DEAR DR. BLONZ: I received information from my doctor's office that read: "Don't take a multivitamin and mineral supplement containing iron. Men can't eliminate iron except through blood loss. High levels of iron in the body may promote cardiovascular disease/cancer." This information goes against the recommendation to take 8 mg of iron daily in my multivitamin. Please explain. -- R.A.

DEAR R.A.: Some context may help here. The human body has a daily requirement for iron, which it needs to make hemoglobin -- the oxygen-carrying compound in blood -- and myoglobin, a related compound that handles oxygen in the muscles. Both compounds are essential to producing energy for our cells. If the body has insufficient iron, the decreased hemoglobin and myoglobin levels can't produce needed energy.

The claim that men cannot eliminate iron is a bit confusing. The non-menstruating adult human body eliminates about 1 milligram of iron per day. Menstruating women lose significantly more. The daily iron requirement for premenopausal women (ages 18 to 50) is 18 mg. For adult men and postmenopausal women, the requirement is 8 mg a day. The dietary requirement is higher than the 1 milligram lost daily because we absorb only about 5% to 10% of the iron in our diet. Meat and poultry contain heme iron, which is the most efficiently absorbed form. (Read more on iron at b.link/z5msxnp.)

If you are an otherwise healthy individual with a healthful diet and lifestyle, you needn't fear taking a supplement that provides up to the recommended daily amount of iron, especially if the supplement is your primary source. There is no advantage, however -- and possible negative reaction -- in consuming excess iron. Discussing your multivitamin with a health professional familiar with your health issues is your best course of action.

Iron's darker side comes from epidemiological studies that found elevated iron stores associated with cardiovascular abnormalities and cancer. Consider how an iron nail will rust if left exposed to the elements, which is the result of iron's reaction with oxygen. Our bodies don't get rusty, per se, but it is thought that iron in our body grabs on to oxygen and then forces it on nearby substances, such as fats or cholesterol. (Oxidized fats and cholesterol are considered the primary villains of disease.) Antioxidants, including vitamins C, E and A, can help prevent errant oxidation and risk of disease.

Another element of iron's "Jekyll and Hyde" personality relates to individuals born with a genetic iron storage disease called hemochromatosis. Also referred to as "iron overload disorder," hemochromatosis is the inability to rid excess iron, which continues to build up, wreaking havoc on systems throughout the body.

Many people are unaware that they have this condition until symptoms emerge. Treatment often involves periodic removals of some of the patient's blood. Hemochromatosis is diagnosed via blood tests and genetic testing. (For more on hemochromatosis, see b.link/v9ndrk2.)

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: 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.