Isn't it hard to get enough protein on a vegetarian diet?
No. Vegetarians typically consume less protein than meat eaters but can still easily meet their needs. Legumes are top sources of protein, but grains, nuts, seeds, and vegetables all provide some. Meat substitutes, such as veggie burgers and soy crumbles, provide about as much protein, ounce for ounce, as meat. If you choose a variety of foods—especially if you eat some dairy and eggs—you should get all the protein you need after going vegetarian.
Do you have to combine certain foods at every vegetarian meal to get "complete" protein?
No. Animal foods (such as poultry, meat and dairy) provide all nine "essential" amino acids needed to make a "complete protein," whereas plant foods, with a few exceptions (notably soy and quinoa), are incomplete, meaning they lack one or more essential amino acids.
"Complementary" protein sources—such as beans and tortillas, or peanut butter and bread—provide adequate amounts of all the essential amino acids. It used to be thought that you had to eat complementary proteins at the same time. But research has shown that you only need to consume all the essential amino acids over the course of a day, which is easy to do if you eat a variety of plant foods after you become a vegetarian.
Source: Originally published in The University of California, Berkeley Wellness Letter (March 2011)