Complete vs. Incomplete Proteins: Do You Really Need to Combine Them? (2025)

Complete Proteins

Complete proteins are foods that contain all nine essential amino acids in sufficient amounts to support the body's functions, including muscle repair, immune health, and overall well-being.

These proteins are vital because the body cannot produce these amino acids independently, making it necessary to obtain them through diet. Common sources of complete proteins include animal-based foods such as:

• Eggs
• Meat
• Poultry
• Dairy
• Fish

These foods naturally provide a well-balanced amino acid profile. However, certain plant-based foods also qualify as complete proteins, including:

• Quinoa
• Soy
• Buckwheat
• Chia seeds

These plant-based options offer valuable alternatives for those following vegetarian or vegan diets while still ensuring adequate protein intake.

Incomplete Proteins

Incomplete proteins are foods that—presumably—lack one or more essential amino acids. Examples include:

• Beans
• Nuts
• Seeds
• Spinach
• Cauliflower

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.

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.

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.

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.