What is an “Incomplete” Protein?

As a result of working on The Bean Bake Blog, I’ve begun to look more closely at beans in general. For example, bean protein is considered an “incomplete” protein.

I don’t know about you, but I’ve never really understood the term: incomplete protein. I know we have to “complete” the protein with other food, but what does that mean, and how are we supposed to do it?

Clearly, it was time to do some research, and here is what I learned.

There are 9 essential amino acids that come only from food.

Your body contains thousands of different proteins that have different metabolic functions. However, all proteins are molecules that include the same amino acids. It’s the arrangement of these amino acids that determines the type and function of a protein.

Your body can make many of these amino acids on its own, but not all. The nine essential amino acids that come from food and their main purposes are

  • Histidine—physical and mental growth
  • Isoleucine—muscle production, blood formation
  • Leucine—growth hormone production, tissue production and repair
  • Lysine—bone development, hormone production
  • Methionine—digestion of fats, removal of plaque
  • Phenylalanine—brain processes and mood
  • Threonine—monitoring of body proteins
  • Tryptophan—mood, pain, and sleep regulation
  • Valine—muscle production

The proportion of these 9 essential amino acids in a food determines whether it is “complete” or “incomplete.”

The “complete” or protein-rich foods: Certain foods contain the nine amino acids in a balanced ratio that enables your body to access them. Another way to put this: these foods deliver the protein to you immediately—in “one bite,” so to speak.

Meat, poultry, fish, eggs, and dairy fall into this category as well as plant products made from quinoa and soy.

The “incomplete” or less protein-rich foods: The plant-based foods—legumes, whole grains, nuts, seeds, fruits, and vegetables—also contain all the essential amino acids.

However, they have a low amount in one or more of these acids. For example, lentils have a low amount of methionine, and brazil nuts have a low amount of lysine.

This lower amount or “limiting amino acid(s)” creates an imbalance that limits the ability of the food to make its other amino acids available to you. To put it another way, you cannot get the protein in “one bite.”

Your body creates a nutrition resource—an “amino acid pool”—from the food you eat during the day.

Let’s say you eat rice cereal in the morning, salad at lunch, and legumes at dinner. Your body “pools” together the various essential amino acids from these foods and uses them, as needed, to create proteins.

In other words, you do not have worry about creating the right balance of essential amino acids at any one meal. Your body handles the “balance-making” for you.

The moral of the story: Eat a wide variety of non-processed, plant-based foods.

Your body does the nutrition work for you, but you must give it the tools to get the job done.

Highly processed foods, e.g., potato chips, and the “white” foods, e.g., white bread, are not good tools because they have been stripped of many good nutrients.

To get the most efficient, effective use of the amino acids stored in your body, make sure your diet includes beans, whole-grain cereals and breads, sweet potatoes, brown rice, nuts and seeds, and fruits and vegetables.

Helpful links for more information:

  • The Savvy Vegetarian: A good overview of this subject along with a helpful chart of complementary foods.
  • Self Nutrition Data: This website displays the amino acid proportions for individual foods.
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5 thoughts on “What is an “Incomplete” Protein?

  1. The other odd detail I have to add is that there some amino acids that can become “conditionally” essential. Situations like illness can render your body less able to convert one or more of the above essential amino acids into the “conditionally” essential one. The old cookbooks that talked about feeding invalids carefully were right about that (if not necessarily about what to feed them)!

    Genetic disorders can also render people unable to convert them properly, but that’s more than your average eater really needs to know or worry about.

  2. Interesting and thanks. Of course, the chemistry is complex and out of my range of understanding but it helped me not to keep feeling that I had to add rice if I was eating any beans. It also explains why, in countries where there’s a lot of poverty, e.g., Latin America, people eat rice and bean meals all the time.

  3. Thanks for answering a question I had. Now that I read your post, I realized I had this exact class 20+ years ago in college Biology. I thought I would never have to use what I was taught in that class ever again 😉

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