Protein is one of the three required macronutrients for humans. Carbohydrates and fats are the other two.
1. Proteins are body builders. Much of our body, except for water, is made up of different kinds of protein. All living tissue contains protein.
2. Protein supports and maintains our blood, organs, muscles, hair, skin, and nails.
E.G. Blood contains plasma proteins; hemoglobin has a protein component; proteins are components of some antibodies; many hormones are proteins (like insulin).
3. Proteins are the body's most abundant organic compound - generally more than fat, and much more than carbohydrate. The protein content of the average cell is 16% of its total mass, and about 65% of the total body protein lies in the skeletal muscles.
4. There are more than 50,000 different proteins in our bodies, made from 20 or more amino acids, which are small organic compounds containing at least one amino group and an organic acid group.
Protein tissues in our bodies are continuously broken down and rebuilt throughout the day. In metaphysical terms, we're constantly dying and being reborn.
In this process the body efficiently recycles the nitrogen from the degraded proteins to use for making new tissue, so that nitrogen isn't lost in tissue turnover. A small amount of nitrogen leaves the body daily in sloughed-off skin, growing hair and nails, and various secretions and excretions.
Protein from food replaces the amino acids and nitrogen we lose. During digestion, this protein is broken down into its component amino acids to form a common amino acid pool, which the body uses to make new proteins for growth and tissue repair.
We must eat regularly to keep adding to this pool, because we don't have a store for proteins like we do for fats or carbohydrates. So our protein requirement is for enough amino acids to replace those that are lost.
Nine of these amino acids are "essential," because they can't be synthesized, but must be brought in from outside. The eight essential amino acids required by adults are: leucine, isoleucine, valine, threonine, methionine, phenylalanine, tryptophan, and lysine. For children, histidine is also considered an essential amino acid, which makes nine. These nine essential amino acids allow our bodies to synthesize the other eleven.
Of the nine essential amino acids, six are abundant in many foods, and three are not, so they're known as the limiting amino acids - lysine, tryptophan, and methionine (or cystine). These three limiting amino acids are critical, as they determine whether a food has complete protein. Without them, the remaining amino acids can't make new protein.
Protein-rich foods with all nine essential amino acids, called "complete protein", are animal foods --- meat, poultry, fish, milk, and eggs. Soybeans and soy products are the exception.
Other plant-based proteins - legumes, grains, nuts or seeds - contain all the the essential amino acids, but one or two will be low enough not to count, so these foods are called "incomplete protein." When two foods can be combined to make complete protein, they're called "complementary proteins" - more about that below.
Complete protein isn't superior to incomplete protein - it's just simpler. Animal protein supplies in one food (e.g. an egg or steak) what vegetable proteins supply in two (e.g. beans and rice). Either way, you're covered. The proteins are made of the same amino acids, and the chemistry is exactly the same, no matter the food source.
We need to get the essential amino acids from our diet and it doesn't matter what foods they come from. Protein quality has nothing to do with amino acid quality - lysine from beans has the same chemical structure as lysine from eggs.
We can't isolate the protein in food from the rest of the nutrient package. Animal foods are good sources of complete protein. But are they healthy? Most of their calories come from cholesterol laden, saturated fat. Even lean meats clog arteries.
Animal foods, high on the food chain, also contain high concentrations of herbicides, pesticides, drugs and hormones from industrial agriculture. They have little fiber, needed for digestive health. All of that, combined with heavy meat consumption, has a lot to do with the high rate of deaths from heart attacks, strokes and cancer in the western world, according to decades of health studies.
The research is clear that protein from plants supplies all our protein needs, while lowering the amount of cholesterol, fat, and pesticide residues we consume. Plant-based proteins are superior not only because of what they don't include, but for positive nutritional factors like complex carbohydrates, minerals, vitamins, and fiber.
Why does it take two or more vegetable proteins, combined, to equal the protein in meat? It's because of the limiting amino acids. Some plant-based proteins are strong on one, but weak in two; some are strong in two, but weak in one.
In 1971, Frances Moore Lappe's book, Diet for a Small Planet, introduced the concept of protein complementarity - combining plant-based foods so their particular amino acid strengths and weaknesses balanced out.
Ms. Lappe's book was precise and specific in amino-acid balancing recommendations, and it was too much to think about and keep straight! Thankfully people quickly discovered that it's enough to balance food families, because the members of food families like grains, legumes, vegetables and dairy share similar amino acid strengths and weaknesses.
For example, grains tend to be short of lysine while legumes (beans and lentils) are short of methionine. If you eat a wide variety of nourishing whole foods, your body will do the combining for you, with no brain strain.
Well known food combinations for complete protein: beans and cornbread, red beans and rice, stir-fried vegetables with tofu over rice, or Boston baked beans with steamed brown bread, whole grain cereal with milk.
According to Dr. Dean Ornish in 'Reversing Heart Disease': "You don't have to be a scientist or nutritionist to combine foods properly. It's easy: just eat any grains and any legumes sometime during the same day. That's all, folks!"
Dr. Ornish says that the ideal proportion is approximately two-thirds grain to one-third legumes, but that "this is not critically important. As long as you consume enough non-sugar calories to maintain your ideal body weight, you will likely be eating enough protein."