[7]"

In this connection, the story of Alain Bombard, which I read as a kid, is interesting too - he sailed across the Atlantic in a small boat, with no provisions, catching fish and plankton to survive. I think I read that he researched nutrition needs and decided to get plankton too, because meat / fish do not contain Vitamin C.

http://en.wikipedia.org/wiki/Alain_Bombard