Essential Amino Acids, and Why Complete Protein Matters

By Courtney Sperlazza, MPH

Protein is a crucial nutrient that plays hundreds of roles in our overall health and well-being. It is responsible for numerous vital functions in the body, such as building and repairing tissues, supporting immune function, and facilitating the production of enzymes and hormones. 

While protein is found in a variety of food sources, not all proteins are created equal. Some are complete proteins, some are incomplete and must be combined with other protein sources to get the full benefit. 

What does all of that mean, exactly? In this article, we will explore the significance of complete protein and why it matters for a balanced and nutritious diet.

First, an overview of protein and its component parts. 

What are amino acids?

Amino acids are organic molecules that contain an amine group (–NH2), a carboxylic acid group (–COOH), and a side chain specific to each individual amino acid. They serve as the building blocks of proteins, which are essential for many biological processes such as enzyme function, cellular communication, and muscle contraction. Amino acids can also act as neurotransmitters in the body and help regulate mood and behavior. In addition, they play a major role in metabolic pathways and energy production. 

Some amino acids are produced naturally within the body while others must be obtained from dietary sources such as meats, dairy products, legumes, grains, nuts, and seeds. Additionally, some supplements provide supplemental amounts of specific amino acids. 

How many amino acids are there?

There are 20 standard amino acids that are commonly found in proteins. These amino acids are the building blocks of proteins and play essential roles in various biological processes, including:

1. Alanine (Ala)
2. Arginine (Arg)
3. Asparagine (Asn)
4. Aspartic acid (Asp)
5. Cysteine (Cys)
6. Glutamic acid (Glu)
7. Glutamine (Gln)
8. Glycine (Gly)
9. Histidine (His)
10. Isoleucine (Ile)
11. Leucine (Leu)
12. Lysine (Lys)
13. Methionine (Met)
14. Phenylalanine (Phe)
15. Proline (Pro)
16. Serine (Ser)
17. Threonine (Thr)
18. Tryptophan (Trp)
19. Tyrosine (Tyr)
20. Valine (Val)

These amino acids differ in their chemical properties, side chains, and functions within proteins. However, it's important to note that some organisms have additional or modified amino acids beyond these 20 standard ones.

Essential vs nonessential amino acids 

Some amino acids are considered essential, others are considered non-essential. Despite the label, our bodies need them all. The difference lies in where we get them. 

Non-essential amino acids

Amino acids are considered non-essential if our bodies can make them, without needing to get them from food sources. Even though they are labeled non-essential, they are necessary for our bodies’ proper functioning.

Non-essential amino acids include:

• Alanine
• Arginine
• Asparagine
• Aspartic acid
• Cysteine
• Glutamic acid
• Glutamine
• Glycine
• Proline
• Serine
• Tyrosine

Essential amino acids

Essential amino acids are the building blocks of proteins and they are essential to human health. They cannot be made by the body, so they must be consumed through the diet. 

9 essential amino acids 

Essential amino acids include: 

• Histidine
• Leucine
• Isoleucine
• Lysine
• Methionine
• Phenylalanine
• Threonine
• Tryptophan
• Valine 

These essential amino acids are important for many bodily functions including growth and development, proper digestion and absorption of nutrients, maintenance of muscle mass and tissue repair. Foods that are rich in protein such as meat, fish, eggs, and dairy provide these essential amino acids in abundance. 

Plant-based sources such as beans, nuts, and grains also contain some essential amino acids but not all nine. Therefore vegetarians and vegans may need to combine a variety of plant foods to get enough of each essential amino acid. 

Conditionally essential amino acids 

Conditionally essential amino acids are amino acids that do not have to be obtained from the diet, as they can usually be synthesized in the body. However, under certain conditions such as illness or injury, they cannot be produced adequately and must therefore be obtained from the diet. 

Examples of conditionally essential amino acids include glutamine, arginine, proline, tyrosine, glycine and cysteine. These types of amino acids are important for optimal health and performance as they play a role in immune function, tissue repair, and other metabolic processes.

Limiting amino acid

A limiting amino acid refers to an essential amino acid that is required for protein synthesis, but it is generally present in the lowest quantity relative to the body's requirements. 

Think of locks and keys. If you have seven locks but five keys, the keys are comparable to the limiting amino acids because they limit the number of locks that you can use. 

When a dietary protein source is deficient in one or more essential amino acids, the body's ability to build new proteins is limited by the availability of the limiting amino acid.

Proteins are composed of various amino acids, including both non-essential amino acids (which the body can synthesize on its own) and essential amino acids (which must be obtained from the diet). In order to synthesize new proteins, all essential amino acids must be present in adequate amounts. If one or more essential amino acids come up short, protein synthesis cannot proceed optimally, leading to a limitation in the production of new proteins.

