Educational Series
The Science of Nutrition
Protein Quality Matters
By Mohammad Ahrar, PhD
Introduction
Protein is an essential macronutrient, which is necessary for growth, tissue repair, and maintaining health. Almost all foods contain protein, but not all food sources of protein are created equally and you may not need as much protein as you think you do. In this article, we will discuss different types of proteins and why some proteins are not considered complete or good quality proteins.
What Is Protein?
Protein is found throughout the body—in muscle, bone, skin, hair, and virtually every other body part or tissue. All cells in your body contain a cell membrane. Proteins are important molecules of the cell membranes. About 50% of protein in the body is found in muscles. All chemical reactions in the cells that produce energy are catalyzed by enzymes, which are specialized protein molecules. The hemoglobin in your blood that carries oxygen to tissues and cells is a complex protein. At least 10,000 different proteins make you what you are, how you behave, and how you interact with the environment.
What Are Proteins Made Of?
All proteins are made of subunits called amino acids. Each amino acid contains a nitrogen component in the form of an amine group (-NH2) and an acid group (-COOH), hence called amino acid. There are 23 known amino acids in nature, but only 20 of them participate in the formation of proteins. When amino acids are put together in an exact sequence, they make a specific protein. Some proteins may contain hundreds of amino acids. Please visit Peyk #162 for more information about proteins.
Can The Human Body Make Proteins?
The simple answer is yes. However, because our body does not store amino acids, the body can make them in two different ways:
By recycling proteins in the body: Some proteins in the body normally break down into amino acids and the body uses them to make new proteins. As an example, if you do not get enough protein from your diet for a long period, the body uses your muscle proteins to make the necessary proteins.
From food: When we eat food, the digestive system breaks down the food proteins into amino acids, which are absorbed as such. Amino acids can bind together in a series of chemical reactions in organs, such as the liver, to make new proteins.
For a protein to be made, two points should be considered. First, the law of all or none applies. For a protein to be synthesized in our body, all 20 amino acids must be available to the cells so that they can be put together to form a specific protein molecule. If one or more of the amino acids are missing, the correct form of a protein cannot be synthesized. Second, there must be the correct sequence of amino acids to form a protein. Otherwise, an abnormal protein may be formed. It is the job of your DNA to control the proper sequence of amino acids in a specific protein.
Can The Human Body Make Amino Acids?
Again, the answer is yes, but there is a trick to it. Nine of the 20 amino acids are considered essential because the human body cannot synthesize them (or not enough of them), so they must be supplied through the diet. They include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The other 11 amino acids are referred to as nonessential amino acids, but are also necessary for protein synthesis. The body can make nonessential amino acids if enough essential ones are present.
Good Quality v. Poor Quality Proteins
High-quality proteins (also known as complete proteins) are the proteins in foods that supply all nine essential amino acids in adequate amounts that the body requires for the synthesis of its proteins. Low-quality proteins (also known as incomplete proteins) are low in or lacking one or more of the essential amino acids. If incomplete proteins are consumed regularly and for a long period, the body cannot synthesize the necessary proteins.
What Foods Provide Essential Amino Acids?
Except for gelatin, almost all animal sources of protein—such as red meat, poultry, fish, eggs, and dairy products— are considered complete or high-quality proteins that provide all essential amino acids. Among the mentioned food sources, egg albumin is the most complete protein. Seafood is an excellent source of lean protein and heart-healthy omega-3 fatty acids. For example, just a 3-ounce serving of salmon provides 22.5 grams of complete protein. (1)
Are Plant Proteins Considered Complete Proteins?
Most plant proteins are not considered complete proteins because one or more essential amino acids are lacking or inadequate in them. Plant-based protein sources—such as quinoa, chia seeds, and hemp seeds—offer a complete set of essential amino acids. Moreover, they are often more easily digestible and have fewer health risks than soy protein, which is another complete protein. Beans, peas, and lentils are also excellent sources of protein. However, they may not provide an adequate amount of one or more essential amino acids. Usually, grains, vegetables, and some seeds are sources of incomplete protein. For example, grains are typically low in lysine, methionine, and cysteine. Corn is low in lysine and tryptophan because concentration of these amino acids declines as the kernels mature. So, you usually don’t get complete protein with one particular type of grain by itself.
