Q: Would you please tell me what the difference is between complete and complementary proteins.

-- Angie

Proteins are the basic building blocks for all living cells.  They are made up from chains of smaller compounds called amino  acids.  There are 20 amino acids that are important to human health.  Of these, your body is able to manufacture most itself from carbohydrates, fats and other amino acids.  That leaves nine amino acids that you need to get from your diet, called the Essential Amino Acids.

Meats and eggs contain all nine of the essential amino acids, and in about the right combinations for use in the human body, and so are called Complete Proteins.  Other protein sources, such as beans, grains and nuts, do not contain all nine essential amino acids, or contain them in quantities that are not in the same proportions as needed, so they are called Incomplete Proteins.

In her book Diet for a Small Planet, originally published in 1971, Frances Moore Lappé popularized the idea of Complementary Proteins. The concept was that, because the body doesn't store amino acids like it does fat, for example, non-meat eaters would need to take care to get all of their essential amino acids together in the same meal in order to have a healthy diet.  Lappé, as have others since, suggested that traditional diets showed the way in which diets with complementary proteins might be built.  Mixing rice or corn with beans, or pita with hummus were given as examples of naturally evolved diets where this was the case.

Think of it as being sort of like building a house.  It doesn't do you much good to get a shipment of bricks if you don't also get a shipment of mortar cement, a shipment of sand, a shipment of reinforcing bar, and so on.  Not all of them will be the same weight or volume, but in the right proportions, and put together at the same time, they can be used to build the house.

Nowadays, however, the thinking is that this approach is overly strict.  While there is no evidence that your body can store amino acids, there is always a certain amount floating around in the body at any given time.  For most people with a reasonably balanced and varied diet, complementarity works itself out, provided that the essential amino acids all show up in the diet and are eaten within a day or so of each other.

Complementary proteins are usually only an issue for people on highly restrictive diets or with special health needs.  If you are thinking of going on a diet, always check with your health care professional first.