Complete vs. Incomplete Proteins: Understanding the Difference
Medically reviewed by Jonathan Purtell, RDN
Historically, protein-containing foods have been divided into two categories: incomplete and complete proteins. "Complete" and "incomplete" refer to the amino acids certain protein foods contain and how they contribute to your daily needs.
Generally, animal-based proteins are considered a higher-quality protein source than plant-based proteins in terms of their amino acid profiles. However, this isn't necessarily the case.
This article examines what complete and incomplete proteins mean for your diet and meeting your protein needs.
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What Are Complete and Incomplete Proteins?
Many have long assumed that protein-containing foods can be categorized as:
Complete proteins: Foods containing all nine essential amino acids. Examples include eggs, meat, poultry, dairy, fish, and certain plant foods like quinoa, soy, buckwheat, and chia seeds.
Incomplete proteins: Foods which—presumably—lack one or more essential amino acids. Examples include beans, nuts, seeds, spinach, and cauliflower.
However, this is a widespread misconception that continues to circulate; the idea of complete and incomplete proteins is misleading.
All foods with protein contain all nine essential amino acids (even those in the "incomplete proteins" list above). The difference is that amino acids are present in varying amounts in those types of foods. This doesn't mean they're incomplete, though.
The only truly incomplete protein in the food system is gelatin, which entirely lacks the amino acid tryptophan.
What Are Amino Acids?
Amino acids are protein's building blocks. Some amino acids are nonessential, meaning that your body makes them itself. Nine amino acids are essential, so you must get them through your diet because your body doesn't make them. Your body combines the amino acids you make and those you consume in various ways to ensure adequate protein. The nine essential amino acids are:
Histidine
Isoleucine
Leucine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
Do You Need to Combine Proteins?
You do not need to combine different proteins in the same meal to ensure amino acid adequacy. Some scientists have tried to develop tools, such as the protein digestibility corrected amino acid score (PDCAAS), to figure out how to combine multiple plant foods in specific ways to reproduce the amino acid profile of animal proteins in one meal. However, this hasn't proved easy.
The PDCAAS is a measurement system for evaluating the quality of a protein based on its amino acid profile and digestibility. Animal proteins typically score higher on the PDCAAS scale than plant proteins due to their more even amino acid profiles and higher digestibility. However, researchers are calling for a better way, stating that PDCAAS has limitations in its ability to accurately evaluate plant protein quality and digestibility.
Now, the standard recommendation, backed by the American Heart Association (AHA), is to eat a variety of foods throughout the day to meet your amino acid needs.
Even if you don't consume animal proteins (i.e., complete proteins), you can still meet your amino acid needs by eating a variety of plant proteins.
When Did the Idea of Combining Proteins Originate?
The idea that plant proteins are inferior to animal proteins stemmed from a study of rats from over 100 years ago.
The authors found that baby rats did not grow as well on plants as they did on their mother's milk, which researchers attributed to the differences in amino acid profiles. But baby rats don't grow well on human milk either because rat milk contains 10 times more protein per serving than human milk does (rats grow significantly faster than humans).
Combining incomplete proteins in the same meal to get all the necessary amino acids was popularized in the 70s. Specifically, it was inadvertently popularized by Frances Moore Lappé in her bestselling book Diet for a Small Planet in 1971 before other major publications circulated the message further.
For example, recommendations suggested pairing rice and beans, peanut butter on whole-grain bread, or oatmeal with chia seeds. These were called "complementary proteins" because they balanced out each other's amino acid profiles.
However, the nutrition science community debunked the need for combining proteins over 20 years ago. Lappé changed her position and apologized for reinforcing the myth in the revised 1981 edition of her book.
Limiting Amino Acids
Instead of categorizing protein foods as complete or incomplete, it is more accurate to say that some contain limiting amounts of certain essential amino acids. For example, even though legumes contain all nine, they contain less of the amino acid methionine compared to other protein foods.
The only instance in which this would be problematic is if your diet was made solely of one food with limited amino acids. Most people eat many different foods, so as long as you incorporate various protein sources into your diet, your body will combine amino acids.
