Incomplete Proteins: Sources, Misconceptions & How To Use Them

There's a prevailing belief in the health and fitness world that plant-based protein sources are inferior to animal-based proteins because they are "incomplete." But it turns out that incomplete proteins should still be a part of a balanced diet because of the important nutrients they bring to the table.

Proteins are made up of amino acids. There are 20 amino acids your body needs to function properly. Eleven are naturally produced by your body and are considered nonessential amino acids. The remaining nine cannot be made by the body and must be obtained through food. These are essential amino acids. 

The term "incomplete protein" refers to foods that don't contain adequate amounts of all the essential amino acids to meet the minimal human requirements. Contrary to popular belief, plant-based proteins aren't missing any essential amino acids1 but are usually low in one or two of them. These are called limiting amino acids.  

Stephan van Vliet, a scientist in the Center for Human Nutrition Studies at Utah State University, previously told mindbodygreen, "All proteins, including plant proteins, contain all nine essential amino acids. They are typically just lower in certain amino acids like lysine, methionine, and/or leucine, but it is a misconception that they are missing essential amino acids." 

So, how do plants live up to this threshold? Let's look at rice protein isolate3, for example: It surpasses all these targets except lysine. For each gram of protein, rice protein isolate provides 31 milligrams of lysine. In this case, lysine is the limiting amino acid keeping rice protein isolate from being considered a complete protein. 

Another way to consider how incomplete protein sources stack up to complete protein sources is to look at amino acid percentages.

The World Health Organization (WHO) and Food and Agriculture Organization of the United Nations (FAO) sets guidelines for each essential amino acid requirement as a percent for various protein sources.

"Many of us are familiar with leucine (the muscle-building amino acid), where the current requirement within a given protein source is set at 5.9%. Some plant-based proteins like hemp (5.1% leucine) and lupin (5.2%) fall short, while others like oat (5.9%) and wheat (6.1%) protein provide close to the recommended leucine content," says Nanci Guest, Ph.D., R.D., CSCS, a plant-based dietitian and nutrition researcher. 

The popularization of the term "incomplete protein" dates back to a 1971 book by Frances Moore Lappé in which she recommended combining complementing plant-based proteins to meet protein requirements. She retracted her position years later, but the damage was done. Many people continue to believe that they either need to eat animal proteins or strategically combine plant foods at every meal to get complete proteins. 

"While protein quality may be a consideration in the case of low intakes, when intakes are sufficiently high, all proteins provide the essential amino acids in sufficient quantities," explains van Vliet. "Outside of a research setting, it is also rare that someone consumes single protein sources in a meal or during the day, especially in the context of developed nations where protein powder use is common."

Not necessarily. Incomplete proteins come from plants, which are some of the most nutritious foods on the planet. "When looking at protein, it's important to look at the entire protein package. Unless you are consuming amino acids supplements, like branched-chain amino acids, you are getting much more than just protein in a food," says Nichole Dandrea-Russert, M.S., RDN, a dietitian nutritionist and the author of The Fiber Effect. 

"As long as these are whole plant foods (with no or limited processing), we can feel confident that these foods are high in vitamins, minerals, and fiber with no saturated fats," says Guest.

Dandrea-Russert explains that plant-based proteins come with fiber and phytochemicals, both of which have been shown to support health, from brain to gut and heart health. "Both fiber and certain phytochemicals have been shown to reduce inflammation, whereas compounds in meat, such as antibiotics, dioxins stored in animal fat, and heterocyclic amines (carcinogenic compounds formed when meat is cooked) promote inflammation7." 

"By focusing on a variety of plant proteins, you're also going to be helping your gut microbiome8," adds Amanda Sauceda, M.S., R.D. The American Gut Society found that people who regularly eat more than 30 different plant foods per week have a more diverse microbiome than those eating 10 or fewer8. 

However, Guest notes, complete proteins may offer the advantage of having higher mineral bioavailability. You absorb minerals like iron and zinc when you eat meat than when you eat plant foods.

At the end of the day, amino acids are amino acids. If you're entirely plant-based, you may just have to eat larger portions in order to reach your protein needs, Layman notes. "It's perfectly fine [to be vegan or vegetarian], but you will always need more total protein, and that means more total calories to be equal," he previously explained on the mindbodygreen podcast.

While the recommended daily allowance for protein is 0.8 gram per kilogram of body weight, many experts and recent studies highlight the importance of eating much more than that for optimal health or to support athletic performance. 

Most active people should aim to eat at least 100 grams of protein per day, but keep in mind that your protein needs will change depending on your age, body composition, activity level, and health goals. For example, women might need to consume more protein during pregnancy and lactation to keep up with their body's needs. Older adults10 can also benefit from more protein. 

The Journal of the International Society of Sports Nutrition (ISSN) recommends active people looking to build or maintain muscle should eat 1.4 to 2.0 grams of protein per kilogram of body weight11. 

Meat eaters can easily eat 100 grams of protein per day by including lean protein and plant-based proteins with each meal, choosing high-protein snacks, and using protein powder to help fill protein gaps in their diet. Variety is key, so we don't think people should pass up on plant proteins and eat strictly animal proteins just because they've been deemed "complete." 

Vegetarians and vegans who eat a wide variety of protein-rich plant-based foods don't have to worry about protein deficiency unless they're eating too few calories overall. Vegans may want to consider increasing their protein intake, though. 

Protein combining or pairing in the same meal13 is another thing vegans no longer have to worry about.

"There was an old theory that you have to 'complement' certain foods to make a 'complete' protein like eating rice (low in lysine, but adequate in methionine) and black beans (low in methionine but adequate in lysine) together in the same meal," explains Dandrea-Russert. "That theory has gone out the window as it's known that our bodies have an amino acid pool14 that maintains all of the amino acids from the foods we eat as well as the amino acids our bodies make." 

So while vegans don't need to eat complete proteins at every meal to get all of their essential amino acids, they should aim to get a balance of those amino acids throughout the day.

Plant proteins are still misunderstood and often passed up by meat-eaters, athletes, and fitness enthusiasts because they are deemed incomplete. In reality, your body treats the amino acids from incomplete proteins the same as those from complete proteins.