Soy milk, tofu, and soy protein powder are all excellent sources of complete protein for vegetarians. In fact, soybeans are one of the few plant-based sources of complete protein. Most sources of complete protein come from meat and dairy.
A complete protein is one which contains an adequate proportion of all nine essential amino acids required for the dietary needs of humans. The nine essential amino acids are tryptophan, threonine, leucine, lysine, isoleucine, methionine+cystine, phenylalanine+tyrosine, valine, and histidine.
Essential amino acids are those which the body cannot produce on its own. They must be obtained directly from dietary sources. On the other hand, nonessential proteins can be produced by the body if they are not obtained from food. Amino acids are the building blocks of proteins. The body combines amino acids to produce a wide range of substances, such as enzymes, hair, hormones, and muscle tissue.
Not every food we consume needs to contain the full spectrum of essential amino acids. But a healthy diet requires the consumption of an adequate combination of all essential amino acids. Amino acids can be mixed and matched from different foods to produce nonessential amino acids, as long as they are consumed within 24 hours of each other. For example, by combining rice and beans, or bread and legumes, one can consume all the essential amino acids, although none of those foods individually is a complete protein.
Health benefits of soy protein
As mentioned, complete proteins generally come from meat and dairy sources. Soy is a notable plant-based exception. Therefore, vegetarians may have more difficulty getting a healthy balance of essential amino acids from their diet than do meat-eaters.
Soy milk and tofu are a great alternative to meat as a source of complete protein, and not just for vegetarians. Studies have shown that diets high in soy protein and low in animal protein provide many health benefits, such as lower risks of breast cancer and prostate cancer. Soy is also lower in saturated fat than meat and dairy, helping to keep cholesterol and triglyceride levels in check.
Something to beware with soy milk, specifically, is the sometimes high sugar content. Check the nutrition label to make sure your soy milk has a reasonable level. Soymilk with lots of sugar may not be any healthier than cow’s milk, after all.
Another benefit of soy milk is its comparatively long shelf life. Soy milk can have an expiration date months in the future, depending on how it is packaged. And even after opening the container, soy milk lasts longer than cow’s milk before going bad, making it more cost effective and convenient.
A final thought
In the end, it is not so important that many of your individual foods are complete proteins. As long as one eats a healthy and balanced diet, it is generally not difficult to obtain the full range of essential amino acids.
So, although it is interesting that soy milk is a complete protein, nutritionally it is not going to be concern unless you eat a very homogenous and unvaried diet. This applies even to vegetarians.