DEAR DR. BLONZ: We often use commercial fat-free salad dressings as an alternative to a simple olive oil and vinegar dressing. They taste fine, but my daughter asked about all the chemicals listed on the ingredients statement on the bottle. I was hoping you would tell me what xanthan gum and cellulose gel are and whether they are OK to consume regularly. -- F.F., Hayward, California

DEAR F.F.: To be successful in the marketplace, a nonfat salad dressing has to be able to perform just like oil and vinegar and bottled, fat-based salad dressings; namely, it must coat the leaves of the lettuce, provide a pleasing mouthfeel and provide a vehicle for flavors.

Xanthan gum and cellulose gel are used precisely for these reasons, and both of these components appear to be quite safe for regular use at the level found in foods.

Xanthan gum was developed by the USDA during a project to find new uses for surplus corn. It is made from a corn syrup specially fermented from cornstarch. Xanthan gum helps to thicken the texture of food, and has proven to be quite versatile. Aside from salad dressings, it is found in puddings, sauces, baked goods and desserts. It is also found in toothpaste. Because it is made from corn, xanthan gum should be avoided by those who are allergic to corn. Aside from that, there do not appear to be problems generally associated with its use.

Cellulose gum is made from the fibrous cell walls of plants; it also functions as a thickener and is found in dressings, ice cream and puddings.

DEAR DR. BLONZ: This summer's heat is softening fruits quickly, but I hesitate to refrigerate fruits such as tomatoes, peaches and melons for fear of losing flavor. Is this a misconception, and should I go ahead and refrigerate them? -- F.M., Linden, New Jersey

DEAR F.M.: Refrigeration can slow the growth of many microorganisms, but it can also affect the quality of some foods. Tomatoes, for example, should be stored at room temperature, preferably away from direct sunlight in an airy location with moderate humidity; under these conditions they should last about five or six days. Storing a tomato in the refrigerator may delay it from going moldy, but some of its flavor and texture will be destroyed. This is particularly an issue with flavorful homegrown tomatoes and tomatoes purchased at a farmer's market. The effect is less clear with store-bought tomatoes, as they might not have as much flavor to begin with as homegrown. Another variable is sellers storing their tomatoes in the cooler.

Peaches and melons can be stored in the refrigerator, but you should wait until they are at the level of ripeness you like. (For more information on the best way to store fruits and vegetables, see b.link/n74hq9t.)

Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: I am inquiring about placebos in research and how they work. I am also interested in whether the placebo effect can affect food sensitivities. -- D.B., Tulsa

DEAR D.B.: We have only begun to appreciate the powers of the placebo -- a treatment with no therapeutic effect. Despite its absence of efficacy, the placebo plays a crucial role in health and research. An instructive example is the "placebo effect," which helps us account for results likely to occur simply because study subjects believe they will.

In research, study volunteers are split into two matched groups, and all participants receive the same instructions; for example, they are told that they will be testing a product to see if it makes them feel a certain way. Half the participants are given the active ingredient, while the other half are given a placebo. When the subjects don't know what they are receiving but the researcher does, it's called a "single-blind" study. A "double-blind" study is when both the researchers and the study participants are unaware of which group receives the active ingredient. (Double-blind, placebo-controlled research is considered the gold standard.)

It's in our best interest to know that what we take is effective, and not just the result of the placebo effect. For hucksters, the placebo effect is a blessing -- sales promotions and peer pressure among users can generate positive attention to an ineffective product. Underlying all of this is the fact that our bodies are wonderful self-healing organisms.

Back to your question. If you have a food sensitivity, allergy or intolerance, you would expect to experience a reaction when consuming the offending substance, whether you are aware of its presence or not. But what if you only think you are sensitive, or only think the food has the bad stuff, when neither of these is the case?

I recall a discussion in graduate school about the case of an individual who had a digestive problem who stopped eating a certain food because they read it might be the cause of their symptoms. They made significant adjustments to their diet, and the reaction went away, reappearing any time they would give it another try. They later learned that a different food they routinely ate -- and did not react to -- included the same substance. After testing, it was learned that they were not sensitive to the suspect food; more tests revealed that a separate, treatable issue was involved.

Nobody likes to think they can be fooled, but it remains a testament to the powers of the mind that we may not always appreciate. It also reinforces the need to find out what is actually occurring. Scientists testing for food allergies can use double-blind and repeat-testing methods to ensure accurate diagnoses. The message here is that it is in our best interest to verify that food reactions are genuine and not the result of expectations. (For more on placebos, see b.link/hk8mj6c.)

Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: Growing up, I was taught that too much white bread caused constipation. As an adult, I learned that wheat bread is good for us because it includes the health benefits of fiber. Does white bread actually cause constipation, and is wheat bread the better choice? -- K.J., New Orleans

DEAR K.J.: "Wheat" as a descriptor can be misleading, because it can refer to white, whole grain or a blend of the two. About 90% of the breads (and cereals) sold in the U.S. are made with refined, not whole-grain, wheat flour that has been enriched with niacin, riboflavin, thiamin and iron up to the levels found in whole-grain wheat flour. Nutrients naturally present in whole-grain flour include magnesium, zinc, vitamin B6, folate and fiber. These nutrients are not present in flour that's not labeled as "whole wheat" unless purposely added.

"Enriched" flour includes nutrients that are lost during processing and returned to their original levels. "Fortification" refers to adding nutrients not initially present in the food; one classic example is milk being fortified with vitamin D.

Foods considered to cause constipation slow their passage through the intestines. While it is made from little more than refined flour, leavening and water, white bread does not contribute to the unabsorbed bulk traveling through the intestines -- it does not slow things down.

When data about the health benefits of dietary fiber began to emerge, some bread manufacturers began to add dietary fiber to their white bread. It is best to check the nutrition label and the ingredient statement to see if fiber is present in your brand of bread. If, in fact, dietary fiber was added to that product, it would help keep things moving through the intestines.

Be aware that bread color can be deceiving. A not-whole-grain bread that looks like a whole-grain bread could be nothing more than enriched-flour white bread with added caramel (brown) coloring -- which would be shown on the ingredient list.

Those interested in a fiber-rich whole-grain experience should look for bread with whole wheat as the first ingredient. Those whole-grain products will contain all of the wheat's nutrients -- fiber included -- and contribute to your desired moving experience. (Read more about whole grains at b.link/fh2j8fw.)

Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.