My pediatrician recommends that I give my 2-year-old an iron supplement because she is slightly anemic. I told a friend about it, and she said I should now be prepared to cope with a constipation problem. I trust my pediatrician, but on the other hand, I don't want to swap one problem for another. Is my friend correct?

ANSWER - However well-meaning, we believe your friend has alarmed you unnecessarily. The evidence agrees. In a study reported several years ago, one group of 1-year-old children was given drops of an iron-sulfate preparation and a second group a "dummy" supplement. Constipation was not a common problem in either group, but it actually occurred less frequently in the group given the iron. Also, there was no difference in the prevalence of various other symptoms that might have been linked to use of the supplement.

You should certainly check with the doctor by phone if you have concerns about the advice he or she has provided. This will do you more good than picking up inaccurate tips from well-intentioned friends.

QUESTION - How long can canned food be kept on the shelf?

ANSWER - According to the Canned Food Information Council, you can keep canned goods for two years after you buy them. If stored under optimal conditions, a temperature of around 70 degrees Fahrenheit, they should keep for twice that long. Higher temperatures speed deterioration.

If you're like most people, you replace some canned goods regularly and use others only on special occasions, so that they sit on the shelf a long time. For the first group, it's a good idea to rotate the stock as you replenish supplies, storing older cans in front. For those you use less frequently, take time to date them with a marker. As you tidy your cabinets, it should help to draw your attention to items that are nearing the date when they should be used up.

While we're on the subject of canned foods, it will interest you to know that the industry has been gradually reducing the number of products with lead-soldered seams. At present, lead-soldered seams are found mainly in cans of pet foods, vegetable oils and fish. The National Food Processors Association has announced that by July 1991 no more lead-soldered cans will be manufactured.

QUESTION - What's the difference between regular mayonnaise and the salad dressing that has a mayonnaise-type texture?

ANSWER - They are prepared quite differently. Mayonnaise is an emulsion containing oil, egg yolk or whole egg, one or more acidifiers such as vinegar or lemon juice, and seasonings. Salad dressing contains considerably less oil and depends in part for its texture on a starchy paste. The result is that as much as 80 percent or more of the weight of mayonnaise is oil, while salad dressing oil usually accounts for no more than 50 percent of the weight.

Nutritionally, these differences translate to a broad caloric gap between the two. A tablespoon of regular mayonnaise, which has 11 grams of fat, provides about 100 calories. The same amount of salad dressing has about half the amount of fat and 60 calories. And remember we're talking about a level tablespoon. People tend to consume considerably more than that, so the total caloric difference mounts up. One calorie-cutting option works well with either regular mayonnaise or salad dressing: Mix it with equal parts of low fat yogurt.

QUESTION - What is "modified food starch," which appears on so many ingredient lists?

ANSWER - There are many types of modified food starches derived from many different foods, including potatoes, tapioca, wheat, rice and corn. The source of the starch influences the way it reacts in food processing. Starches can affect clarity, gelling properties, and the ability to tolerate acid. They also can be used to adjust certain taste properties, to prevent ingredients from separating, and to keep dry ingredients from caking. Despite starch's many uses, estimates of total consumption of modified food starch made several years ago indicate that a person weighing 150 pounds consumes only a quarter teaspoon a day.