What Determines If a Protein is Complete? The Science of Essential Amino Acids

The human body requires 20 different amino acids to function, but it can only produce 11 of them on its own. This biological fact is at the core of what determines if a protein is complete, a food's classification based on whether it supplies the nine essential amino acids we must get from our diet.

Quick Summary

A protein is deemed complete if it contains all nine essential amino acids needed by the body. Animal products are typically complete, while most plant foods are incomplete and can be paired throughout the day to meet nutritional needs.

Key Points

• Nine Essential Amino Acids: A protein is only complete if it contains all nine essential amino acids that the body cannot produce on its own.

• Animal vs. Plant Sources: Most animal products like meat, eggs, and dairy are complete proteins, while most plant foods like grains and legumes are incomplete.

• Complementary Proteins: You don't need to combine incomplete proteins in a single meal; eating a variety of plant foods throughout the day will provide all the necessary essential amino acids.

• Protein Quality Measurement: The Digestible Indispensable Amino Acid Score (DIAAS) is now the gold standard for accurately measuring a protein's quality and bioavailability, replacing the older PDCAAS.

• Dietary Diversity is Key: The most effective way to ensure adequate protein and amino acid intake is to consume a wide variety of foods, regardless of whether you eat animal products or follow a plant-based diet.

• Meeting Needs for All Diets: Vegetarians and vegans can easily meet their complete protein needs by combining diverse plant sources like pairing grains with legumes.

The Building Blocks of Life: Amino Acids

Proteins are fundamental to life, serving as the building blocks for muscles, tissues, hormones, and enzymes. These complex molecules are constructed from smaller units called amino acids. Of the approximately 20 amino acids the body uses, nine are classified as "essential." This means the body cannot synthesize them in sufficient quantities, and they must therefore be obtained from food. The presence and proportion of these nine essential amino acids are precisely what determines if a protein is complete.

The nine essential amino acids are:

• Histidine: Important for the production of histamine, a neurotransmitter involved in immune function, digestion, and sleep.
• Isoleucine: A branched-chain amino acid (BCAA) crucial for muscle metabolism, immune function, and hemoglobin production.
• Leucine: Another BCAA that plays a major role in muscle protein synthesis, growth hormone production, and blood sugar regulation.
• Lysine: Essential for hormone and energy production, as well as calcium absorption and immune function.
• Methionine: Key for metabolism, detoxification, and tissue growth.
• Phenylalanine: A precursor for other important molecules, including neurotransmitters like dopamine, epinephrine, and norepinephrine.
• Threonine: A structural component of proteins like collagen and elastin, important for healthy skin and connective tissue.
• Tryptophan: Known for its role in producing the neurotransmitter serotonin, which regulates mood, sleep, and appetite.
• Valine: The third BCAA, involved in muscle growth, tissue regeneration, and energy production.

Complete Protein Sources: Animal vs. Plant

Animal-Based Complete Proteins

Animal products are the most straightforward sources of complete protein because they naturally contain all nine essential amino acids in sufficient quantities. This makes them highly bioavailable and easy for the body to absorb and utilize.

Examples include:

• Meat (beef, pork, chicken)
• Poultry
• Fish and seafood
• Eggs
• Dairy products (milk, cheese, yogurt)

Plant-Based Complete Proteins

For those following vegetarian or vegan diets, obtaining a complete protein profile requires more attention. While most plant-based foods are considered "incomplete," a handful stand out as excellent complete protein sources.

Examples include:

• Soy products: Tofu, tempeh, and edamame are notable for their complete amino acid profile.
• Quinoa: A gluten-free grain that is a fantastic source of all nine essentials.
• Buckwheat: Another nutrient-rich pseudo-cereal that contains all essential amino acids.
• Hemp seeds: A great source of complete protein and healthy fats.
• Pistachios: Recent studies have confirmed that roasted pistachios contain all nine essential amino acids.

