Introduction
If you have ever heard that plant proteins are "incomplete" and therefore inferior, it is time to set the record straight. The terms "complete" and "incomplete" protein are technically correct, but their common interpretation often leads to wrong conclusions. Let us look at what these terms actually mean and why you do not need to worry about "protein combining" at every meal.
What Does Complete Protein Mean?
A complete protein is one that contains all nine essential amino acids in sufficient amounts (Young & Pellett, 1994). These nine amino acids are:
- Leucine
- Isoleucine
- Valine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Tryptophan
- Histidine
Your body cannot produce these on its own, so you must get them from food.
Complete protein sources:
- Meat, fish, seafood
- Eggs
- Milk and dairy products
- Whey protein and casein
- Soy (the only widely recognized complete plant protein)
- Quinoa (contains all essential amino acids)
- Buckwheat (also a complete plant protein)
What Does Incomplete Protein Mean?
"Incomplete" protein does not mean amino acids are missing. It means one or more essential amino acids are present at levels below the minimum requirement. This is called the limiting amino acid.
Examples:
- Pea protein: Lower methionine and cysteine, but still contains them
- Rice protein: Lower lysine
- Hemp protein: Lower lysine and leucine
- Grains: Lower lysine and threonine
- Legumes: Lower methionine
The key point: "lower" does not mean "absent." Even in an "incomplete" protein, all nine amino acids are present -- some are simply in smaller amounts.
The Complementary Protein Myth
For decades, people were taught that plant proteins must be "combined" at every meal -- for example, eating rice and beans together -- to get a complete protein. This is called the "complementary protein" theory.
Modern science has corrected this view. Your body does not require all essential amino acids in a single meal. It is sufficient to get them from various sources throughout the day.
In practice, this means:
- You do NOT need to eat rice and beans together at every meal
- It is enough for your daily diet as a whole to cover all essential amino acids
- With a varied diet, this is usually automatically achieved
That said, combining protein powders (such as pea and rice protein) is still a smart choice, as it ensures an optimal amino acid profile in every serving (Gorissen et al., 2018).
Practical Recommendations for Plant Protein Consumers
Good plant protein combinations:
- Pea protein + rice protein (the most popular combo)
- Legumes + grains (the classic combination)
- Soy + any other protein (soy is already complete)
How much protein do you need? When consuming plant protein, it makes sense to aim for slightly higher intake (2.0-2.4g/kg body weight), as lower digestibility means less reaches your muscles. Protein contributes to the growth and maintenance of muscle mass.
Hitting the leucine threshold: To trigger muscle protein synthesis, you need approximately 2.5-3g of leucine per meal. From plant sources, this is easier to achieve with pea protein (rich in leucine) than with hemp, for example.
Do not overthink it: If you eat a varied and sufficient diet, the risk of amino acid deficiency is extremely low. The "incomplete protein" label is not a reason to avoid plant-based eating.
Frequently Asked Questions
Can vegans get enough protein? Yes, if the diet is varied and includes sufficient legumes, grains, nuts, seeds, and/or protein powders.
Is pea protein a complete protein? Technically, pea protein contains all nine essential amino acids, though methionine is slightly below optimal levels. In practice, it is one of the highest quality plant protein sources.
Do I need to combine proteins at every meal? No. Modern science confirms that getting all essential amino acids throughout the day is sufficient. However, combining protein powders (pea + rice) is a convenient way to ensure an optimal profile.
Is animal protein always better than plant protein? Animal proteins typically have higher PDCAAS scores and more complete amino acid profiles, but plant proteins are also effective, especially when properly combined.
What is the best plant protein source for athletes? Pea protein is typically the best single choice due to its high BCAA and leucine content. Even better is a pea-rice protein combination.
References
- Young, V.R., & Pellett, P.L. (1994). Plant proteins in relation to human protein and amino acid nutrition. American Journal of Clinical Nutrition, 59(5 Suppl), 1203S-1212S.
- van Vliet, S., Burd, N.A., & van Loon, L.J. (2015). The skeletal muscle anabolic response to plant- versus animal-based protein consumption. (van Vliet et al., 2015) The Journal of Nutrition, 145(9), 1981-1991.
- Gorissen, S.H., Crombag, J.J., Senden, J.M., et al. (2018). Protein content and amino acid composition of commercially available plant-based protein isolates. Amino Acids, 50(12), 1685-1695.
- Berrazaga, I., Micard, V., Gueugneau, M., & Walrand, S. (2019). The role of the anabolic properties of plant- versus animal-based protein sources in supporting muscle mass maintenance: a critical review. Nutrients, 11(8), 1825.
- Marinangeli, C.P., & House, J.D. (2017). Potential impact of the digestible indispensable amino acid score as a measure of protein quality on dietary regulations and health. Nutrition Reviews, 75(8), 658-667.
See also:
Browse protein products at MaxFit →
Disclaimer
A food supplement is not a substitute for a varied and balanced diet and a healthy lifestyle.
