Q. While listening to a TV talk show that took viewers' calls, I heard a mother call in and ask if it was safe to let her two daughters, ages 10 and 12, become vegetarians. The answer given was that it was fine, especially if the diet contained dairy products and eggs. However, the person on the show was a health reporter, not a nutritionist. Was the advice sound?

A. Indeed it was, as far as it went. A lacto-ovo vegetarian diet can provide enough of the essential nutrients young people need for growth and development. Contrary to popular belief, however, the nutrient that deserves special attention is not protein. Almost certainly, it will be present in sufficient amounts -- not only from dairy products and eggs, but also from the vegetables and grains the youngsters eat to get the calories they need.

Iron is a different story. In vegetarian diets, the amount of iron is more limited. Iron from vegetable sources also is absorbed less efficiently that iron from animal flesh. For these reasons, it's important to pay particular attention both to getting enough iron into the diet and to making sure that it's well absorbed.

Iron-rich foods include enriched and fortified breads and cereals, dark-green leafy vegetables and dried beans. The key to enhancing absorption from these foods is vitamin C. That's found in generous amounts in many fruits and vegetables, among them citrus fruits, tomatoes, cantaloupe, honeydew, broccoli, green pepper and cabbage. Serving a C-source along with an iron source makes good nutritional sense.

Q. I bought a tube of concentrated tomato paste in an Italian grocery. The packaging had no instructions about how much to use in place of regular tomato paste in a can, and nobody in the store could tell me. Do you know?

A. Whether it comes in a tube or a can, we're talking about the same product. Either way, that means about 15 calories per tablespoon, virtually all from carbohydrate. Use the same amount as you would if the paste came in a can. The advantage of the tube is when the recipe calls for a tablespoon or two. Of course, it can be more expensive to buy it in this form. You could open a can of paste when you need a small amount, and divide the rest into small portions to store in the freezer until needed.

In this country, tubes of tomato paste and other seasoning are only beginning to become popular. In Europe, this form of packaging has been in use for a long time.

Are there any differences in the vitamin-C content of frozen orange juice prepared from concentrate versus the juice available in the dairy case?

The differences are insignificant. Researchers from the Florida Department of Citrus and the Citrus Research and Education Center recently analyzed numerous samples of frozen concentrated orange juice, pasteurized orange juice (sold mostly in the dairy case as chilled juice), and orange juice from concentrate (sold either in the dairy case or as canned or bottled juice).

In six ounces of juice, the average amount of vitamin C ranged from 74 to 76 milligrams (mg.), considerably more than the Recommended Daily Allowance of 60 mg. It's true that vitamin C losses occur as the juice sits in the refrigerator. To minimize losses, keep the juice tightly covered. Refrigerate it promptly after use, and buy amounts that you can use up within a few days.

In this study, incidentally, juice samples were collected monthly over a period of two years. And there were significant variations in vitamin C content, depending on the type of oranges being harvested. Early season Hamlin oranges, for example, are richer in ascorbic acid (vitamin C) than Valencias.

While this fact is interesting, it's of no practical concern to consumers: The average amount of vitamin C in six ounces of orange juice exceeds what we need. And we also get vitamin C from many other fruits and vegetables.

Q. I love crawfish, but I know nothing about their nutritional value. I'm on a low-fat diet so I'm afraid to eat them until you give me the green light. I'd especially like to know their fat and caloric content.

A. When it comes to concerns about fat, crawfish are an excellent choice. Three-and-a-half ounces of crawfish (or crayfish; the spellings are used interchangeably) provide about one gram of fat and only about 75 calories. Information about their cholesterol content is hard to come by in standard nutrition tables, but it's reasonable to assume that they're similar to their saltwater relative, the lobster, and therefore have about 100 milligrams per 3 1/2 ounces -- less than half the amount in a single egg.

In this country we tend to associate crawfish with the South, particularly Louisiana, where they are harvested from farms. But it's also true that crawfish were probably the only crustacean widely consumed in Europe during the Middle Ages, most likely because they were plentiful and easy to collect. They achieved their height of popularity in 18th-century France. In dishes with sauce Nantua (named for a French city), you can expect a garnish of crawfish.

Q. I recently read the ingredient list on a container of yogurt and was surprised to see "beet juice" listed as a coloring agent. What other fruit and vegetable juices are being used these days to color foods red?

A. The food industry uses several, including cranberries and grapes. While these natural colors may produce the desired hues, they have drawbacks. They tend to be unstable in heat and light and are affected by the relative acidity of the food in which they're used. And adding enough to obtain the desired color can have a bad effect on flavor or scent.

One recent report suggests that a new colorant derived from anthocyanins, the major pigment in red cabbage, may expand the repertoire of red colors available to the food industry. Laboratory tests show it to be much less vulnerable to heat and sunlight than either beet or grape juice. And because of its potency, the amount needed to produce a desired depth of color is small enough to affect neither the flavor nor odor of foods. To date, the cabbage pigment has been used alone or in combination with other colors to create strawberry, raspberry and blueberry tones.