The Difference Between Incomplete and Complete Proteins, Explained

Dietary sources of protein not only help keep you full and balance your blood sugar, but they’re also required to grow and repair muscles, create white blood cells to support your immunity, and form various hormones and neurotransmitters essential for physical and mental health—and those are just a few examples. Needless to say, without enough protein, you’d feel pretty crummy. 

But it’s not just the quantity of protein in your diet that’s important: The quality and variety matter, too. That’s because only certain protein-containing foods are considered “complete proteins,” meaning they contain all nine essential amino acids that the body requires but can’t produce on its own. 

Keep reading to learn more about complete proteins, including how they’re different from incomplete proteins and tips for mixing and matching proteins to cover your nutritional bases. 

What Is a Complete Protein?

A food is considered a complete protein if it contains adequate levels of all nine essential amino acids and is readily digestible and absorbed, according to Jennifer McDaniel, RDN, a registered dietitian and board-certified specialist in sports dietetics. But what are essential amino acids and why are they important? Let’s back up a bit.

When you eat and digest protein—whether from tofu, beans, or chicken—your body breaks that protein down into its smallest components: amino acids. These amino acids are then absorbed by the body and used in processes such as muscle growth and hormone synthesis. 

The human body requires 20 amino acids to support optimal health, growth, and development. Eleven of these amino acids can be made by the body in most situations, so they’re considered non-essential. Those amino acids are:

The remaining nine amino acids are considered essential, as they cannot be made by the body and therefore must be obtained from protein-containing foods. Again, if a protein contains adequate levels of all nine of these essential amino acids, it’s considered a complete protein. 

Each amino acid has a slightly different function. For example, tryptophan helps make serotonin, a neurotransmitter that supports mood, sleep, and appetite; methionine supports detoxification and the absorption of minerals like zinc; and leucine, isoleucine, and valine are key for muscle growth. 

Complete vs. Incomplete Proteins

A complete protein contains at least some of all nine essential amino acids that the body can’t make, while an incomplete protein is missing at least one of those nine essential amino acids. 

Most complete proteins come from animal sources (think: meat, fish, eggs, and dairy), but some plants, such as soy-derived foods, are also complete. 

Meanwhile, plant proteins (with the exception of soy), are more likely to be incomplete. These include many legumes, whole grains, nuts, and seeds. For example, “beans are frequently low in the sulfur-containing amino acids methionine and cysteine, while lysine is typically limited in grains,” McDaniel says. One animal protein, collagen, is also incomplete, as it’s missing tryptophan. 

While a few other plant foods do technically qualify as complete proteins, they’re not as complete as others. “Soy is widely accepted as a complete protein, but other plant proteins such as hemp, chia, nutritional yeast, quinoa, amaranth, buckwheat, and spirulina are considered [by some experts] to be ‘nearly complete’—they contain all nine essential amino acids, but some of the amino acids are very low in quantity,” McDaniel says.

Because plant proteins may lack or be low in certain amino acids, it’s important for plant-based eaters to mix and match their protein sources over the course of a day, as this variety of proteins will cumulatively provide all essential amino acids the body needs, McDaniel notes. (More on this later.) 

Don’t assume incomplete proteins are less nutritious than complete proteins, though. Many incomplete proteins happen to be great sources of fiber, energizing complex carbohydrates, vitamins and minerals, and antioxidants. We can all benefit from these nutrients, which underscores the importance of including a variety of them in your diet, whether you’re a plant-based eater or an omnivore.

Examples of Complete Protein Foods

So what does a complete protein actually look like? Below, find 13 sources of complete protein and seven sources of “nearly complete” protein, which you can use as inspiration to switch up your protein sources throughout the day or week: 

“Nearly Complete” Protein Foods 

These proteins contain all nine essential amino acids, but some may be present in lower quantities, per McDaniel—so, depending on who you ask, they might be called “complete” or “nearly complete” proteins. Remember, these (and many incomplete proteins) are still highly nutritious foods, and as long as you aim for a variety of protein-containing foods each day, you’re golden.

“Variety is important to help us cover our nutritional bases,” says Jessica Cording, RD, a registered dietitian and health coach. And each of the above foods has unique benefits beyond protein. Case in point: Salmon has loads of anti-inflammatory omega-3s, tofu has a surprising amount of calcium, and edamame is packed with fiber. 

Can You Combine Incomplete Proteins to Get All the Amino Acids You Need?

If you eat predominantly incomplete protein sources (such as if you’re a vegetarian or vegan), you have to be a bit more strategic about your intake. Because incomplete proteins differ in their amino acid profile (with some containing high, moderate, or low amounts of certain amino acids compared to others), consuming a variety of these incomplete protein sources is key to ensuring you get adequate amounts of all nine essential amino acids. 

The good news: While it was once thought that you needed to pair incomplete proteins at every single meal to reap the benefits of complete protein, McDaniel says this advice is outdated. As long as you eat a variety of incomplete protein sources (e.g. legumes, vegetables, seeds, nuts, and whole grains) over the course of the day—and get enough total daily protein—you will get all the amino acids you need. 

That said, we tend to naturally pair incomplete proteins together in a way that creates a complete protein anyway, McDaniel says. Think: hummus and whole wheat pita, cornbread and kidney bean chili, peanut butter on whole wheat toast, or beans and rice.

“A few of my favorite nutrient-dense incomplete protein sources are legumes, collagen, and nuts and seeds,” Cording adds. “Whole grains and vegetables offer smaller amounts of protein, so having a variety of these foods as part of regular meals and snacks can help you cover your bases.”

How Much Protein Do You Need Every Day, Again?

Both complete and incomplete proteins count toward your total daily protein intake. At a minimum, you want to meet the Recommended Dietary Allowance (RDA) for protein, which is 0.8 grams of protein per kilogram of body weight per day, or 0.36 grams per pound. That means a person who weighs 150 pounds (68 kilograms) would typically need at least 54 grams of protein per day. 

That said, nutrition experts say the RDA for protein probably isn’t enough to support optimal health. “The RDA was established to prevent deficiencies, but most people will benefit from higher amounts,” McDaniel explains. Protein needs may vary based on factors such as age, sex, weight, activity level, muscle mass, and overall health.

“Depending on someone’s activity level and wellness goals, active adults may need anywhere from 1.2–2 grams [of protein] per kilogram per day,” McDaniel says. That translates to 82–136 grams of protein per day for a 150-pound person. 

However, it’s important to know that with certain health conditions, such as kidney disease, you may need less. Consult your physician or a registered dietitian if you need further guidance. 

The Takeaway

Protein is vital for everything from maintaining stable blood sugar and appetite to synthesizing neurotransmitters and hormones to building strong muscles. To benefit from its full range of functions, we must consume sufficient amounts of all nine essential amino acids in our daily diet. Complete proteins contain all essential amino acids in one place, while incomplete proteins don’t—but you can combine incomplete proteins throughout the day to cover your bases. And while it’s true that most complete proteins are animal-derived, there are a surprising number of plant-based options, too. To promote overall health and diversity of nutrient intake, use the information and food list above as inspiration to create protein-rich meals utilizing a variety of different complete and incomplete protein sources.

This content is for informational and educational purposes only and does not constitute individualized advice. It is not intended to replace professional medical evaluation, diagnosis, or treatment. Seek the advice of your physician for questions you may have regarding your health or a medical condition. If you are having a medical emergency, call your physician or 911 immediately.