Potential Downsides of Too Much Protein
Despite all the frequent attention health influencers and convincing marketing give to getting enough protein, the truth is that most people get more than enough in their diet. True protein deficiency is rare in Western countries as long as you eat adequate calories. What's more, there are potential downsides to getting too much protein.
Excessive protein intake is associated with a higher risk of:
Kidney stones
Worsened preexisting kidney conditions or a higher risk of kidney disease
Heart disease
Colon cancer, particularly with high processed and red meat intake
These health risks appear more likely when a high-protein diet is based on animal foods versus plant proteins. Recent studies have found that a higher plant protein intake has a more protective effect on heart health and lifespan than animal protein intake.
How Much Protein Do You Need in a Day?
Daily protein needs for adults depend on factors like age, sex, activity level, medical conditions, and health goals. However, baseline recommendations are as follows:
Males, 19+ years: 56 grams daily
Females, 19+ years: 46 grams daily
Pregnancy and lactation: 71 grams daily
Baseline protein needs (not accounting for physical activity) may also be calculated by multiplying body weight in kilograms by 0.8. Active individuals and older adults may need to use a multiplication factor of at least 1.2 grams per kilogram instead. The best way to determine your needs is to work with a registered dietitian.
Related: Increasing Plant-Based Protein by Just 3% May Help You Age Healthier. Here's How to Do It
Protein Considerations
Protein is widely available in the U.S. food system, whether you consume animal sources, plant sources, or a mix of both. Most people in developed countries will have no trouble meeting their protein needs. Still, some individuals may need to pay extra attention to the protein sources in their diet.
Vegetarian or Vegan Diets
Following a vegetarian or vegan diet means that most, if not all, of your nutrition comes from plant versus animal sources. It is possible to meet your daily protein needs while eating a plant-based diet.
All plant foods contain all nine essential amino acids, but some contain limiting amounts of certain ones. On a plant-based diet, the following four essential amino acids may be limited:
Lysine
Threonine
Methionine
Tryptophan
This doesn't mean a plant-based diet provides inadequate protein or nutrition. Instead, it means you should eat various plant foods regularly to ensure optimal amino acid intake.
Sources of these limiting amino acids on a plant-based diet include:
Lysine: Pistachios, soy foods, hemp seeds, chia seeds, and quinoa
Threonine: Legumes (foods that grow in a pod) such as peas, beans, lentils, peanuts, barley, sunflower seeds, and almonds
Methionine: Chickpeas, cashews, walnuts, lentils, wheat germ, and Brazil nuts
Tryptophan: Corn, pumpkin seeds, black beans, barley, and tofu
Amino Acid Metabolic Disorders
Amino acid metabolic disorders are a group of genetic conditions wherein your body can't break down amino acids properly. These disorders often result from deficiencies in specific enzymes involved in amino acid metabolism.
Early detection and management are important because amino acid metabolism disorders can disrupt the balance of amino acids necessary for making protein. This can lead to a deficiency in proteins that are essential for various bodily functions.
Left untreated, amino acid metabolic disorders can lead to a buildup of toxic by-products or a deficiency of essential nutrients. People can experience symptoms like developmental delays and disabilities, seizures, and organ damage.
Treatment for amino acid metabolic disorders often includes a specialized diet pattern and supplements to help ensure adequate amino acid intake. If you have an amino acid metabolic disorder, work closely with a healthcare provider to determine the best plan for you.
Related: 8 Fruits and Vegetables That Can Help You Get More Protein
Summary
There is no such thing as incomplete proteins, nor is it necessary to combine plant proteins at meals to ensure you get all the essential amino acids. Meet your daily needs by eating a wide variety of foods, whether they're plant-based, animal-derived, or a combination of both. There's also no need to obsess over protein intake because excessive intake (particularly from animal sources) may pose certain health risks. Enjoy a balance of healthy protein-rich foods, such as nuts, seeds, legumes, eggs, lean meats, soy foods, and whole grains, and your body will take care of the rest.
Read the original article on Verywell Health.
