Vegetarian friends have mentioned complete and incomplete proteins. What do those terms mean? If I have lentil soup, for instance, do I need bread to go with it to make a complete protein? —F.F., Missoula, Montana
Ready for a short course in Science 101? Proteins are composed of amino acids, of which 12 are manufactured by the body. Another nine, called essential amino acids, must be obtained from food.
Most animal products, such as meat and dairy products, contain all of the essential amino acids; thus they are designated as containing complete proteins. So if you include some animal products (meat, milk, cheese, etc.) in your diet regularly, you should be getting those essential amino acids easily.
Most proteins that come from vegetables contain some, but not all, essential amino acids. In that case, an incomplete protein must combine with another one that contains the missing amino acid (or two or more) to complete the protein.
For instance, if you are a vegetarian, two incomplete proteins that combine nicely to make a complete protein are red beans and rice or whole wheat and peanut butter.
Previously, nutrition experts believed that proteins needed to complement one another within a single meal. However, it is now known that the body stores a short-term supply of the essential amino acids. So as long as you eat a variety of different foods, they need not be taken in at one meal or even in the same day.
As long as all essential amino acids are a part of your diet regularly, it does not matter if the proteins are complete or incomplete. The key is to eat a variety of good foods from all food groups.