DEAR DR. BLONZ: My 25-year-old daughter eats about 4 to 5 ounces of canned tuna packed in water every day for lunch during the week. Should there be any concern about the mercury in the tuna? -- C.M., Los Angeles

DEAR C.M.: I also enjoy eating tuna. It offers great protein, and is a source of essential fatty acids (EPA and DHA). But doing it every weekday is definitely pushing it. Your tuna-holic daughter should probably be expanding her menu for lunchtime sustenance, and some additional information will help with this realization.

The Environmental Protection Agency posts a list of resources at b.link/fish22. Next, the Food and Drug Administration has a page on mercury levels in commercial fish and shellfish at tinyurl.com/y5s3prt4.

More articles can be found online, from various sources, on other tuna-related topics: the environmental impact of overfishing, for example, and the effects of mercury on the nervous system. Knowledge is power, so you should consider all this information. Even if your daughter switches to a lower-mercury fish, keep in mind that a “lower level” of mercury is still not “no” mercury. So the advice to rotate foods still makes sense.

DEAR DR. BLONZ: There are many patches that one can buy to place on the bottom of their feet to cleanse the body. Please advise me if these can really work to help detoxify the body. Various ones say they remove heavy metals, improve circulation, combat fatigue and stress, jump-start the metabolism and promote a better night’s sleep. There is even mention of some sort of FDA approval. I bought one, and according to the literature, you place one on each foot every other night for 30 nights, then use about once every three weeks. -- S.F., Chicago

DEAR S.F.: What, no cure for cancer, obesity, high blood pressure and arthritis? Based on the lack of objective evidence supporting such claims, these products are unadulterated garbage. The idea that those claims have any approval from the FDA is also nonsense. Check out the discussion about the “The Detox Food Pad Scam” at b.link/foot28.

DEAR DR. BLONZ: What is it in asparagus that makes urine smell? It is now in season, and both my husband and I enjoy it, but I always notice a telltale odor the next time I visit the bathroom. I notice it only after eating asparagus. My husband doesn’t have the same problem, and we both eat the same asparagus. -- G.A., Augusta, Georgia

DEAR G.A.: Asparagus, garlic and onions are members of the lily family. These vegetables all have sulfur-containing compounds that can give rise to distinctive odors. With asparagus, the peculiar odor isn’t noticeable in the raw vegetable. Rather, it comes from metabolic byproducts that are eliminated through urine after the asparagus is eaten.

While these harmless, yet odorous, compounds are produced in everyone, the ability to perceive the odor is determined by one’s genes.

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: When my friends, also parents, discuss foods, the topic of simple carbs such as high-fructose corn syrup always seems to evoke universal scorn. I recall your concerns are more moderate but was hoping for some perspectives that I can share. -- F.P., Charlotte, North Carolina

DEAR F.P.: There are definite concerns related to excess consumption, but this tends to apply to fats, protein, vitamins and minerals, as well as the simple carbohydrates. The issue is when there’s an “excess.” The studies that examine these issues look at large populations, breaking them into levels of consumption from low to high. If the data show the group eating the most (of whatever is being studied) experiences significantly more of the health problem than the group eating the least, that component tends to be branded -- at least in the media and popular culture -- as causing the problem. It must be realized that this often is in population studies not designed to tell you what’s causing what (for more on this see b.link/studies77). Let’s now discuss high-fructose corn syrup (HFCS).

Fructose is a simple carbohydrate that’s about 1.4 times as sweet as glucose. Table sugar, also called sucrose, is a 1:1 mixture of glucose and fructose bound together. Honey has the same 1:1 ratio, but there, the glucose and fructose are separate. Honey tastes sweeter than sucrose because you can get the full benefit of the separate fructose's extra sweetness.

HFCS begins as starch, which is a long chain of glucose molecules bound together. Starch doesn’t have a sweet taste because the glucose is bound together. (This also explains why a cracker or piece of bread -- also mainly glucose -- is not sweet.)

