Protein is one of the major nutrients found in our diet. About 10 to 35 percent of our total calories should come from proteins: the recommended daily amount is a minimum of 0.8 g/kg a day. The most prevailing source of protein is animal products such as meat and dairy, but it can also be found in plant-based foods. Because the plant structure is mainly carbohydrates, however, they are not a high quality source.
Function
Proteins are needed by living organisms to carry out many of the important physiological functions that sustain life. They form enzymes to facilitate chemical reactions and help comprise many important transportation molecules such as the oxygen-carrying hemoglobin in the blood. They also serve as structural components in skin and bones and are used for muscle contractions. Other functions include antibodies in immune protection, providing a source of energy to the body, the transmission of nerve impulses and the control of growth. Because protein is so plentiful in living creatures, it is also present in nearly all types of foods.
Amino Acids
Amino acids are the building blocks of proteins. There are 20 of them total. Half of these, depending upon your age and health, are known as essential amino acids: they cannot be synthesized by the body and must be consumed in the diet. A food item that contains all of the essential amino acids is known as a complete protein. Among the plants, only the soybean, a type of legume, is a complete protein. Nevertheless, plant sources are very low or non-existent in saturated fat and contain an abundant supply of dietary fiber, so they are healthy alternatives.
Amount
Asparagus is the best source of protein in vegetables and fruit. Protein constitutes 36 percent of its dry weight. Other good sources include the 33 percent in broccoli, the 27 percent in cauliflower and peas, the 11 percent in potatoes and celery and the 10 percent in avocados. Fruits contain very little protein. A strawberry, for example, is only made up of 7.5 percent protein. This is followed by the 6 percent in watermelon, the 5 percent in bananas, the 3 percent in blueberries and peaches and the 0.3 percent in apples.
Warning
Plant-based proteins are often referred to as low quality proteins because they contain a diminished amount of certain amino acids. The United States Department of Agriculture reports that strawberries, for example, are nearly absent of the amino acid phenylalanine, which is used as a precursor for the skin pigment melanin and important signaling molecules in the brain such as dopamine and epinephrine. Most plant proteins are missing at least some vital amino acids.
Considerations
Because of their absence, amino acids from fruits and vegetables must be mixed and matched to arrive at a complete protein in the diet. If you are embarking on a vegetarian diet, then you should also eat legumes, beans and nuts, all of which are decent sources of protein. If not, then you should also pick a few types of food from the meat, seafood, eggs and dairy groups, all of which are complete proteins.
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