Certainly one of the most delightful things about being a vegetarian is having everyone ask you "but where do you get your protein??"
Implicit in this question is the belief that protein comes primarily, or even exclusively from meat. It doesn't.
Meat does contain a lot of protein, but in completely comparable amounts to lentils, tofu, seitan, tempeh, beans, quinoa, spinach and broccoli.
How much protein does a person need?
Multiply your body weight in pounds x 0.36 grams, or 0.013 ounces. (for kilograms, see below)
For example, someone weighing 150 lbs. will need 54 g of protein per day. Someone at 110 lbs. will need 40 g. Someone at 185 lbs. will need 67 g.
That translates to:
40g = half a cup of tofu, a cup of black beans, a cup of broccoli
54g = what's above, and add a cup of soy milk and half a cup of edamame
67g = and add half a cup of cooked spinach, half a cup of quinoa and two tablespoons of nut butter
(Protein needs will increase for pregnant women and athletes, and especially for pregnant athletes)
The belief that various plant foods have to be eaten together in order to get their protein ("combining proteins") is a myth. A diet containing a variety of grains, legumes and vegetables will easily meet your protein needs.
Why is it better to get your protein from plant sources than animal?
-less saturated fat
-lower risk of heart disease
-easier on your kidneys (high protein intake is largely responsible for kidney stones - a friend of mine who had testicular cancer and kidney stones in succession said that if he had to have one of them again, he'd choose testicular cancer)
-grilling and frying meats produces carcinogenic compounds: "hereorcyclic amines," linked to colon and breast cancer
-lower risk of osteoporosis
My source for all of this is a pamphlet published by Earthsave Canada, written by Carolyn Victoria Mill. Check out Earthsave's website for more information, as well as references and recipes. Also check out the site for the Physicians Committee for Responsible Medicine.
Multiply your body weight in kilograms by 0.8 to get your daily protein needs in grams.