Q. In answering a question about dietary sources of calcium, you listed several vegetables that contained considerable amounts. Soon afterwards I read another list that mentioned bok choy as a good source. I am unfamiliar with it and would be glad if you would describe it.

A. Bok choy, or Chinese cabbage, is one of the milder members of the cabbage family. It has long, smooth snow-white stems with large, crinkly dark-green leaves. Sometimes the heart, which has yellow florets and is tender, is sold separately.

Bok choy cooks quickly. It can be stir-fried or sliced and cooked in soup. Nutritionally it is relatively low in calories. And a cup would provide 160 milligrams (mg.) of calcium, or about as much as a half-cup of milk. It would also have small amounts of a number of B vitamins and appreciable amounts of vitamin C. The leaves are an excellent source of beta carotene, which the body converts to vitamin A.

Q. I read that eggs can inhibit iron absorbtion. Is this true?

A. Yes. More than 20 years ago a group of British researchers were studying the effectiveness of various iron compounds for use in enriching flour. They stumbled on the fact that when eggs were omitted from the test breakfast, absorption increased. The negative effect was greater with some iron supplements than with others.

Eggs are not the only foods which can inhibit iron absorption. Phytic acid, a compound found in whole grains, can tie up iron and make it unavailable to the body.

Large amounts of cellulose affect iron absorption negatively. Tannins in tea and polyphenol in coffee are also capable of tying iron up and making it unavailable. For this reason it is sometimes suggested that these beverages be consumed well apart from meals. However, those who look forward to a cup of hot coffee or tea at the end of a meal find this impractical.

Alternatively, for many people it seems easier to focus menu planning as often as possible around those foods (including meat, fish and poultry) that increase the absorption of so-called "non-heme" iron in vegetable foods and/or a good source of vitamin C. To give you an idea of just how effective these measures are in increasing iron absorption: if the meal contains three ounces of meat and 75 milligrams of vitamin C (the amount in six ounces of orange juice or three-fourths of a cup of broccoli), about 8 percent of the iron from vegetable sources will be absorbed.

In a meal containing neither meat, fish, poultry nor any good source of vitamin C, absorption will be less than half of that.

Q. Is it true that sulfites can appear in commercially processed, prepared mixed dishes without being identified on the label?

A. The use of sulfiting agents in meat products is not allowed. However, it is possible to use sulfur-containing additives in preparing fruits or vegetables which are then used in mixed dishes with meat or poultry.

Example: the potatoes in canned beef stew. Until now it has not been specified that if these additives are used they must be declared on the ingredients list. Because it has been discovered that suflites pose a risk to some individuals, the United States Department of Agriculture plans to amend the regulation for meat and poultry dishes to specify that when these additives are used in the prior processing of fruits and vegetables they must appear on the label.

Until the regulation is made final, the Standards and Labeling Division of the USDA's Food Safety and Inspection Service has prepared a policy statement. It specifies that any sulfites used in preparing USDA-inspected meat and poultry products must be declared on the label.