The Power of Complementary Proteins

The traditional advice for vegetarians and vegans was to combine specific incomplete proteins at every meal. However, modern nutritional science has debunked this, showing that the body can assemble a complete protein profile by consuming a variety of plant proteins throughout the day. This is known as the concept of "complementary proteins".

Common complementary food pairings include:

• Rice and beans
• Hummus and whole-grain pita
• Peanut butter on whole-wheat bread
• Corn and black beans

Measuring Protein Quality: The DIAAS Standard

To standardize the evaluation of protein quality, international health organizations have developed scoring systems. The former standard, the Protein Digestibility Corrected Amino Acid Score (PDCAAS), has been largely replaced by a more accurate method known as the Digestible Indispensable Amino Acid Score (DIAAS).

DIAAS is considered the gold standard for evaluating protein quality because it measures the digestibility of individual amino acids at the end of the small intestine, providing a more accurate reflection of what the body actually absorbs. It also provides a non-truncated score, meaning it can show the full superior quality of higher-tier proteins. For example, studies have shown that high-quality whey protein scores significantly higher on the DIAAS scale than on the older PDCAAS scale.

Complete vs. Incomplete Protein Sources

Feature	Complete Proteins	Incomplete Proteins
Definition	Contains all nine essential amino acids.	Lacks or is low in one or more essential amino acids.
Primary Sources	Animal products (meat, eggs, dairy), soy, quinoa, hemp seeds, pistachios.	Most plant foods: grains, legumes, nuts, seeds, vegetables (excluding those listed as complete).
Requirement to Combine	Not necessary for a complete amino acid profile.	Must be combined over the course of the day for a full amino acid profile.
Biological Value	Generally higher; easily digested and absorbed by the body.	Lower; absorption can vary due to factors like antinutrients.
Associated Nutrients	Often rich in Vitamin B12, iron, and zinc. Can be high in saturated fat.	Contain beneficial fiber, phytonutrients, and antioxidants. Low in saturated fat.

Conclusion

Ultimately, what determines if a protein is complete is its amino acid composition. Foods containing all nine essential amino acids are considered complete, while those lacking one or more are incomplete. It is a myth that plant-based eaters must combine specific complementary proteins at every single meal. Instead, consuming a diverse range of plant-based foods throughout the day is sufficient to meet all essential amino acid needs. For everyone, prioritizing varied protein sources ensures a balanced diet, but for those with specific goals like muscle building or those following restrictive diets, understanding the principles of complete protein and modern quality metrics like DIAAS offers valuable nutritional insight.

To learn more about the different types and functions of amino acids, consult the MedlinePlus Medical Encyclopedia at https://medlineplus.gov/ency/article/002222.htm.

Frequently Asked Questions

Do I need to eat complete proteins at every meal?

No, it is a common misconception. Your body can store a pool of amino acids and combine them from different foods eaten throughout the day to meet its needs.

Are all plant-based proteins incomplete?

No, a few plant-based foods are naturally complete proteins, including soy products (tofu, tempeh), quinoa, buckwheat, and hemp seeds.

What are some common plant-based combinations to make a complete protein?

Popular examples include rice and beans, hummus and pita bread, and peanut butter on whole-grain toast.

Is a complete protein always better than an incomplete one?

Not necessarily. While complete proteins are convenient, incomplete proteins can be combined to achieve a complete amino acid profile and often come with other beneficial nutrients like fiber and antioxidants.

What is a limiting amino acid?

A limiting amino acid is the essential amino acid found in the smallest quantity within a food, which limits the body's ability to synthesize new protein from that source alone.

Who needs to pay special attention to complete proteins?

Individuals following a strict vegetarian or vegan diet, growing children, older adults, and those with specific health conditions or high athletic demands should ensure they consume a variety of protein sources.

How does the DIAAS protein quality score work?

The DIAAS measures the digestibility of individual essential amino acids at the end of the small intestine, providing a more accurate score of a food's protein quality compared to older methods.