Protein

Wikipedia defines proteins as, "polymer chains made of amino acids linked together by peptide bonds."  What that basically means is that the word "protein" is a catch-all for about 30 different amino acids. Most of those your body can produce itself, but 9 (the "essential" ones) you need to get from external sources.  Protein is the the major structural component of all cells in the body -- including muscles, organs, skin, and hair.

There are three "urban myths" around protein:
1) Animal protein is "better" (or more complete) than vegetable protein
2) Vegetables don't contain enough protein
3) Without sufficient protein, you will feel tired and listless

I'll tackle these individually:

1) In 1971, in the book "Diet for a Small Planet," the author advocated a vegetarian diet but, referencing an old study on rats, stated that vegetable proteins were "incomplete" -- in other words, they did not contain all the "essential" amino acids.  This had actually been proven false years before, but the idea caught on, mostly by people who promoted meat!  The author has since recanted this claim but the myth still lives on.

2) By weight, meat is 23-33% protein (depending on cut), fish is 18-23%, chicken breast 25%, and lamb 27%.  By comparison, fruit is 7% protein, rice is 7.5%, nuts are 11%, wheat 14%, vegetables 22%, beans 28%, and soybeans are an amazing 38% protein.

That said, the US Dietary Reference Intake guidelines recommend 50 grams of protein per day for a normal sedentary person (although some estimates think this is too high, and may be as low as 12 grams per day).  At 4 calories per gram, that's about 10% of the "average" 2,000 calorie diet.  More importantly, since most food contains more than 10% protein, as long as you eat 2,000 calories per day, you will get more than the recommended amount of protein!  The problem with iceberg lettuce (for example) isn't that it is deficient in protein, but it is deficient in calories -- at 1 calorie per leaf, you'd have to eat 31 heads of lettuce to get 2,000 calories!

According to a National Health and Nutrition Examination Survey from 2007-2008, the typical American diet provides a net protein intake of 102 grams to males and 70 grams to females a day.  The body is unable to store excess protein, and so it is processed by the liver and kidneys, and excreted through urine. It also causes the body to excrete calcium, although no one is sure why.  High animal protein intake increases the probability of forming kidney stones by 250 percent.
So the real problem isn't that vegetarians don't get enough protein, but meat eaters get too much.

3) Proteins have four calories per gram, while fats and carbohydrates have nine. That's why exercise burns fat, not muscle--your body is selectively choosing the best energy source.  If a person consumes too few calories, and has no fat reserves, the body will start to eat muscle (and then organs); however, as you can imagine, this is pretty rare in developed countries.

To sum up, there is *no* relation between protein and energy; if you eat enough calories, you will get enough protein; there is no difference between animal protein and vegetable protein; and if you eat too much protein, your body will quietly throw it away (and you may get heart disease, cancer, and/or kidney stones for your trouble).

Now I have to figure out why, on some days, I have no energy.  (Which is why I started researching protein.  When I started this, I believed all three myths!)

You can read more here: http://michaelbluejay.com/veg/protein.html