DEAR DR. BLONZ: What is behind the advice that saving oranges until they are sweeter may result in them losing their vitamin C? Oranges that are picked too early may not have any. Should I avoid oranges with a greenish tint, which indicates they were picked before they were ripe? -- S.B., Scottsdale, Arizona

DEAR S.B.: Some fruits, such as bananas and stone fruits, including avocados, contain enzymes that remain active after harvest; they break down complex carbohydrates into simpler, sweeter-tasting sugars. This isn't the case with oranges, however, which are harvested at the peak of ripeness. What that means is that when it is picked, an orange is about as sweet as it will ever be. It is also when its vitamin C content is the highest.

Don't look at the color as being an indication of ripeness. Oranges can often "regreen," which is a process unrelated to ripeness or taste -- the overall color is more a reflection of the temperature before harvest than of the flavors locked inside. Although oranges will keep for a few days at room temperature, the best storage method is to keep them in the refrigerator. When an orange begins to shrivel, its nutrient value has begun to decline. The juiciest fruits, incidentally, are the ones that are heaviest for their size.

DEAR DR. BLONZ: A friend told me to take guarana pills for alertness, better concentration and faster learning. But I have heard that guarana is also used as a cure for erectile dysfunction. Could you tell me if I should try this? -- D.P., Nashville, Tennessee

DEAR D.P.: Guarana comes from the seeds of a South American shrub. The claimed effects you mention -- at least the ones associated with alertness and better concentration -- stem from the fact that guarana has a high concentration of naturally occurring caffeine -- higher even than the coffee bean, the kola nut or cacao (cocoa).

There is no reliable evidence to support the use of guarana (or caffeine) for erectile dysfunction (ED). Given the financial success of pharmaceutical drugs designed for ED, it is not surprising to find herbal supplements claiming efficacy in this area, lack of evidence notwithstanding. For a review of dietary supplements touted as effective for erectile dysfunction, see b.link/54hps9e. Guarana is not even on the list.

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: There are always discussions this time of year about the annoyance of mosquitoes and how I seem to be their favorite target. A while ago, I was advised to take 100 milligrams of vitamin B1 twice a day to help repel the critters, and it seems to have worked. I received similar positive feedback at the time from others when they tried it. But I am now concerned about whether this is something I could recommend for my 10-year-old granddaughter, who is about to head to overnight camp. -- S.F., Anderson, South Carolina

DEAR S.F.: Some people seem more attractive to mosquitoes than others. To make oneself uninviting to mosquitoes would be an asset, no doubt. There are anecdotal reports that some people achieved benefits from taking vitamin B1 (thiamine); however, these are casual reports and not the result of any rigorous, objective data collection. To achieve any level of reliability, studies examining the efficacy of a mosquito repellant should compare the treatment with an inactive placebo in a blind study.

Thiamine had its moment, but objective, double-blind studies failed to affirm that it works to prevent mosquito bites with any reliability. You asked whether to recommend thiamine for your granddaughter at camp. The RDA (recommended dietary allowance) for a 10-year-old is 0.9 mg per day, so the 100 mg twice-a-day regimen you mention represents a megadose -- 222 times her recommended level. So the answer to your query is "Nope, not recommended." I'm glad you asked.

Unfortunately, no dietary supplement has proven effective at repelling mosquitoes. Mosquitoes and other biting insects are attracted to perfumes, food and brightly colored clothing. Eliminating stagnant water -- which is a mosquito breeding ground -- from around your house and yard can help. (For more information on how to choose an insect repellant, check out b.link/vtpbxnf. If possible, get a sample before purchasing a repellant, as some people find certain scents to be disagreeable.)

DEAR DR. BLONZ: I have read articles that say combining calcium with vitamin C at the same meal is not good, as one counteracts the other. This notion creates a problem for me at breakfast. I like to enjoy my fresh-squeezed orange juice with a serving of yogurt or cottage cheese. If these nutrients are incompatible, why are some orange juices fortified with calcium? I appreciate your answer. -- D.W., Scottsdale, Arizona

DEAR D.W.: No incompatibility here; these nutrients go well together. If anything, the vitamin C helps keep the calcium in solution and facilitate its absorption.

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: Is it reasonable that pharmacists filling my prescriptions are trained in drug, nutrient and supplement interactions and that I can expect them to alert me if there are potential problems? What if I get my prescriptions filled at a large chain drugstore? -- D.N., Walnut Creek, California

DEAR D.N.: Clinical knowledge in such interactions is expanding as researchers continue to collect pertinent information. All parties involved in your health care should be aware of how a prescribed drug works in your body.

A pharmacist's approach is shaped by the school where they received their training, as well as the pharmacy's policies. In some cases, a patient will receive their prescription along with multiple pages of accompanying information that can be daunting to read through. At some pharmacies, you may not get a consultation with a trained pharmacist unless you request it.

I work on faculty at the University of California, San Francisco (UCSF), and its Doctor of Pharmacy degree students learn how medications work, along with possible interactions with other drugs, supplements and health conditions.

Health professionals must ask the right questions, and patients and their caregivers must provide complete information in response, so that the best care can be offered. Pharmacists may need to contact the prescribing physician for clarification when conflicting medications are prescribed. (Read more about how common medications interact with essential vitamins and minerals at b.link/tvvyrxfl. Online drug interaction checkers are available; inquire with your pharmacy chain, or check out b.link/quwac8.)

We must act as our own health advocates, which includes making a complete list of concerns to discuss with those entrusted with our health care.

DEAR DR. BLONZ: I read about a company that sells water-soluble minerals that they promote as angstrom-sized, elemental minerals that are 99% pure in water. If true, wouldn't this be a far better way to get our minerals? I hope you can help shed some light on this subject for me. -- W.P., Anderson, South Carolina

DEAR W.P.: The angstrom is a tiny unit of measure (1 inch = 254 million angstroms) named after a Swedish physicist that is often used for measuring wavelengths of light. The promotion of the product you mention smacks of sales-talk puffery with little clinical evidence affirming that this form has been shown to be significantly better than others on the market.

Our bodies absorb minerals from foods -- people have been doing that for centuries! A healthful diet is primary, but a wide variety of mineral supplements can help fill any nutritional gaps. Don't be swayed by products promoted with such confusing language. Demand evidence from independent researchers -- ones who don't stand to profit from the sale -- before you buy.

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.