Dear Doctors: What does it mean when something is a “complete protein”? A friend said that when I count beans or grains as a protein, it’s not the same nutritional value as meat or cheese. Are there really different kinds of protein? Is he right that it matters which kind of protein you eat?
Dear Reader: Proteins are vital to the health and function of our bodies. That makes it important to understand what they are and how they fit into the diet.
Dietary protein, along with carbohydrates and fats, is what is known as a macronutrient. These are nutrients the body requires in large quantities to maintain health and function. Macronutrients have unique properties that distinguish them from one another, and they are all used by the body as sources of energy.
When it comes to proteins, they are found in every cell in the human body. They are used for cell structure, growth and repair, and they play a central role in numerous metabolic processes. The antibodies that defend the body are made up of proteins, as are the enzymes that carry out virtually every chemical reaction within the cells. Small wonder, then, that proteins are often referred to as the building blocks of life.
You are correct that beans and grains are among a wide range of plant-based proteins. But your friend is also accurate in saying many of these proteins are incomplete. To explain, we need to dip a toe into biochemistry. Proteins are made up of long chains of molecules called amino acids. In carrying out the countless functions that maintain life, our bodies use 20 different amino acids. These are joined together in varying lengths and combinations to make thousands of different types of proteins, each with a unique job.
Although our bodies produce some of the amino acids it needs, it can’t make them all. The nine it can’t make are known as essential amino acids, and they must be obtained through diet. They are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Foods such as meat, fish, poultry, eggs and dairy products contain all of the amino acids that the body needs. They are also present in the exact proportions required for optimal health and body functions. That makes them “complete” proteins.
Other than soy, quinoa and buckwheat, which are complete proteins in themselves, most plant-based proteins are either low in certain amino acids or are missing them altogether. That makes them “incomplete” proteins. The good news is that by eating a variety of plant-based proteins, you can provide your body with combinations of amino acids that add up to a complete protein. For instance, beans and rice each lack certain essential amino acids and are thus incomplete proteins. But eaten together, they form a complete protein, because each contributes the amino acids the other is missing.
While this concept of combining plant-based proteins is particularly important for those on a meat-free diet, it applies to all of us. By eating from a wide range of foods, including vegetables, grains, rice, nuts, seeds and legumes, you’ll get the protein your body needs.
(Send your questions to [email protected], or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)
