Corn is an incomplete protein. It contains all nine essential amino acids, but its levels of lysine and tryptophan are too low to meet human nutritional needs on their own. Lysine is the primary limiting amino acid in every variety of corn tested, which means it runs out long before your body gets enough of the others.
What Makes Corn’s Protein Incomplete
A protein is considered “complete” when it supplies all nine essential amino acids in the quantities and ratios your body requires. These nine (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) can’t be manufactured internally, so they have to come from food. Animal proteins like eggs, meat, and fish consistently deliver all nine in adequate amounts. Corn does not.
The standard way to measure protein quality is the Digestible Indispensable Amino Acid Score, or DIAAS. A score of 100 means the food fully meets amino acid requirements; anything below 100 means at least one amino acid falls short. Across seven maize varieties tested in a 2023 study published in Heliyon, DIAAS values ranged from just 29 to 53 for older children and adults, with lysine identified as the limiting amino acid in every single variety. That means corn protein delivers roughly a third to half of what you’d need from a complete source.
How Much Protein Corn Actually Provides
Corn kernels contain around 8 to 10% protein by weight, which is modest compared to legumes or animal foods. But the bigger issue isn’t quantity, it’s quality. The main storage protein in corn, called zein, is particularly low in lysine and tryptophan. Standard field corn contains about 0.29 to 0.31% lysine, a fraction of what’s found in higher-quality protein sources.
Digestibility also plays a role. Whole grain corn flour has an amino acid digestibility of roughly 51 to 61%, meaning your body only absorbs about half to two-thirds of the protein present. Removing the hull (the outer fiber layer) bumps digestibility up to around 66%, but still doesn’t fix the lysine shortfall.
Quality Protein Maize: A Better Variety
Plant breeders have developed a biofortified type called Quality Protein Maize (QPM) specifically to address corn’s amino acid weakness. QPM contains about double the lysine and 1.9 times the tryptophan of conventional corn. According to research published in Foods, QPM can supply 70 to 80% of human protein requirements, compared to a maximum of 46% for standard varieties.
QPM’s DIAAS score is higher than regular corn (48 vs. 32 to 46 for adults), but it still falls below the threshold for a complete protein. It’s a significant improvement, especially in regions where corn is a dietary staple, but it doesn’t eliminate the need for complementary protein sources.
Does Nixtamalization Help
Nixtamalization is the traditional lime-cooking process used to make tortillas, tamales, and hominy. It does improve corn’s nutritional profile in some ways. The alkaline treatment reduces compounds called phytates by up to 21%, which helps your body absorb more of the protein that’s present. Tortillas made from processed dry masa flour showed protein digestibility of about 82%, a noticeable jump from the 65 to 74% seen in less processed varieties.
However, nixtamalization doesn’t increase the actual lysine content. Lysine remains the limiting amino acid in tortillas just as it is in raw corn. The process makes more of corn’s existing protein available to your body, but it can’t create amino acids that aren’t there.
How to Complement Corn’s Protein
The classic pairing exists for a reason: corn and beans together form a complete protein. Beans are rich in lysine (corn’s weak point) but low in methionine and cysteine (which corn provides in adequate amounts). You don’t need to eat them in the same meal. As long as you consume a variety of protein sources throughout the day, your body pools the amino acids and uses them as needed.
Other good complements to corn include lentils, chickpeas, peanuts, and dairy. If you eat eggs, meat, or fish regularly, the lysine gap from corn is easily covered without any special planning. The incomplete protein issue really only matters if corn or other cereals make up the vast majority of your diet, which is the case for some populations in sub-Saharan Africa and Central America where QPM has had the most public health impact.
For most people eating a varied diet, corn’s protein quality isn’t something to worry about. It contributes useful amino acids, fiber, and energy. It just shouldn’t be your only protein source.