What Is a Complete Protein? Complete vs Incomplete Proteins

steak as an example of a complete protein

A complete protein is a protein source that contains all eight essential amino acids in the sufficient proportions to support normal biological functions. In adults, the eight essential amino acids are:

  • Phenylalanine
  • Valine
  • ​Threonine
  • ​Tryptophan
  • ​Isoleucine
  • Methionine
  • ​Leucine
  • Lysine

These amino acids are called “essential” because the body cannot make them, so they have to be supplied through diet. Recently, histadine has been added to this list as well, as scientists discovered that adults cannot synthesize it.

Incomplete proteins are proteins that lack one or more of the eight essential amino acids. Most plant-based proteins will meet the criteria for an incomplete protein, while animal-based foods are considered the primary sources of complete protein.

Examples of Complete Proteins from Animal Sources

The most common examples of complete proteins are foods that come from animals. These sources of protein include things like:

  • Fish
  • Seafood/Shellfish
  • ​Poultry
  • ​Milk
  • ​Eggs
  • Cheese
  • Meat

When you consume these foods, you take in all eight essential amino acids in a single food source. There is generally no need for additional amino acid supplementation.

Examples of Complete Proteins from Plant Sources

There are only a handful of plant-sources of protein that provide all eight amino acids in the necessary proportions to qualify as a ”complete protein.” These sources of complete protein include:

  • Quinoa
  • Buckwheat
  • ​Hempseed
  • Soybeans

Most people who eat a variety of plant-based and animal-based foods typically do not have to worry about whether they are consuming complete proteins — the variety in their diet will typically take care of that naturally.

Complete Protein for Vegetarians


In terms of complete proteins for vegetarians, conventional wisdom has stressed protein combining of certain foods together in the same meal to ensure that a person is receiving adequate amounts of all eight essential amino acids. Examples of complete protein food combinations include consuming beans and rice in the same meal or eating corn and wheat together.

The idea here is to combine a food that is lacking in one or more amino acids with another food that contains those missing essential amino acids. In doing so, you create a complete protein.

However, recent research has indicated that it may not be necessary to consume complementary proteins in the same meal. A small amount of amino acids are ”pooled” by the body – so for most moderately active people eating a variety of plant-based proteins across the day should suffice. For very active people, athletes or even fitness buffs who are trying to add lean mass, protein combining may be more desirable.

While protein combining may be unnecessary for the average vegetarian, there are some exceptions: If a person is on a vegetarian diet that is heavily dependent on fruit, tubers or is high in junk food, they may not be consuming sufficient amounts of essential amino acids. In these cases, diet modification may be necessary.

Complete Proteins and Vegetarian Athletes


While protein deficiency in vegetarians is generally rare — especially for people of average activity levels –vegetarian or vegan athletes may need to pay extra attention to the amount of protein they consume, even if the protein combinations are less important.

Because protein requirements are typically higher in athletes or very active individuals in general (regardless of their type of diet), if you are focusing on building additional lean tissue, you may need to up your protein slightly on a plant-based diet to ensure recovery, muscle growth and support the added demands of your sport or physical activities.

Many plant-based sources of protein such as certain types of beans, pluses and grains (for example wheat) are not as easily digested as animal-sources of protein, so you may require a slightly higher RDA of protein versus a person who includes meat, eggs, milk or seafood in their diet.

Even this is a bit controversial, since scientists continue to debate whether the Protein Digestibility Corrected Amino Acid Score (PDCAAS) accurately measures true protein digestibility in humans – especially since there are a number of variables that can influence how much protein from a food is absorbed by your body (including cooking methods, other foods consumed alongside it, your age and even flora and fauna in the gut.)

While the USDA and World Health Organization (WHO) does not differentiate between athletes and non-athletes when it comes to recommended daily allowances for protein, a good rule of thumb for endurance and strength athletes in good health is to have a protein intake of around 10 – 20% of your total calories.

Recommended ranges are typically out 1.4 – 1.8 grams of protein per kilogram of body-weight per day, or roughly 150 grams of protein for a 180 lb male, or about 110 grams of protein for a 130 lb female.

Should I Worry About Getting Complete Proteins?

Depending on the type of vegetarianism that you follow, you may already be consuming plenty of sources of complete proteins.

Lacto-Ovo vegetarians who eat eggs and/or dairy products should have few concerns about complete proteins. Vegetarians who only eat plants probably have little to worry about, but there are many vegetarian athletes and body builders who choose to protein combine just to be safe.

Whether this is necessary is open to debate. The belief among these individuals is that protein combining may give them a performance and recovery edge — especially immediately following weight training when complete proteins may be more vital to recovery and growth. There is very little direct clinical research around this, however.

The research that does exist has looked not specifically at protein combining in vegetarian athletes, but rather at the use of sources of complete protein — typically whey and/or casein protein from diary products — as a post-workout recovery meal. In these cases, individuals consuming this combination of protein and carbohydrates have shown increases in lean body mass over those who consumed only whey or carbohydrates.

The reasons for this are still open to interpretation. It may be simply that whey protein is quickly digested by the body, making it more readily available for recovery. Or it may be that consuming a complete protein itself assists with protein synthesis. More than likely, it is a combination of the two.

If muscle building on a vegetarian diet is one of your goals, go ahead and protein combine — especially immediately following your workouts. It won’t hurt, and may possibly help. While this is a bit easier and more convenient for Lacto-Ovo vegetarians, if you are on a strictly plant-based diet, you can always try soy protein isolates or grains like quinoa.

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