Are Plant Proteins Complete Proteins?

People who are trying to eat more plant protein—as nearly 30 percent of Americans are, according to market-research firm Mintel—often wonder how to put the right foods together in the right combinations to make sure they’re getting the "complete" protein they need. But you really don’t need to worry about that, says Maxine Siegel, R.D., who heads CR’s food-testing lab. Though plant proteins technically aren’t "complete," as long as you’re eating a variety of plant protein sources, your body does the work of “completing” the proteins for you.

Complete proteins are protein foods that contain all of the essential amino acids—those your body can’t produce itself. Amino acids are often called the building blocks of protein because your body puts them into different combinations to create the various types of proteins it needs. They’re required for digestion, muscle and hair growth, and the production of various enzymes and antibodies, among other things.

Animal protein—beef, pork, poultry, fish, dairy products, and eggs—contain all nine essential amino acids. Some plant proteins, such as those found in buckwheat, quinoa, and soy, are complete proteins as well. But the majority of plants contain just some of the essential amino acids.

“We used to think you had to combine certain incomplete proteins, like the ones in rice and beans, in the same meal to get all the essential amino acids,” says Dana Hunnes, R.D., Ph.D., an adjunct assistant professor at the Fielding School of Public Health at UCLA. “Now we know that you can meet your needs by eating a variety of plants throughout the day.” And if you’re eating any animal protein at all, you’re also taking in all the essential amino acids.

Getting more of your protein from plants actually has health benefits, Siegel says. (Beans, grains, lentils, nuts, and tofu are all good sources of plant protein.) In a Harvard Medical School study, researchers found that substituting 3 percent of calories from animal protein with plant protein was linked to a 12 percent reduced risk of dying from heart disease and a 10 percent reduced risk of dying from any cause during the 32-year study period. “Plus sources of plant protein also supply fiber, antioxidants, and other nutrients that we need more of in our diet,” Siegel says. 

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