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Protein Quality & Completeness: Types & Examples

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  • 0:02 Protein Quality
  • 1:20 Complete vs. Incomplete
  • 2:49 Complementary Proteins
  • 3:50 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Protein quality is a measure of how good a food is at providing the essential amino acids. See examples of high and low-quality proteins in this lesson, and learn the difference between complete, incomplete and complementary proteins.

Protein Quality

Have you ever spent days putting a jigsaw puzzle together only to find that the last piece was missing? In order to complete a jigsaw puzzle, you must have all the pieces. This makes a jigsaw puzzle much like protein because in order to provide the body with the protein needed for the repair, growth and maintenance of the cells, you must have all of the building blocks of proteins, which are substances called amino acids.

There are 20 amino acids needed by the body. Half of these amino acids are referred to as nonessential amino acids, which means it's not essential to get them from the foods you eat because they can be made by your body using available substances or through metabolic processes. The remaining amino acids are referred to as essential amino acids because they cannot be made by your body so it is essential that you eat foods that contain them.

Some foods are better than others at providing the body with all the essential amino acids it needs and we can rate foods based on their protein quality, which is a measure of how good a protein-containing food is at providing the essential amino acids. In this lesson, we will look at the high and low-quality proteins and how this relates to their completeness.

Complete vs. Incomplete Protein

When you eat foods that contain protein, your digestive system breaks down the protein molecules into amino acids. Your body can now reassemble these digested amino acids into new body proteins. This breakdown and build up happens in the same way that a builder might remove bricks from a demolished building and use them to build a new structure.

Most high-quality proteins come from animal products. This is because animals and humans share similar body compositions; therefore, animal proteins, such as meat, fish, poultry and eggs, are complete protein sources, meaning they provide all the essential amino acids a human body needs. Proteins obtained from animal sources are also easier for humans to digest than those obtained from plants, so we have a better chance of being able to extract the needed amino acids from animal proteins.

As you may have guessed, most plant foods are incomplete protein sources that lack one or more of the essential amino acids needed to properly synthesize proteins in the body. Because of this, we can also say that most plant foods, such as grains, legumes (which are similar to beans), nuts and seeds, are low-quality proteins. There are a few exceptions, including quinoa and soybeans, which are high-quality plant proteins, but these plant foods are certainly the exception, not the rule.

Complementary Proteins

The fact that most plant proteins are low-quality and incomplete does not mean they cannot be used to help you meet your complete protein needs. In fact, even a vegetarian diet, which is a diet that contains mostly plant-based foods with only limited animal products, can provide all the needed amino acids by combining two or more incomplete proteins. In a way, you can think of incomplete proteins helping each other out.

For example, beans are low in the amino acid methionine, but this amino acid is abundant in rice, so by combining beans with rice, a complete protein is created. These are called complementary proteins, or two or more incomplete proteins that become complete when combined. Incomplete proteins do not need to be combined in the same meal to be considered complimentary. Eating a variety of plant foods over the course of the day is sufficient to provide all the essential amino acids your body needs to work properly.

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