The concept of limiting amino acids is particularly important in the context of dietary protein sources. Different protein sources, such as animal-based proteins (e.g., meat, eggs, dairy) and plant-based proteins (e.g., legumes, grains, nuts), have different amino acid profiles. Some protein sources may be deficient in certain essential amino acids, making them the limiting amino acids in those sources. To ensure a balanced intake of essential amino acids, it is important to consume a varied diet that includes different protein sources.

Nutritional recommendations often focus on identifying and addressing limiting amino acids to ensure that the body receives an adequate supply of all essential amino acids for optimal protein synthesis and overall health.

What is a complete protein?

A complete protein is a dietary protein source that contains all nine essential amino acids in adequate amounts that are required by the human body. These essential amino acids cannot be synthesized by the body and must be obtained through the diet.

Proteins are composed of chains of amino acids, and they play a crucial role in various bodily functions, such as building and repairing tissues, producing enzymes and hormones, and supporting the immune system. However, not all protein sources contain all essential amino acids in sufficient quantities.

Animal-based protein sources such as meat, poultry, fish, eggs, and dairy products are considered complete proteins because they contain all essential amino acids in the right proportions for human needs. These sources are usually considered excellent protein sources due to their high biological value and amino acid profile.

On the other hand, most plant-based protein sources tend to be incomplete proteins, as they may lack or have low levels of one or more essential amino acids. Examples of plant-based protein sources include legumes (such as beans, lentils, and peas), grains (such as rice, wheat, and oats), nuts, and seeds. However, by combining different plant-based protein sources, it is possible to obtain a complete protein profile. For instance, combining legumes with grains or seeds can provide a complementary amino acid profile and create a complete protein source.

Do you need to consume complete proteins every time you eat? 

It's worth noting that while consuming complete proteins is beneficial, it is not necessary to obtain all essential amino acids in a single meal or within one specific food item. As long as a person's diet includes a variety of protein sources over the course of the day, the body can obtain all the essential amino acids it needs.

For example, Equip protein contains 1 g of leucine, and 2-3 g are required for muscle protein synthesis. Does that mean you won’t create muscle out of the amino acids you consumed? No—your body can still synthesize muscle protein when there’s a limiting amino acid present. 

Equip protein is made of beef, which is a slow-digesting protein. That means, it takes a while to make its way through the digestive system. Slow digesting proteins allow lots of time to round out your amino acids, so if you have some salmon, eggs, beef, or nuts later, your muscles will have what they need to strengthen and grow. 

There is more information on fast vs slow digesting proteins below. 

Vegan complete protein

Vegans can obtain complete proteins by combining different plant-based protein sources throughout the day to ensure an adequate intake of all essential amino acids. This practice is commonly known as protein complementation or protein combining.

Here are some examples of plant-based protein sources that, when combined, can provide a complete protein profile:

• Legumes (beans, lentils, chickpeas) + Grains (rice, quinoa): Legumes are generally rich in the amino acid lysine but lower in methionine, while grains are higher in methionine but lower in lysine. Combining legumes with grains creates a complementary amino acid profile, resulting in a complete protein. Examples include beans and rice, lentils with bread or rice, or hummus (made from chickpeas) with pita bread.
• Legumes + Seeds/Nuts: Legumes can also be combined with seeds or nuts to achieve a complete protein profile. For instance, peanut butter (from peanuts) on whole-grain bread or a salad with chickpeas and sesame seeds.
• Grains + Seeds/Nuts: Combining grains with seeds or nuts can also provide a complete protein. Examples include a salad with quinoa and sunflower seeds or oatmeal topped with almonds.

It's important to note that the concept of protein complementation has evolved, and it is now recognized that the body can store amino acids and utilize them efficiently throughout the day. As long as a vegan consumes a varied diet that includes a range of protein sources, it is likely to provide all the essential amino acids needed for optimal health.

As with animal proteins, it's not necessary to consume complete proteins in every meal or even at the same time. As long as a person's diet is well-balanced and includes a variety of plant-based protein sources, the body can effectively utilize the amino acids over time to meet its protein requirements.

Is there an essential amino acid supplement?

Yes, there are essential amino acid (EAA) supplements available in the market. These supplements are designed to provide a concentrated dose of all nine essential amino acids that the body requires for protein synthesis and various physiological functions.

Essential amino acid supplements typically come in the form of powders or capsules. They can be derived from animal sources (such as whey protein) or plant sources (such as soy protein or pea protein) to cater to different dietary preferences.

