Proteins: Complete, Incomplete and Complementary

Published:  01/30/2018

Proteins are made up of amino acids. Amino acids are the building blocks of protein. There are 20 different amino acids that join together to make all types of protein.

Some of these amino acids can't be made by our bodies, so these nine amino acids are known as essential amino acids. It's essential that our diet provide these.

Humans can produce 11 of the 20 amino acids. The other nine must be supplied by food. If we don't get enough of even 1 of the 9 essential amino acids the body's proteins degrade. Unlike carbohydrate and fat, the human body does not store excess amino acids for later use—the amino acids must be in the food every day.

The 11 amino acids that we can produce (which are called non essential amino acids) are alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine. Tyrosine is produced from phenylalanine, so if the diet is deficient in phenylalanine, tyrosine will be required as well. The essential amino acids are arginine (required for the young, but not for adults), histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These amino acids are required in the diet.

In the diet, protein sources are labeled according to whether or not they have all the essential amino acids or not. They are called complete or incomplete.

A complete protein source is one that provides all of the essential amino acids.  Animal-based foods such as meat, poultry, fish, milk, eggs, and cheese are complete protein sources.

Plant sources of protein are incomplete or have very little of some amino acids, with the exception of soybeans, amaranth and quinoa. If a vegan or vegetarian has a limited diet, they can potentially not be getting their daily protein needs met. They must be careful to eat a wide a variety of protein rich plant foods.

Vegans and vegetarians can get their protein needs met if they eat a variety of what is called complementary proteins which are two or more incomplete protein sources that together provide adequate amounts of all the essential amino acids.

Lentils for example are low in certain essential amino acids; however, these same essential amino acids are found in greater amounts in barley. Similarly, barley contain lower amounts of other essential amino acids that can be found in larger amounts in lentils. Together, these two foods can provide adequate amounts of all the essential amino acids the body needs. A soup that contains both lentils and barley is an example of a complete protein made by combining two complementary protein sources.

In the past, it was thought that these complementary proteins needed to be eaten at the same meal for your body to use them together. Now studies show that your body can combine complementary proteins that are eaten within the same day. You do not HAVE TO, but I still like to educate people on combining protein so that they can if they wish and so that they understand that if you are eating plant-based it is important to get a wide variety of foods.

Protein Quality Scoring

Protein quality is the digestibility and amount of essential amino acids for providing the proteins in correct ratios for human consumption.

Protein quality can be determined by the PDCAAS (Protein Digestibility Corrected Amino Acid Score) method, is a measure of the quality of a protein based on a human's ability to digest it and the amount of amino acids.

Using the PDCAAS method, the protein quality rankings are determined by comparing the amino acid profile of the specific food protein against a standard amino acid profile with the highest possible score being a 1.0. Medium quality is .75 or above. This score means, after digestion of the protein, it provides per unit of protein 100% or more of the nine essential amino acids required. There are some limitations as it doesn't factor "anti-nutrients" in food and how much protein our bodies can digest. Because of this there is another method called DIAAS described below.

 A PDCAAS value of 1 is the highest, and 0 the lowest.

These are rated a 1: Cow's milk, eggs, casein milk protein, soy protein isolate, whey protein.

Beef: .92

Chicken .95

Soy: .91 to 1

Pea protein concentrate: .893 - .93

Edamame (soybeans) and chickpeas: .78

Beans and lentils in general: .70-.75

Quinoa: .667

Peanuts: .52

Potato: .74

Rice: .42

Tofu: .56

Whole Wheat: .4

Wheat gluten: .25

Gelatin: 0

Collagen (beef protein isolate): 0

Another newer protein quality measure called DIAAS (digestible indispensable amino acid score) is recommended to replace PDCAAS. It measures the digestibility of each amino acid at the end of the small intestine.

DIAAS is defined as: DIAAS % = 100 x [(mg of digestible dietary indispensable amino acid in 1 g of the dietary protein) / (mg of the same dietary indispensable amino acid in 1g of the reference protein)]. This gets complicated as you can see in this report here. 

Take Home Message

Eat a varied diet of different plants and protein sources.

If you are eating a lower calorie diet, eat high-quality proteins such as soy, and make sure you are following the recommendations per the Academy of Nutrition and Dietetics Vegetarian position on vegetarian diets. The position states that protein needs may be somewhat higher, especially when consuming protein sources that are less well digested. Some evidence also suggests that protein is used less efficiently with aging, which may translate to higher protein requirements. Thus, it is important for older vegetarians and vegans to include protein-rich foods such as legumes and soy foods in their diets. Meat analogs may be helpful as protein sources. Try to focus on eating some of those protein sources at a PDCAAS of 0.75 or above.

Complementary Proteins

Many studies have shown that it is not necessary to combine protein foods at one meal but my intuition tells me that there is an advantage of eating complementary proteins in the same meal as it makes it easier to ensure that all the necessary amino acids are available to the body. I think this is especially important when I am counseling vegans or vegetarians that I have determined through their food intake journals, are not getting enough protein or variety of proteins. Focusing their attention on combining complementary proteins helps teach them variety and adequacy that then becomes instinctive.

  • Grains plus Dairy 
  • Legumes plus Nuts or Seeds
  • Grains plus Legumes

And here are some common meal items that naturally complement each others' proteins:

  • Beans and rice or tortillas
  • Brown rice and beans
  • Brown rice and green peas
  • Brown rice with almonds, cashews or pecans
  • Chickpea hummus on pita bread
  • Corn and beans
  • Grilled cheese sandwich
  • Lasagna
  • Lentil soup or dairy-based soup with bread
  • Macaroni and cheese
  • Noodle stir-fry with peanut or sesame seed sauce
  • Oat bran and soy milk
  • Peanut butter sandwich
  • Pizza
  • Quinoa salad with black beans and feta
  • Tacos filled with beans or lentils
  • Tofu or Tempeh on whole wheat bread
  • Tofu with rice (or any grain)
  • Tofu with tahini (sesame seed paste)
  • Whole grain bread and peanut butter
  • Whole grain cereal with milk
  • Yogurt with nuts
  • Yogurt with walnuts

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Maria Faires, RD

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