In the making of HFCS, an enzyme gets mixed with the cornstarch that can break it into its glucose pieces. At this point, you have corn syrup -- which is sweet. Another enzyme then converts a portion of the glucose into fructose. The percentage will depend on the intended use. The typical HFCS is about 55% fructose (45% glucose), which makes it pretty close to honey in terms of sweetness.

Is HFCS innately unhealthful? One study in the respected American Journal of Clinical Nutrition looked at whether HFCS might not satisfy like other sweeteners, which could then lead to excess consumption (and an increased risk of obesity), but found no differences between HFCS, corn syrup and sucrose. Another study in a later issue of the same journal looked at the effects of beverages sweetened with HFCS, sucrose, fructose and glucose. That study reported no differences in several physiological measures, including 24-hour blood glucose, insulin and triglyceride levels.

Comparing HFCS to sucrose, you get more sweetness per unit weight. It is also less expensive than cane or beet sugar, which explains why it's found in so many processed foods. Using a sweeter sweetener means fewer grams needed and fewer calories for the equivalent level of sweetness. HFCS is simply another sweetener, and I consider it to be no worse than regular sugar. The issue with sweeteners, whether sucrose, HFCS or even honey, is the level of consumption. Also, keep in mind that any food that would use a high level of HFCS -- or any added sugar -- should only be a bit player, not a star, in an otherwise healthful diet. Don’t be afraid of the stuff; just don’t overdo it. Realize that processed food that adds excessive simple sugars will never equal fresh whole foods.

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 need a clarification about the meaning of “elemental” as it applies to the minerals in the diet. My confusion comes from the lack of consistent terminology in books, online, and on products I have seen. I have also asked “knowledgeable” clerks in supplement stores, and the responses tend to be all over the place. I am anxious to hear a reliable answer, especially regarding calcium and how much potassium we need. -- R.Q., Phoenix

DEAR R.Q.: Minerals are often referred to as “mineral elements.” They are found in foods and dietary supplements as compounds made up of the mineral, plus an “escort” substance.

The escort has an opposite charge to the mineral, as this is how mineral elements exist in biological systems. For example, calcium can be paired with carbonate, phosphate, citrate, gluconate or other escorts. Calcium compounds have differing characteristics, including bioavailability (how effectively the calcium is absorbed), how well they will function in a given food system, and, of course, cost. There are practical implications, as well. Calcium carbonate is approximately 40 percent calcium by weight. Compare this with calcium citrate, which is approximately 20 percent calcium. For a given amount of calcium, you would have to take twice as many pills (or have pills twice as large) if you took calcium in the citrate form, as opposed to the carbonate.

In terms of bioavailability, the calcium has to separate from its escort before it can be absorbed; carbonate comes apart best in an acid medium, while calcium citrate (and a related form known as calcium citrate-malate) do not need acid to separate. That explains why calcium carbonate is best taken at mealtimes, but calcium citrate can be efficiently absorbed either during or between meals. Because of their higher solubility, calcium citrate and calcium citrate-malate are often recommended for those who do not produce sufficient stomach acid. There have been some issues of late on the amount of calcium in our diets, and possible dangers from excessive intakes. For more on this, see b.link/calcium29.

Potassium is involved in the transmission of nerve signals -- for example, it helps keep the heart beating, and it is also important for regulating the water balance inside the cells and the body’s acid-base balance. A reasonable goal is about 4,700 milligrams a day. Potassium is in a variety of fresh, whole foods such as potatoes, greens, tomatoes, bananas, citrus fruits and avocados. Avoid having too much potassium; supplementation is not usually needed unless there is a specific health problem, or medications are being taken that drain the body’s supply of the mineral. For more on potassium, see b.link/potassium21.

On a separate issue, you are right to be wary about health-related answers from online sites and store clerks. It’s a bit of a wild card, as some sources provide information based on facts and an understanding of the issues, while others can be of a lesser stripe. It’s always best to have an understanding of the basic concepts in mind. A reasonable strategy when looking online is to seek out information from recognized educational institutions or government agencies.

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.