Protein, a required part of everyone’s diet, accounts for 10-20 percent of the daily total calorie intake for most Americans. When we talk about protein, it’s often in the context of its source – red versus white meat, plant-based versus animal-based – but not often in terms of its quality.
This is partly because protein quality is complicated and can be difficult to interpret, but that doesn’t make protein quality unimportant. You might have heard the term “complete protein” or seen a food label claiming the food is “high in protein” or a “good source of protein” – but what does that actually mean?
Whether you’re following a particular diet or just trying to eat more deliberately, having more information about what you’re eating will help you make wise choices for your health and dietary goals.
What is protein?
Protein is one of three macronutrients the body needs in large quantities, along with carbohydrates and fats. Amino acids are the “building blocks” of proteins. Much like you build with and take apart LEGO bricks to build different things, the body regularly and non-destructively disassembles proteins to release individual amino acids the body then uses to build new proteins.
Amino acids can also be used destructively by the body, such as when it uses amino acids to supply nitrogen for other body processes. These destructive processes, along with fluctuations in the body’s protein-building demands, create the need to replenish amino acids on a daily basis.
Why is protein important?
Proteins in the body come in a wide range of sizes and perform a vast array of functions, from the largest known protein – titin (made up of about 34,000 amino acids) that contributes to the strength and elasticity of skeletal muscle – to the smallest known protein, glutathione, an important antioxidant (made up of just three amino acids). Proteins are a major component of hair and fingernails, act as enzymes, provide structure, hold cells together, transmit and receive signals, regulate the movement of other molecules, protect you from germs, and much more.
Thus, the primary reason to eat protein is to preserve the many functions proteins and amino acids perform by replenishing the amino acids that are lost each day.
Is all protein the same?
Yes and no. All proteins are built from just 20 amino acids that are arranged in different combinations. However, the ratios of these 20 amino acids can vary dramatically among protein sources (animal versus plant, for example). The body’s ability to break down proteins and extract amino acids also varies.
Of these 20 amino acids, only 11 are made by the body (alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine). The remaining nine are called “essential” amino acids. Because your body can’t make them, it is “essential” that they are in your diet.
If your body runs out of an amino acid, then the proteins that require that amino acid can’t be built. This results in either the loss of that protein’s function in the body, or the breakdown of other proteins to free up the needed amino acid, which can then compromise a different function.
The nine essential amino acids (and some examples of why they’re important) are:
- Histidine (inflammatory response, antioxidant, metal chelator)
- Isoleucine (muscle endurance and repair)
- Leucine (blood sugar, growth, muscle recovery)
- Lysine (absorption of calcium, formation of collagen and carnitine)
- Methionine (cellular energy, provides sulfur)
- Phenylalanine (neurotransmitters, mood, appetite, memory)
- Threonine (tooth enamel, fat metabolism, nervous system)
- Tryptophan (serotonin, melatonin)
- Valine (muscle, nerves, immune function)
What is a “complete protein”?
A complete protein is a protein source that provides all nine of the essential amino acids in a form the body can use and in the quantities the body needs. Although all animal-derived proteins (including fish, dairy, and eggs) are complete proteins, most plant-based protein sources are not; i.e., most plant-based protein sources lack one or more of the nine essential amino acids.
However, soybeans, quinoa, chia, and hemp seeds are among the few plant-based proteins that are complete proteins. Other non-animal sources of complete protein include spirulina (a blue-green algae) and nutritional yeast.
Different protein sources can be combined in the diet to create a complete protein. Such combinations aren’t necessary at every meal, but they should be present in the daily diet.
A well-known combination that forms a complete protein is rice and beans, but other combinations, like whole wheat bread and peanut butter, achieve the same result. Although there are many such combinations, mixing legumes, like beans, peas, or lentils, with nuts, seeds, or whole grains can provide complete proteins from non-animal sources.
What is protein quality and why is it important?
A protein source is only as useful as the body’s ability to extract – through digestion – its amino acids. Not all proteins are equally digestible. “Protein quality” is a term that describes the combined completeness and digestibility of a protein. So, if a protein source lacks one or more of the essential amino acids or it is difficult to digest, then its quality is considered lower than a protein source that is both complete and easy to digest.
Understanding the quality of the proteins you eat helps you evaluate whether you are getting the nutrition you need and are expecting. This is especially important for individuals who choose not to eat animal-derived proteins.
How can I tell if a protein is high quality?
Several methods to classify protein quality exist; each one has its strengths and weaknesses. The method currently accepted by the U.S. Food and Drug Administration (FDA), the World Health Organization, and the Food and Agriculture Organization, is called the “Protein Digestibility-Corrected Amino Acid Score” or PDCAAS.
PDCAAS scores are calculated by combining the relative amino acid content in the food compared to the amounts needed by humans and how well the protein is digested based on laboratory studies. The highest quality proteins have a PDCAAS score of 1.0 and the lowest quality have a PDCAAS score of 0.1.
Generally, milk, egg whites, whey, casein, and soy proteins have the highest PDCAAS scores – at or near 1.0. Soy scores vary but are generally high. Animal proteins – meat, poultry, and fish – tend to be classified as high quality because of the ratios of essential amino acids they contain and the ease with which the human body generally digests and absorbs them. Plant-derived proteins fall into a wide range of scores, from high (like soy), to very low (like sunflower protein or wheat gluten).
The FDA has regulations that limit the use of terms like “High,” “Rich In,” and “Excellent Source Of,” that are also helpful to understand. In general, these terms can only be used when one serving of a food provides at least 20 percent of the Reference Daily Intake (RDI) or Daily Reference Value (DRV) of protein based on an FDA-reference diet. PDCAAS scores are used to calculate these amounts.
Is a high quality, complete protein enough?
Almost. From an amino acid supply perspective, yes. From a total health and nutrition perspective, not necessarily. Proteins in food are not pure. They are accompanied by other macronutrients (carbohydrates and fats) and micronutrients (vitamins and minerals needed in smaller quantities).
These other components of food impact how your body uses the protein, plus they can affect other aspects of your health and wellness. For instance, red meat is a complete protein, but it typically includes higher quantities of saturated fats; whereas, most plant proteins are not complete, but often provide significant amounts of dietary fiber. Generally, consuming a variety of proteins is good, whether plant or animal-based.
What about protein supplements?
Although supplemental protein should not replace a healthy diet that includes a variety of protein sources, even the healthiest diet can have nutritional gaps. High-quality protein supplements are a convenient way to increase protein intake in an efficient manner, especially for exercise recovery or when circumstances make it difficult to get enough whole food at the right times.
Whether you’re looking to supplement a vegan diet or to just boost your protein intake to support good health and lean muscle,* Thorne offers a number of products that contain high-quality proteins and amino acids. Thorne’s offering is Amino Complex, which contains essential amino acids (EAAs), including branched-chain amino acids (BCAAs) with clinically-validated formula .