What Are Essential Amino Acids?
Amino acids are the building blocks of protein, which are crucial for repairing tissue, building muscle, and performing other vital body functions. The human body can produce 11 of the 20 necessary amino acids, which are classified as non-essential. The remaining nine, however, cannot be synthesized by the body and must be obtained through food. These are known as essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. A food is considered a “complete protein” if it contains all nine essential amino acids in sufficient quantities, while a food that is low in or lacking one or more of these is an “incomplete protein”.
Common Incomplete Protein Sources
Most plant-based foods, with a few notable exceptions, are considered incomplete protein sources. This does not mean they are unhealthy or lack value; it simply means they need to be consumed in combination with other protein sources over the course of a day to ensure all essential amino acids are acquired.
Legumes
Legumes, which include beans, lentils, and peas, are an excellent source of plant-based protein but are typically low in the essential amino acid methionine. Classic examples of legume-based foods include:
- Beans (black, kidney, pinto): Primarily deficient in methionine.
- Lentils: Also have a limiting supply of methionine.
- Peas: Low in methionine and cysteine.
Grains
Whole grains are a staple in many diets, providing carbohydrates, fiber, and protein. However, most grains, including wheat, oats, and rice, are low in the essential amino acid lysine. The lysine content is even lower in more processed, refined grains.
- Wheat (bread, pasta): Low in lysine and threonine.
- Rice (brown and white): Also has a limiting amount of lysine.
- Corn: Lacks lysine and tryptophan.
Nuts and Seeds
While healthy fats and protein powerhouses, many nuts and seeds are incomplete proteins. Most are limited in lysine, and some also have other deficiencies.
- Tree Nuts (walnuts, cashews): Limiting in lysine.
- Sunflower Seeds: Low in lysine.
- Sesame Seeds: Limiting in lysine.
Vegetables
Though they offer a range of nutrients, most vegetables provide only small amounts of protein and are typically low in methionine.
- Broccoli: Contains some protein but is not a complete source.
- Spinach: Similarly contains protein but is not nutritionally complete on its own.
- Leafy Greens: Many leafy greens contain incomplete protein profiles.
Creating Complete Proteins Through Complementation
The good news for those relying on plant-based diets is that you don't have to get all nine essential amino acids in a single meal. The concept of “complementary proteins” involves combining different incomplete protein sources within a 24-hour period to create a complete amino acid profile. The body stores amino acids in a pool, allowing it to draw from different foods throughout the day.
Classic examples of complementary pairings include:
- Grains + Legumes: The lysine missing in grains is abundant in legumes, while the methionine limited in legumes is plentiful in grains. Examples include rice and beans, hummus with pita, or a lentil and barley soup.
- Legumes + Nuts/Seeds: A peanut butter sandwich on whole wheat bread is a familiar example of this combination.
A Few Plant-Based Exceptions
Some plant-based foods do contain all nine essential amino acids, making them complete proteins. These include:
- Soy and Soy Products: Tofu, tempeh, edamame, and soy milk are all complete protein sources.
- Quinoa: This grain-like seed is a popular complete protein.
- Buckwheat: Another pseudo-cereal that contains all essential amino acids.
Incomplete vs. Complete Protein: A Comparison
| Feature | Incomplete Proteins | Complete Proteins |
|---|---|---|
| Essential Amino Acids | Missing or limited in one or more. | Contain all nine in sufficient amounts. |
| Primary Sources | Mostly plant-based: legumes, grains, nuts, seeds, most vegetables. | Mostly animal-based: meat, eggs, dairy, fish. Also includes soy, quinoa. |
| Dietary Strategy | Combine with other foods over a day for a full profile. | Can be consumed on their own to meet daily essential amino acid needs. |
| Limiting Amino Acid | Often lysine (grains) or methionine (legumes). | None. All essential amino acids are present. |
| Example | Rice is incomplete, low in lysine. | Chicken is complete, with all essential amino acids. |
A Misleading Dichotomy
The terms “complete” and “incomplete” can be misleading, suggesting that some protein sources are less valuable. In reality, a diet with a wide variety of plant foods can easily provide all the essential amino acids needed for optimal health. Focusing on dietary diversity rather than meal-by-meal protein pairing is the modern, evidence-based approach to nutrition. Millions of people thrive on plant-based diets by simply eating a range of vegetables, grains, and legumes throughout the day.
Conclusion
Many plant-based foods, such as grains, legumes, and nuts, are typically missing or limited in one or more essential amino acids. Grains are most often low in lysine, while legumes are limited by methionine. However, this does not pose a nutritional problem for those following a plant-based diet, as long as a variety of protein sources are consumed throughout the day to ensure amino acid complementation. Foods like soy and quinoa offer complete protein profiles, simplifying meal planning. Ultimately, understanding which foods are typically missing one or more essential amino acids empowers individuals to create balanced and nutritious diets by combining different food groups effectively, without the stress of strict meal-by-meal pairing.
Further reading
For more detailed information on protein and amino acid nutrition, consult reliable sources like the Harvard T.H. Chan School of Public Health: https://nutritionsource.hsph.harvard.edu/what-should-you-eat/protein/.