How Can Vegetarians Get Complete Proteins?
People who do not eat animal products can receive enough essential amino acids when they eat a variety of protein-containing foods each day. One cup of lentils provides about 18 grams of protein. One cup of brown rice provides 5.5 grams of protein. Quinoa is a plant-based protein source that contains complete protein and one cup of quinoa contains 8 grams of protein. Because some foods may be low in one or more amino acids, but higher in other amino acids, two incomplete proteins when eaten together, can complement each other and provide a complete protein. For example, a combination of legumes (beans, peas, lentils, soy) with grains (oats, wheat, quinoa) plus the consumption of a variety of vegetables can make a balanced diet.
How Much Protein Do You Need?
The National Academy of Medicine recommends that adults get a minimum of 0.8 grams of protein for every kilogram of body weight per day. (2) For a 140-pound (60 Kg.) person, that means about 48 grams of protein each day. For a 200-pound (90 Kg.) person, that means about 72 grams of protein each day. Bodybuilders and athletes need more good quality protein than non-athletes. Protein requirements for different age groups also affect the amount of protein needed in diets. Many people in the U.S. are consuming more than enough protein that they need, especially from animal-based foods.
Protein Sources Come In A “Package”
According to the Harvard School of Public Health, when we eat protein foods, we also eat everything that comes alongside it: fats, fiber, sodium, and more. It’s this protein “package” that’s likely to make a difference in health. (2) For example, a 4-ounce broiled sirloin steak is a great source of protein—about 33 grams worth. But it also delivers about 5 grams of saturated fat. A 4-ounce ham steak with 22 grams of protein has only 1.6 grams of saturated fat, but it’s loaded with 1,500 milligrams worth of sodium. On the other hand, salmon and other fatty fish are excellent sources of Omega-3 fats that are good for the heart. A cup of cooked lentils provides about 18 grams of protein and 15 grams of fiber, and it has virtually no saturated fat or sodium. (3)
Side Effects Of Too Much Protein Consumption
Studies at the Harvard T. H. Chan School of Public Health have found that eating even small amounts of red meat—especially processed red meat—regularly is linked to an increased risk of heart disease. (2) People who eat just two servings of red meat per week may have an increased risk of developing type 2 diabetes compared to people who eat fewer servings—the risk increases with greater consumption. Replacing red meat with healthy plant-based protein sources, such as nuts and legumes, or with modest amounts of dairy foods was associated with reduced risk of type 2 diabetes. (4) Excessive consumption of soy products may increase the risk of certain health problems, including breast cancer in women and prostate cancer in men. (5)
Other Considerations Involving Protein
• Specific proteins in food may cause allergies, which are overreactions of the immune system.
• Eggs, fish, milk, peanuts, tree nuts, and soybeans can cause allergic reactions in some people.
• Individuals diagnosed with certain diseases (such as kidney and liver disease) need to monitor their protein intake according to their physician’s guidelines.
• Most worrisome of all, processed meats are considered carcinogenic. Many contain nitrates or nitrites, which studies link to several cancers. Regularly eating processed meats is correlated with an increased risk of colorectal cancer. (2, 5)
Summary
Protein is a key part of any diet. All proteins are made of subunits called amino acids. Of the 20 amino acids that make proteins, nine of them are considered essential because the human body cannot synthesize them (or not enough of them), so they must be supplied through the diet. The average person needs about 7 grams of quality protein every day for every 20 pounds of body weight. Most plant-based foods are deficient in at least one essential amino acid and therefore are incomplete protein sources. Because grains and legumes are not deficient in the same amino acids, they can complement each other in a diet. Eating a variety of foods is the key to a healthy and balanced diet because each food has a unique mix of nutrients.
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Selected References:
1- https://www.webmd.com/fitness-exercise/good-protein-sources
2- https://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/protein/
3- https://health.clevelandclinic.org/best-and-worst-sources-of-protein
4- https://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/whole-grains/
5- https://pressbooks.calstate.edu/nutritionandfitness/chapter/7-4-dietary-sources-of-protein/
