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.

DEAR DR. BLONZ: I'm inquiring about the use of activated charcoal to help get toxins out of the body: how much to take and whether it works as a blood purifier. -- L.N., Anderson, North Carolina

DEAR L.N.: Activated charcoal, a purified form of charcoal ground into a fine powder, needs some explanation. Activated charcoal is not absorbed into the body; instead, it acts locally with whatever substance it comes in direct contact with. Its fine powder consistency gives it a large surface area to do its job, and its pure carbon composition attracts and holds onto substances using a process known as adsorbence (not to be confused with absorbance).

Adsorbency is why activated charcoal is used in water filters: It grabs and holds heavy metals, unwanted minerals and odors. Activated charcoal is often used in first aid after accidental poisonings, as it holds on to the poison and prevents it from getting into the blood. (According to a position statement of the American Academy of Clinical Toxicology, activated charcoal is more beneficial if administered within one hour of poison ingestion.)

It is unclear how activated charcoal could work as a blood purifier, because it isn't absorbed into the body. It can keep things out, but there is no evidence it can grab on to toxins already in the body. So unless your doctor instructs you otherwise, there is no reason to consume activated charcoal regularly. It can reduce the absorption of nutrients, but even more concerning, activated charcoal can adsorb medications, rendering them unavailable for absorption. (For more on activated charcoal, see b.link/r65ey55.)

DEAR DR. BLONZ: I drink three to four liters of sparkling (carbonated) water daily that I make from filtered tap water in a home device. I don't like regular water, so I figured this would give me my daily requirements. I often add fresh lemon or lime juice, but no sweetener. I don't drink any sodas.

I was on a plane recently, and after ordering soda water, the man sitting next to me stated that drinking carbonated water affects the nutrients the body absorbs because of the CO2 it contains, and now I'm concerned. I am health-conscious, eat well and work out regularly. Are even unsweetened sparkling beverages bad? -- A.H., Pomona, California

DEAR A.H.: Outside of causing a burp now and then, there is no evidence that the carbon dioxide (or CO2) in carbonated water -- also known as sparkling water or club soda -- causes any nutrient deficits. Albeit a bit more costly, drinking a carbonated mineral or spring water is a perfectly fine way to get your daily water requirements.

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.