The nutrition column in the Food section March 19 did not give correct credit for a study of the costs of frozen dinners and entrees. The study was conducted by the Nutrition Education Division of the Human Nutrition Information Service of the Department of Agriculture.

Q: Are Teflon pans safe?

A: People have been concerned about the safety of Teflon since it appeared on the market 25 years ago. But happily, there is no reason to suspect it is unsafe. One worry related to the possible toxicity of fumes if Teflon decomposed at high temperatures. Empty dry pans do decompose to some degree if heated to temperatures well above the smoking point. But the toxicity of the resulting fumes was found to be less than that from overheated cooking oils.

A second fear was that Teflon, if used over extended periods, might migrate to the foods. Again, controlled studies indicated this was not a significant risk.

Teflon was discovered accidentally by a chemist at Du Pont who was working with refrigeration gases. He observed that one of the cylinders he used to store dry ice appeared to be empty and did not register pressure -- a sign that gases are present. But when he weighed it, the cylinder was as heavy as the others. Inside he found a white powdery fluorocarbon residue. That was in 1938, more than 20 years before Teflon cookware was approved by the FDA for sale in this country.

Because Teflon was highly resistant to corrosion even at high temperatures and was unaffected by acids, alkalis, fuel oils and other substances, it was pressed into a number of wartime uses where less durable materials had proved unsuccessful.

Q: What is meant by the term "megavitamin therapy"? Are there times when large doses of vitamins are appropriate?

A: Megavitamin therapy refers to the use of vitamins in amounts 10 or more times the Recommended Daily Allowance for a particular nutrient. There are medical situations in which very large doses of vitamins are indicated. Sometimes they must be prescribed for individuals who are being treated with drugs that act as antivitamins or vitamin antagonists. That is, they interfere with the body's ability to use vitamins.

Other individuals suffer from diseases where they cannot always transport vitamins across cell membranes effectively. They may take in enough vitamins, but the vitamins may not be able to get to where they are needed. People who suffer from malabsorption disorders fall into this category.

Finally, there are some people who have genetic disorders that do not allow them to use vitamins normally. In these instances, though not in all cases, very large doses of vitamins can counteract the defect.

These are all situations in which vitamin therapy is used under close medical supervision. Despite claims to the contrary, there are no good reasons for the self-prescription of large doses of vitamins.

Q: I realize that the fancy-frozen main dishes that seem to be getting so popular cost more than preparing similar dishes at home. But can you tell me just how much more?

A: Staff members from the U.S. Food and Drug Administration's nutrition education division recently compared the cost of three of the newer-style dinners and five entrees with dishes they produced to resemble the convenience foods as closely as possible. The dinners (turkey breast, beef tips and cod) each included a gravy or sauce, a rice or potato side dish, and a vegetable mixture. The five entrees, including two beef, two chicken and one shrimp, came with rice or noodles and/or a vegetable component.

Each convenience product and the ingredients for the home-prepared counterparts were priced in three Washington, D.C., area markets for three consecutive weeks during 1985. They found that the cost of a serving of frozen dinners and entrees ranged from $1.99 to $3.99, while the ingredients to make the homemade versions ranged from 62 cents to $1.29. The frozen products thus cost between three and four times as much as similar recipes prepared at home.

Because convenience foods require little effort to prepare, they have a role in many busy households. But if you are looking to trim your food budget, this is the place to begin.

In evaluating these products, which usually include fancy names, colorful packaging and a luxury image, the serving size of meat, fish or poultry in the dinners ranged between 2.8 and 3 ounces, while the meat in the entrees was considerably less, only between 1.1 and 2.5 ounces.