Understanding the Difference Between Complete and Incomplete Protein Is Critical for Everyone (but Particularly Those Who Eat Plant-Based)

It's an annoying yet well-intentioned question. After all, the most common forms of protein—you know, the macronutrient shown to be responsible for building muscle, repairing tissue, and assisting in vital bodily functions like blood clotting and the immune response—come from animal foods like chicken, fish, beef, and eggs. What do you do if you don't eat those foods?

Well, you can tell your mom/co-worker/random IG follower to calm down with that concern. It is completely possible—nay, easy—to get enough protein on a plant-based diet. (To determine your Recommended Dietary Allowance (RDA) for protein, just multiply your weight in pounds by 0.336 or use this online calculator.) Meat is not the protein end-all, be-all, people!

However, there is a slight catch that plant-based eaters need to know. Beyond the whole "did it come from an animal or not" distinction, not all protein is the same from a nutritional standpoint. There are complete proteins, incomplete proteins, essential amino acids... it's kind of complicated. And not knowing the difference does matter, especially when you don't eat animal-based foods.

What is a complete protein?

First, a nutrition science refresher. Dietary protein is made up of different combinations of 20 different amino acids, says Scarlett Full, RD and nutrition scientist at Growing Naturals. "The human body is able to produce 11 out of the 20 amino acids on its own. But the other nine must be obtained from daily food sources," she says. These amino acids are lysine, cystine, methionine, threonine, histidine, leucine, tryptophan, valine, and phenylalanine.

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Those exclusive nine are considered "essential" amino acids because your body needs them to function properly. "Although you can’t see or feel it, the body’s cells are constantly being repaired or replaced with new ones," Full says. "In order to build or rebuild tissue, the body needs a specific combination of the nine essential amino acids."

For a food to be considered a complete protein, it needs to contain adequate amounts of all of those nine essential amino acids. An incomplete protein, as you've likely deduced, doesn't contain all nine essential amino acids. This is where things get tricky for plant-based eaters. Animal foods like meat, eggs, and even Greek yogurt are complete proteins; a number plant-based proteins (like beans, brown rice, and lentils), however, are considered incomplete proteins.

Notable exceptions: Quinoa, amaranth, hemp, chia, and soy, among others, says Amy Shapiro MS, RD, CDN and founder of Real Nutrition. For reference, here is an outline a longer list of plant-based foods that contain adequate amounts of all nine essential amino acids.

Vegan sources of complete protein:

• Tempeh: A 3-ounce serving of tempeh contain 11 grams of protein.
• Hemp seeds: 3 tablespoons of raw, hulled hemp seeds has 10 grams of protein.
• Amaranth: 1 cup of cooked amaranth provides approximately 9 grams of protein.
• Quinoa: 1 cup of cooked quinoa provides approximately 8 grams of protein.
• Tofu: A 3-ounce serving of tofu provides approximately 8 grams of protein.
• Edamame: A 1/2 cup of whole edamame provides 8 grams of protein.
• Ezekiel bread: A 2 slice serving contains 8 grams of protein.
• Nutritional yeast: A 1/4-cup serving of nutritional yeast provides 8 grams of complete protein.
• Buckwheat: 1 cup of cooked buckwheat groats provides approximately 6 grams of protein.
• Spirulina: 1 tablespoon of dried spirulina provides 4 grams of protein.
• Chia seeds: 2 tablespoons of chia seeds contain 4 grams of protein.

How to hack complete and incomplete proteins for vegetarians and vegans

This whole incomplete proteins thing sounds a lot scarier than it is. Yes, not getting enough protein (or the right kind) can lead to some gnarly side-effects like hair loss and weak nails, and it'll be that much harder to build muscle or recover from workouts. But plant-based eaters can easily get all the necessary aminos acids by eating the above-mentioned vegetarian complete proteins as well as combining different incomplete proteins to create a complete protein. (Which is good, because there's only so much chia pudding a person can eat.)

Complementary proteins

This is not nearly as complicated as it may sound. "Grain-based protein sources like rice, wheat, and oats are typically low in the [essential] amino acid lysine but high in cystine and methionine," says Full. That makes them a great complement to legume-based proteins, she says, which are conversely high in lysine but low in cystine and methionine. Full also recommends pairing nuts and seeds, which are usually low in lysine and threonine, with legume-based proteins.

Some easy-to-remember examples of complementary plant-based proteins:

• Beans and rice
• Nut butter and whole grain bread
• Lentil and barley
• Hummus and pita bread
• Peanut butter on whole wheat bread
• Black (or other) beans on corn tortillas
• Tofu with barley (or any grain)
• Lentil soup with bread
• Lettuce greens topped with beans and seeds

It's also not necessary to eat all nine essential amino acids every time you eat. "No need to stress about balancing out every meal—if you eat complementary foods throughout the day, you’ll get all the amino acids you need to stay healthy," Shapiro says. So almonds as a snack plus a lentil soup later on helps you get that complete picture of protein throughout the day.

However, you do need to make sure you eat complementary foods every day. "Dietary protein cannot be stored in the body, so amino acids need to be 'replenished' on a daily basis," Full explains. Each day is an amino acid blank slate, so to speak. This is true for everyone, BTW, not just vegans and vegetarians.

So there you have it—all the knowledge you need to explain to inquiring/annoying minds how, yes, you're doing absolutely fine in the protein department. And also to live your healthiest plant-based life, of course.

Originally published on January 28, 2019 with additional reporting by Betty Gold.

Hey, fun fact: It's totally possible to do keto on a vegetarian diet. Here's how to live that high-fat life sans meat.  More of a carb-leaning vegetarian? Try this sweet potato chili recipe right now.

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