Proteins have important, widespread roles in our bodies. They are part of many biological structures, for example, hormones, enzymes and red blood cells. They also make up our hair, skin, nails, organs and muscles and are involved in the growth, repair and maintenance of tissues. The structure of food proteins determines if they are complete or incomplete in term of their usability in the body.
Food Sources
Referring to the MyPyramid food guide, the principal sources of protein come from the following food groups:
Meat & Beans, including all meats, poultry, fish, seafood, eggs, nuts, seeds, lentils, dry beans, and foods made from them, e.g., hummus and tofu
Milk, including all milks, cheese, yogurt and milk-based desserts
Grains, vegetables, and fruits also provide some protein.
Amino Acids
Proteins are made up of chains of amino acids. There are 20 different types of amino acids; their sequence and combinations determine the specific proteins. If words and letters are used as an analogy, amino acids are the letters and words are proteins. The letters s-t-e-p make the word "step," but if the sequence is changed to p-e-s-t, a new word is formed. According to "Understanding Nutrition" by Eleanor Noss Whitney and Sharon Rady Rolfes, the human body has 10,000 to 50,000 different proteins.
Complete vs. Incomplete
There are nine essential amino acids. They are essential because the body either cannot produce them or cannot produce them in enough quantities to meet its needs. Complete proteins contain all the essential amino acids, whereas incomplete proteins lack one or more of them. All animal sources of protein, including milk, do not lack any essential amino acids and are therefore considered complete.
Incomplete sources include plant-based foods, for example, legumes, such as dry beans and lentils, grains, cereals, nuts and seeds. An exception is soy beans, which are considered complete.
Misconceptions
In the past, it was believed that specific plant-based foods needed to be combined in the same meal to form a complete protein. For example, kidney beans and rice would be complementary proteins, one providing the essential amino acid that the other is lacking. However, in a 2009 position paper on vegetarian diets published in the "Journal of the American Dietetic Association," it is stated that having a variety of plant foods throughout the day is sufficient in providing all essential amino acids and that combining them in the same meal is not necessary.
Considerations
Consuming a variety of foods from all food groups will easily meet your protein and essential fatty acid needs. If you are considering becoming vegetarian--avoiding some or all animal-based foods--you are advised to consult a registered dietitian to ensure your protein and all your vitamin and mineral intakes are adequate for your particular stage of life, whether you are a child, teen, pregnant, elderly or an athlete.
References
- USDA: MyPyramid
- "Understanding Nutrition," 6th. edition; Eleanor Noss Whitney, Ph.D., R.D., and Sharon Rady Rolfes, M.S., R.D.; 1993
- Medline Plus: Protein in Diet
- American Dietetic Association: Vegetarian Diets
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