By Roberta H. Anding, MS, Baylor College of Medicine and Texas Children’s Hospital

Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

Even if you’re eating a high-protein diet, you might not be getting the most out of your diet. Professor Anding explains what complete proteins are, why we need to combine proteins, and how to manage as a vegan.

Why We Need Complete Proteins

Before learning how to combine proteins, first it’s important to understand why this is necessary. As human beings, we have, unfortunately, an endless ability to store fat. And, we have a small ability to store carbohydrate in our muscles and our liver.

We cannot, however, store protein. In order to make new protein, the body needs a daily supply of amino acids.

If we can’t store protein, any loss of body protein is going to represent loss of function. All the functions of protein, including the benefits that protein provides for post-surgery recovery, infant growth, and immune system support, will be lost as well. Thus, amino acids are needed to replenish our body’s supply of protein.

Some proteins contain all of the essential amino acids that we need to build new proteins. They’re often called complete proteins. Usually, they are of animal origin, such as milk, cheese, chicken, fish, and red meat. 

An exception to the rule is soybean, which is a plant protein and can be found in tofu. Soybean is as equally nutritious as the other sources of complete proteins that contain all of the essential amino acids.

Combining Proteins

Other proteins can be missing an essential amino acid, or they’re not contained in an adequate amount. These are called “incomplete proteins.” They’re lacking one or more essential amino acid. 

Most breads contain between two and three grams of protein per serving, but it’s not a complete protein. Similarly, nuts, rice, beans, and vegetables are all good sources of protein, but are incomplete by themselves. 

They’re going to need something else to balance out the essential amino acid that’s missing. This is called “combining proteins.” You combine a protein that complements the missing amino acid in an alternative way, making a complete protein. 

If you have a missing amino acid, or that amino acid is in short supply, protein synthesis stops. It doesn’t slow. It stops, because you’re missing the structures to complete that protein. 

This is called a “limiting amino acid.” That’s why it’s important to make sure you’re getting a balance of protein-containing foods.

Protein for Vegans

“The only time I ever really see an issue with this in the United States is in people who decide to become vegan,” Professor Anding said.

Vegans eliminate all sources of animal protein, and you can be very healthy on a vegan diet. You just have to be wise in the way that you’re combining proteins. 

For example, cereal grains are low in the essential amino acid lysine, and soybeans and other beans can be used in place of low-lysine foods to complement that. Red beans and rice is a great example of complementary proteins. Peanut butter—a nut—and bread—a grain—are also complementary proteins.

We used to believe that you had to have two incomplete proteins, like rice and beans, at the same meal. Science doesn’t support that belief any longer. You should still have them within the same day, but you don’t have to eat them within the same meal. 

Therefore, if you like to start out your day with a spoonful of peanut butter and a banana, you have an incomplete protein, but if you have a granola bar later in the morning, you’ve now had that missing essential amino acid. It wasn’t at the same meal, but it was on the same day. It’s not difficult to balance amino acids if you’re eating on a frequent basis.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.