Protein is made up of carbon, hydrogen, and oxygen (as are the other macronutrients) and one more element none of the other nutrients have - nitrogen. If you ever hear somebody talking about being in positive nitrogen balance or negative nitrogen balance what they are referring to is whether they are in an anabolic state (able to build muscle), or in a catabolic state (losing muscle).
Some foods contain what is called complete protein - that is, they provide all the amino acids necessary to produce usuable protein. Examples of these foods are milk, eggs, meat, fish and various vegetable products, such as soybeans. But even these foods contain differing amounts of usable protein per weight. That is, even though a food might contain 10 grams of protein, your body is only able to use only a certain percentage of it - for example, only 7 or 8.5 grams. Below is a chart that shows on the left what percentage protein makes up various foods commonly used as protein sources and on the right what percentage of that protein your body is actually able to use for building muscle: N.B. Whey, a milk derivative, which is a refined product, has even more net protein than eggs).
This chart tells us, for example, that an egg contains only 12% protein by weight. Yet because of the specific balance of amino acids present in that protein, 94 percent of it can be used by your body. In contrast, 42 percent of soybean flour is protein, but the make up of that protein is such that your body is able to use only 61 percent of it. So there is a big difference between how much protein a food contains and how much of that protein you can actually use to build muscle. Eggs are such a good source of quality protein that they are used as a basis of comparison in rating the protein quality of other foods, with eggs given an arbitrary value of a "perfect" 100.
Incidentally, notice that I have given the value of whole eggs. It is increasingly fashionable these days to eat only the egg whites because the yolk contains some fat and the egg white does not. However, I never do this (although I would certainly have additional egg whites to supplement my protein intake without taking in excessive fat and cholesterol). The yolk actually contains as much protein as the egg white, as well as the majority of the vitamins and minerals. If you feel the need to eliminate fat in your diet, I recommend you do so by eliminating other foods, not by throwing away what is in many ways the best part of the egg.
You can see that foods like rice, potatoes and beans give you considerably less usable protein than eggs and fish. The reason is they have some, but too few, of the essential amino acids that are required for complete protein. You can, however, combine two or more sources of this low-quality (incomplete protein) to obtain high-quality, complete protein. That is, one food lacks certain aminos that are supplied by the other food, so in combination, they give you what you need.
The need to assemble a complete combination of amino acids means that adding just a small amount of the right food to your eating plan can make a big difference. Combining incomplete protein in this way is useful because it usually involves eating foods that are relatively low in fat, and thus contain fewer calories than many common complete protein sources. When you are trying to build maximum muscle mass with as little body fat as possible, this can be a great advantage. The following are some recommended combinations of incomplete proteins:
Grains plus seeds
- Breads with added seed meals
- Breads with sesame or sunflower seed spread
- Rice with sesame seeds
Grains plus milk products
- Cereal with milk
- Pasta with milk or cheese
- Bread with milk or cheese
Grains plus Legumes
- Rice and beans (a fundamental part of many diets around the world)
- What bread and baked beans
- Corn soy or wheat-soy bread
- Legume soup with bread
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