Donna Dyke, age 14, of Kings Mountain. North Carolina, for her question:

Which foods have proteins?

A proper diet is a balanced assortment of fats, carbohydrates and proteins, plus various vitamins and minerals. Proteins are our most important foods because they can take over some of the work done by fats and carbohydrates. Besides, every living cell must have its quota of different proteins to stay healthy. For this reason, it is wise to know all we can about proteins.

The list of protein rich foods includes meat and dairy products, eggs and fish. This sounds simple enough, but there is nothing simple about the way in which the body uses the proteins available in these foods. For example, it would seem logical for the digestive system to separate the proteins in, say, a piece of cheese, and put them to work right away. But this does not happen.

Actually, proteins are large molecules built up from small units called amino acids. And each cell insists on building the particular proteins it happens to need from these basic ingredients. For this rea¬son, when protein rich foods are eaten, the digestive system breaks the big protein molecules apart. From this process, supplies of assorted amino acids are circulated around to the living cells. The DNA in the cell nucleus directs the assembling of the amino acids to rebuild the amino acids into the required proteins. These may or may not reassemble the proteins you ate for dinner.

Amino acids are small molecule packages made from atoms of carbon and hydrogen, oxygen and nitrogen. Most of them also contain atoms of sulfur. So far, more than 60 different amino acids have been found in living cells of the plant and animal kingdoms. But only about 20 of them are used by living cells to build proteins.

A protein molecule is a polymer, a long chain of basic units strung together. Its shape is either a coil or a straight string. Here and there, long chains are linked to form a web of tough protein material.

The smallest protein molecules contain perhaps 40 amino acid units. Large ones contain many hundreds, including a number of different amino acids.

Some of the amino acids we need to model our needed proteins can be manufactured in the liver and in other parts of the body. These are called the nonessential amino acids because we don’t have to take them from our foods. The so called essential amino acids are the ones that the human body cannot manufacture for itself. These we must get from either plants or animals.

Plant foods contain a large variety of proteins—but their proteins tend to lack one or more of the essential amino acids. Unless all of these are included, the human body cannot use them to build its own pro¬teins. For this reason, when we serve soybeans and other protein rich plant food, it is wise to include a cheesy dairy product, a little meat or some other animal food.