Proteins are one of the macro nutrients in our diets. Every gram yields 4 calories of energy to the body, but its real essence lies in the fact that it provides the building blocks of every tissue, hormone, antibody and enzyme in the body. The way these are manufactured explains the importance of consuming the so called complete proteins.
The smallest unit of a protein is an amino acid. All proteins consist of all amino acids. Proteins are only distinguished from one another by the sequence in which they are stringed together and the structure it forms. While the body makes most of its amino acids, there are some that have to be consumed - the essential amino acids. The absence of even one of the essential amino acids has as a consequence that protein could not be synthesized. If so, no tissue could be replaced, no hormone or enzyme could be synthesized. The body will start breaking down some of the tissues, like muscles, at first to try to keep up with the demand for enzymes and hormones, but eventually will just stop trying and use the rest of the amino acids as energy. This could have severe consequences to your health.
The most extreme example of a protein deficiency is in children of hunger struck countries and the development of a condition called Kwashiorkor. Typically, their muscles are wasted away, their bellies are swollen and they have a red tone to their hair. Eventually they die of the condition. Before this condition becomes as pronounced as this, we see lack of energy, dull and brittle hair, weakness, poor concentration, anemia and premature aging. Injuries and illness also take longer to heal or that do not heal at all.
Anyone consuming animal proteins, even in modest amounts, will not likely develop a deficiency in amino acids as animal proteins are readily absorbed by the body and always consist of all essential amino acids. Also, you do not have to consume it on a daily basis, as amino acids are stored in the form of protein in tissues like muscles and will not deplete quickly. The problem starts for some people who are vegan or vegetarian because not all plant based foods consist of all essential amino acids. And even if they do, like in the case of amaranth and quinoa, it is more difficult for some individuals to absorb them from the fibrous plant material. The ability to absorb all amino acids seems to be an individual one. Some people do it very well and even some ultra-marathon athletes seem to do just fine on a vegan diet.
Keep in mind that the best sources of plant protein are seaweed, legumes, nuts, seeds and grains as well as fermented food. The protein in the latter come from the bacteria and good examples are miso, unpasteurized sauerkraut, kimchi and pickles.
Supplements and shake powders of essential amino acids can also be found in any health store to ensure complete protein in the diet. So, if you prefer a plant based diet, be sure to consume a wide variety of protein sources and be vigilant for the signs of protein deficiency as they develop over time and are not always an obvious connection to your diet.