The unfortunate truth is that anyone wishing to be a vegetarian has a lot of dietary work cut out for him or her. It’s easier to understand one component to why this is so if you know a little basic biochemistry, thus:
Almost every single process in a human cell involves proteins. These proteins are mostly manufactured inside the cell itself from building blocks called polypeptides, which are in turn made up of long chains of amino acids. These chains are made to order according to blueprints in every single cell’s DNA. Which proteins are made in cells helps determine your hair colour, your eye colour, your tendency to add fat around your middle before your hips, just about every unique physical characteristic about you.
Most organisms concentrate amino acids around their young, storing them up so that their progeny have enough protein to survive the difficult growing process. That’s why eggs, milk, nuts, seeds, beans, and grains are so high in protein. (Meat is a good protein source because animals also store protein in their muscle.) Leaving out animal products for a moment, what’s wrong with eating a diet composed entirely of nuts? Ok, ignoring scurvy, you’d also be limiting the amount of amino acids you can absorb from those nuts. Why? Because we make certain essential amino acids out of other essential amino acids, and nuts are deficient in two of those key protein components (lysine and isoleucine, but you probably don’t have to know that on your next test). Fortunately for vegans everywhere, legumes (beans) are not limiting in either of the amino acids that make nuts a problem food-but are deficient in two others (tryptophan and methionine), which can be provided by nuts or grains (which are limited in lysine, isoleucine, and threonine). Thus, if you eat beans and rice, you’ve got a complete protein source.
What effect could this have on vegetarians? Meat provides a complete protein source without any supplementation. It also provides a good amount of extra calories from fat and sometimes higher risks of foodborne illness (bacteria love meat precisely because it’s so protein-rich). But if a vegetarian wants a high-quality food source, they must combine their protein sources and eat them at generally the same time to get the same benefits Calvin the Carnivore gets from his slab of hamburger. (I think it was within the same day, but will have to find evidence and document for you.)
To all you brave vegetarians and especially vegans out there, I salute you. It takes a lot of effort and planning to eat like you do and stay healthy.