Tonya Woods, age 12, of Peoria, Illinois, for her question:

What exactly is cholesterol?

Modern young persons are very interested in diet. This is fine because obviously what we eat has a lot to do with whether we feel and look healthy. However, the assimilation of foods is a highly complex chemical business. Sometimes we eat too much of one food while cutting out another which our bodies really need. Nowadays, there is quite a bit of confusion about certain fatty foods called cholesterols.

First, let's remember that the scientific study of diet is fairly new and involves extremely complex chemical activities. Even the simplest food contains a wide variety of chemical nutrients. The body has a multitude of purposes for each of these nutrients and a multitude of varied processes to put these nutrients to work.

So let's not expect to learn the last word about diet in the near future. In the meantime, let's not go for the latest diet fad which may go out of style when the experts learn more about it. Something like this has occurred in the case of cholesterol.

Cholesterol is classed as a steroid biochemical, closely related to a large family of fatty substances. Some years ago, medical researchers found large deposits of cholesterol in the blood vessels of patients with heart troubles and related diseases. When this was made known, the conclusion was reached by many that cholesterol is some sort of poison that can do us all in.

Many people cut out saturated oils and tried to avoid all foods containing cholesterol. Later, medical research revealed that though large quantities may be unhealthy, small amounts of cholesterol are necessary. We get some in certain foods and the body makes some for itself.

We now know that cholesterol is needed for a variety of vital functions and processes. The highest concentrations are in the fatty sheath material of the spinal cord, the nervous system and the brain. Apparently cholesterol pays a role in the body's fabulous system of communications. And this is not all.

The presence of cholesterol in the skin makes it possible for the body to process vitamin D from sunshine. Cholesterol also is used to manufacture bile acids, needed in the digestive processes. And this busy biochemical also is used in the manufacture of certain hormones that govern sex and growth.

We now know that small amounts of cholesterol are needed, but let's not gorge ourselves on cholesterol rich foods. Enough is enough and too much of anything tends to be disastrous. Most of us already know that foods high in cholesterol include butter and eggs, fatty meats and certain oils. But the secret of a healthy diet is variety a menu that includes a lot of smallish helpings of many different foods. An overdose of anything, even water, can be fatal.