Athletes, bodybuilders, and individuals with specific dietary needs or certain medical conditions may consider using EAA supplements to support their protein intake or enhance muscle recovery and growth. These supplements can be useful for individuals who have limited access to quality protein sources or who struggle to meet their protein requirements through diet alone.

It's important to note that while EAA supplements can be a convenient option, they should not replace a well-balanced diet that includes a variety of whole food protein sources. Whole foods provide additional nutrients, fiber, and phytochemicals that are beneficial for overall health.

If you are considering using EAA supplements, it is advisable to consult with a healthcare professional or a registered dietitian who can assess your individual needs and provide guidance on appropriate supplementation. They can help determine whether a supplement is necessary, recommend a suitable product, and provide advice on proper dosage and usage.

Fast digesting proteins (1-2 hours)

Fast-digesting proteins are proteins that are quickly broken down and absorbed by the body, providing a rapid supply of amino acids to support muscle recovery and growth. These proteins are often consumed around workouts or when there is a need for fast nutrient delivery.

Here are some examples of fast-digesting proteins:

• Whey Protein. Whey protein is derived from milk and is considered one of the fastest digesting proteins available. It is rich in essential amino acids and has a high biological value. Whey protein isolate and whey protein hydrolysate are particularly fast-digesting forms of whey protein.
• Pea Protein. Pea protein is derived from yellow peas and is a popular plant-based protein source. While it is not as fast-digesting as whey protein, it is still relatively quickly absorbed compared to other plant-based proteins.
• Egg Protein. Egg protein, derived from egg whites, is a highly bioavailable protein that is also considered fast digesting. It contains all the essential amino acids in appropriate ratios.
• Fish Protein. Fish protein, such as from sources like salmon or tuna, is also known for its fast digestibility. It is rich in essential amino acids and omega-3 fatty acids.
• Soy Protein. Soy protein is a plant-based protein derived from soybeans. While it is not as fast-digesting as whey protein, it is still considered a relatively rapid protein source.
• Collagen Protein. Collagen protein is primarily known for its benefits to skin, hair, and joint health. While it is not a complete protein, it is quickly absorbed by the body due to its unique composition.

It's important to note that the speed of protein digestion can be influenced by various factors, including the presence of other macronutrients, the form of protein (powdered vs. whole food), and individual differences in digestion. Additionally, the overall protein content and quality of your diet are more important factors for muscle growth and recovery than the speed of digestion alone.

Slow digesting proteins (4+ hours)

Slow-digesting proteins are proteins that are broken down and absorbed by the body at a slower rate compared to fast-digesting proteins. These proteins provide a sustained release of amino acids over an extended period, which can be beneficial for promoting prolonged muscle protein synthesis and preventing muscle breakdown.

Here are some examples of slow-digesting proteins:

• Casein Protein. Casein is the main protein found in milk and is known for its slow digestion rate. It forms a gel-like consistency in the stomach, resulting in a gradual release of amino acids into the bloodstream over several hours. Micellar casein is a popular form of casein protein. Some people are sensitive to casein and may not do well with casein protein sources. Greek yogurt and cottage cheese contain casein protein.
• Beef Protein. Beef protein is derived from beef and is considered slower digesting compared to some other animal-based proteins. It provides a sustained release of amino acids. Equip protein supplements are beef protein. 
• Milk Protein. Milk protein, which contains both casein and whey protein, is considered a slow-digesting protein. It provides a combination of fast and slow release amino acids due to the presence of both whey and casein.
• Hemp Protein. Hemp protein is derived from hemp seeds and is a plant-based protein source that is generally digested more slowly than some other plant proteins.

The speed of protein digestion can vary depending on factors such as the presence of other nutrients, individual differences in digestion, and the form of protein consumed (powdered vs. whole food). A well-balanced diet that includes a mix of fast- and slow-digesting proteins is generally recommended to support optimal muscle protein synthesis and overall health.

Key Takeaways

Amino acids play a crucial role in protein synthesis. Essential amino acids are vital for the body but cannot be produced internally, necessitating their intake from dietary sources. Limiting amino acids are those present in the lowest quantity relative to the body's needs and can potentially hinder protein synthesis. Slow-digesting proteins release amino acids gradually, providing a sustained supply for muscle repair and growth. On the other hand, fast-digesting proteins are quickly broken down, offering an immediate influx of amino acids for post-workout recovery. Understanding these concepts can help individuals optimize their protein intake based on their health and fitness goals. With a varied diet and targeted supplementation, you can optimize your nutrition to get the most out of the amino acids and proteins you consume.