Dr. Fuhrman Dr. Fuhrman

Complete Protein in a Vegetarian Diet

In previous articles about protein, I mostly talked about the total amount of protein in a diet, as if all protein were created equal. People who are concerned about protein in a vegan diet usually are not only concerned about the amount of protein, but also about the quality of protein. Vegetable protein is said to be of lesser quality than animal protein. It is often referred to as "incomplete" whereas animal protein is said to be "complete". In this article I will discuss what is meant by the term "complete protein" and we will look at whether this is something to worry about.

Proteins are made up of amino acids. They are the building blocks of protein. There are twenty different amino acids in protein, nine of which are essential, which means that the body needs to get them from food because it cannot make them itself. The other amino acids can be made by the body so they do not have to be present in our food. Of course there is some controversy about the ideal or minimum amounts of the essential amino acids that we need. An often cited standard is that of the Institute of Medicine's Food and Nutrition Board. They give the following recommended amino acid profile (amounts given are percentages of the total amount of protein in the food):

Amino Acid Percentage of Protein
Tryptophan 0.7
Threonine 2.7
Isoleucine 2.5
Leucine 5.5
Lysine 5.1
Valine 3.2
Arginine 1.8
Methionine+Cystine 2.5
Phenylalanine+Tyrosine 4.7

A protein is said to be complete if the amino acids are present in at least these amounts. The body can make methionine from cystine and phenylalanine from tyrosine and vice versa, so that is why they are listed together. If a food does not have all the amino acids in the recommended amounts, it is said to be deficient in that amino acid, and the amino acid is called the limiting amino acid. This is not a problem at all, since people eat a variety of foods. Sometimes people (even self proclaimed experts) still recommend that vegans combine foods in a single meal to make up for limiting amino acids. This is not necessary. Just like we do not need to get optimum amounts of all vitamins in every single meal, we also do not need optimum amounts of amino acids in every single meal. The body will be able to use them if we eat a variety of foods every day.

Even though the food combining theory has long been discredited, many people still consider it a fact that plant based foods have incomplete amino acid profiles by definition. This is false. Grains often (but not always) have a limiting amino acid, but most vegetables and many kinds of beans have complete proteins. Green vegetables are no brainers for people who read this site, but even potatoes and carrots have a good amino acid profile, as have chickpeas, white, black and kidney beans. It has been stated again and again by vegan nutrition experts, but I think it bears repeating: if you eat a healthy, varied diet, you do not need to worry about complete proteins. If you do not eat a healthy varied diet, you have more important things to worry about than amino acids.

It was surprising to me that not all animal foods are complete proteins. I wonder why we never hear that people should always combine ground beef or cured pork with a food that is high in tryptophan, because ground beef and cured pork are severely deficient in this essential amino acid.

I made a simple search tool that allows you to search for the amino acid profile of all foods in the USDA database: Essential Amino Acids Search. The score you see in the tables is simply the percentage protein divided by the reference value from the Institute of Medicine. For example: the reference value for lysine is 5.1. If a food has 10.2% of its protein in the form of lysine, it gets a score of 2. If it has 2.55% lysine, it gets a score of 0.5, which means that this food is deficient in lysine.

March 15, 2008