I’m not a dietitian, nor an expert on the subject. I’m just some guy who’s interested in nutrition and reads a lot. So my overview may be oversimplified, incomplete, or possibly just plain wrong. You should take my post as a starting point for your further research.
OK, that disclaimer out of the way, so let’s get some terminology straight.
[ul]
[li]Protein: A big molecule that consists of a chain or chains of…[/li][li]Amino acid: A smaller, organic molecule. Often called the “building block” of proteins. There are a lot of different kinds—on the order of 500—but only twenty-one are relevant to nutrition. Our body knows how to produce twelve of these from other components, so these are classified as “nonessential”. The other nine are…[/li][li]Essential amino acid¹: Amino acids the body does not know how to produce, and so must acquire through protein in food. This is the idea I think you read about.[/li][li]Complete protein: A protein that contains all nine essential amino acids.² This can be either a single food source (e.g. eggs), or a meal (e.g. lentils and rice).[/li][/ul]
Now, to be healthy, your intake of protein must be adequate in those essential amino acids. Pretty much no one in the developed world has a problem getting adequate protein; meat, eggs, fish, and dairy are complete sources all by themselves. Most plant sources are deficient in at least one essential amino acid (soy is an exception; it’s a complete protein), but luckily they’re not always the same one. So it’s easy to combine plant foods to get the full smörgåsbord of amino acids: Beans and rice, for instance, or succotash³. So as you can see, you’re not getting “more protein” than the labels say from a dinner of black beans and rice, but you are, at least, getting a more balanced intake than you’d get from eating either one separately.
Once you’ve reached your body’s requirement for essential amino acids—which is not difficult to do, even for strict vegans (soy products are everywhere and are good protein sources)—you start to get into murkier waters, scientifically. Higher protein intake seems to allow for more muscular hypertrophy (i.e. bigger muscles) when combined with strength training, which is why bodybuilders tend to eat lots of protein, to the point of adding supplements like those giant tubs of whey or soy protein. Recommendations vary, so this number is not without controversy. But most people who aren’t bodybuilders should probably be getting about 15% of their Calories from protein⁴ (protein provides 4 Calories per gram⁵, so on a 2000 Calorie diet, that’s 75 g).
Footnotes:
[ol][li] The essential amino acids are phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, lysine, and histidine. You make recognize a couple of these; tryptophan is the stuff some people incorrectly say makes you sleepy after a big turkey dinner on Thanksgiving. Phenylalanine is mentioned on the label of aspartame or any aspartame-containing food, as there is a rare disease called phenylketonuria which makes people incapable of metabolizing phenylalanine.[/li][li] There’s a thing called the protein digestibility-corrected amino acid score which looks at the completeness and digestibility of protein sources. Eggs, milk, and soy score high on this; beans, vegetables, fruits, and grains tend to score lower.[/li][li] A combination of corn and beans. Sylvester the Cat’s “sufferin’ succotash” is probably a minced oath of “suffering savior”.[/li][li] The Dietary Guidelines for America say 10-35% for healthy adults. I think most people can agree that this is a large, unhelpful range.[/li][*] Per the Atwater system.[/ol]
I was just pointing out that the government was very much hedging when it came up with that range.