We recently wrote about the hazards of this year's "in" fad diet - the [WORD ILLEGIBLE] - called protein-sparing, or modified, fast diet. We have since been deloged with letters, especially from teenagers, asking about the various protein and vitamin-mineral supplements that are now sold widely in drug and health-food stores. An untold number of people are apparently using these supplements for a do-it-yourself modified fast without bothering to consult a physician.

As we have often said, if you want to lose more than a couple of pounds, you should be sure to see a doctor before going on any diet. This warning is even more important for anyone tempted to fast.

Fasting is simply not suitable for the great majority of individuals, teenagers in particular. Moreover, it should be undertaken ONLY with the careful supervision of a physician.

Why is the fasting diet dangerous? For one thing, all fasting diets involve a loss of muscle tissue as well as fat. While this loss may not be as severe when protein supplements are taken, there is still a significant danger, especially during adolescence, when the body is undergoing a rapid growth spurt and should be adding lean tissue. Also, there is evidence that afasting may produce a loss of bone, and young people again run a serious risk of stunting their normal growth by trying this do-it-yourself fasting diet.

There are other important reasons to avoid this diet. For instance, overeating often represents an emotional shield, and fasting may lay bare psychological problems and produce sevree depression. Fasting also puts an extra strain on the liver and kidnesy, and can lead to gouty arthritis and hepatitis. Women on fasting diets may develop menstrual irregularities and even experience extensive loss of hair.

In extreme, selected cases of obesity, a physician may put a person, even an older adolescent, on a fasting diet. But when they do, many doctors hospitalize the patient for periods ranging from a week to a month or more. Such patients require close medical monitoring in order to spot and correct any medical difficulties before they become full-blown problems. With hospital costs ranging from $100 to $200 a day, it becomes a very expensive way to lose weight.

Even when done under professional supervision, the modified fasting diet presents a problem that is common to all weight-loss schemes - how to maintain your proper weight once you go off the diet. The dieter must learn new eating habits, which the doctor and nutritionist undertake to explain before the patient is discharged from the hospital.

At home, the task of following a modified fast can be very difficult unless the family also has learned to modify the home environment and eating patterns. (If you have ever dieted, you know how tempting it is to watch someone else eat a favorite dish that is taboo for you.) Therefore, while a patient is hospitalized, his family also should be educated in dieting so they can help the patient continue treatment at home. Every few weeks, the patient must check back with the doctor to make sure that growth and general health are maintained.

We cannot over-emphasize the warning that the fasting diet is only suitable for a very few people and should be undertaken only after consulting a doctor.

What is particularly troubling to us is the fact that many people who want to lose 5 or 10 pounds are turning to do-it-yourself fasting. Our anser is don't! The best way for most people to lose weight is to count calories and increase physical activity. By reducing your consumption of food, and at the same time increasing your exercise, you burn up stored calories (fat) without endangering your health in the process.