On Nutrition

DEAR DR. BLONZ: I use canned tuna to make tuna salad for sandwiches. How much of a difference does it make whether the tuna is packed in oil or water? Is there more of the omega-3 fats present in the oil-packed kind? I also wanted to know about the risks of mercury in tuna. -- S.S., Phoenix

DEAR S.S.: The oil used for canned tuna is usually soybean or canola oil, not fish oil. Oil-packed tuna will contain more fat, but no extra amounts of the healthful long-chain omega-3 fats (abbreviated as EPA and DHA).

One cup of light tuna, canned in oil (drained), will contain 12 grams of fat, of which 0.3 grams will be omega-3 fatty acids and 0.4 grams will be omega-6 fatty acids (the omega-6 coming primarily from the oil). By contrast, 1 cup of light tuna, canned in water, contains 1 gram of fat, of which 0.4 grams will be omega-3 fatty acids and 0.14 grams will be omega-6 fatty acids. (What could explain there being less omega-3 in the oil-packed? Consider that a bit of the fish's omega-3 fat mixes with the oil while they are hanging out in the can, and then exits when the oil gets drained.)

Albacore (white) tuna contains more omega-3 than the yellowfin and skipjack varieties used to make light tuna. One cup of albacore tuna, canned in oil (drained), will contain 14 grams of fat, of which 0.8 grams will be omega-3 fatty acids and 0.4 grams will be omega-6 fatty acids. One cup of albacore, canned in water, will contain 5 grams of fat, of which 1.5 grams will be omega-3 fatty acids and 0.1 grams will be omega-6 fatty acids. You can see the difference.

Your concern about mercury in fish is valid. Because tuna is relatively high on the food chain, it tends to contain higher contaminants such as mercury, a dangerous toxin. A study in the April 2013 issue of the journal Environmental Science and Technology reported that over 70% of the mercury in the human body comes from that present as a contaminant in fish. Since mercury distributes itself throughout the fish, cleaning and method of preparation can't solve the problem.

The most commonly consumed fish found to have higher levels of mercury contamination include swordfish, shark, orange roughy, tilefish and certain, but not all, types of tuna. (Note: Salmon is not of serious concern in this regard.) Size matters with tuna, because the longer the animal lives, the more mercury it can accumulate. When selecting canned tuna, consider opting for tuna caught via troll, or pole-and-line.

Two resources for follow-up: Check out the EPA's advice regarding fish; the site's information is targeted to children and pregnant women, which are the most vulnerable populations in this regard. This page has a downloadable table: b.link/h96jx6. Then, check Seafood Watch (seafoodwatch.org) for more on harvest methods for all types of fish.

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 read your recent column on cans, solder and BPA. For years, I have been boiling water for tea or coffee in an old kettle, which has green stuff all around its exterior bottom surface. It is obvious that part was soldered. Does the boiling of the water act as a deterrent to any toxins from the kettle's solder? If not, what would the health consequences be in the short or long term? -- H.D., Rio Vista, California

DEAR H.D.: Although the green substance is on the exterior of the kettle, it may have resulted from minerals in your water accumulating over the years. The green color can be from limescale and mineral elements such as copper. You can see whether lead is involved by using a test kit, available online or at most hardware stores.

If you haven't already, review the mineral content of the water coming to your house.

Six ground wells supply the water in your city of Rio Vista, California. If your home is on that municipal supply, you can access your local water report. (Check with your supplier, whose name and contact information should be on your water bill.) The next element to consider is the state of your home's plumbing, which will take more research as it will not be reflected on a municipal water report.

Boiling can destroy microbial elements such as bacteria, viruses, protozoa and parasites. But it does not destroy or remove potentially toxic minerals such as lead, chlorine, arsenic, copper, nitrates or radon, nor does it destroy other toxic chemicals. The only way to eliminate them is through a quality water filtration system.

It isn't easy to generalize about short- and long-term health impacts, as that will depend on the exposure type, level and duration, and your overall state of health. But it is best not to be in the dark about your water.

The U.S. Environmental Protection Agency publishes an annual consumer confidence report on drinking water quality. Check that out (visit b.link/hslf3y), and then look at the CDC's page on water quality and testing (visit b.link/6zbdbg).

DEAR DR. BLONZ: Please settle a dispute at our house. Does removing the skin from chicken before you eat it get rid of a good proportion of the cholesterol? -- F.R., Anderson, South Carolina

DEAR F.R.: No. There is a small amount of cholesterol in the skin and its layer of fat. Once this is removed, however, the greater amount of cholesterol remains; it is present in every muscle cell of both light and dark meat.

This holds true for all animal products, including beef, pork, lamb and seafood. Buying leaner cuts of meats can reduce total fat, but it only slightly affects the dietary cholesterol in the meal.

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: A famous medical doctor does these videos where he recommends the elimination of all corn from the diet for optimal mental and physical health. He doesn't refer merely to corn syrup, which I would understand needing to limit, but all corn, in any amount. One of the reasons he gave was the fatty acids in corn. This is all a bit shocking, as this is the time of year I look forward to enjoying fresh, sweet corn on the cob. Is there any basis for such concern? -- H.K., Tulsa, Oklahoma

DEAR H.K.: With the obvious exception of individuals who are allergic to it, the answer is no: You do not need to eliminate corn from your diet. Just like you, I consider it a harvest treasure that can be enjoyed in any healthful diet.

Fresh, sweet corn is a whole food that provides fiber, vitamins and minerals, as well as phytochemicals that are good for our vision (lutein and zeaxanthin). Are corn's fatty acids an issue? Maybe, if your diet was 100% corn. As with most individual foods, corn does not provide every fatty acid and all other nutrients the body needs. But that is a disingenuous argument, and it makes one wonder where these concepts originate -- certainly not with any evidence I have seen.

DEAR DR. BLONZ: I am wondering about nutrient losses when cooking carrots. In one class I took, the instructor mentioned that, when cooking carrots in water to soften them for a recipe, the best way to retain their nutrients was to keep the temperature just below boiling and keep the lid on tight to prevent evaporation. I wanted to question this, but it was presented as family advice passed down through generations, so it did not seem appropriate. -- J.S., Anderson, South Carolina

DEAR J.S.: The primary nutrient that is lost through evaporation is ... water. Higher temperatures can cause some nutrient destruction; the most heat-sensitive nutrients include vitamin B12, thiamine (vitamin B1), riboflavin (B2), niacin (B3), pyridoxine (B6), folate, pantothenic acid and vitamin C.

During cooking, water-soluble nutrients are distributed in the total volume of water, including that found naturally in the food. With increases in water temperature and food surface area, more nutrients end up in the cooking water. For example, being in water helps keep plant cells hydrated and intact. Virtually all the nutrients in intact carrots (greens removed) sitting in ice water remain in the vegetables. Some slicing and steaming, just until tender, will bring more nutrients into the water, but most still remain in the carrot. More will be lost if you do some fine dicing, followed by boiling, and then discard the cooking water. This is one reason that some recipes suggest saving the cooking water for use in a sauce.

Most fat-soluble substances, including vitamin A, vitamin K and any minerals, will remain in the carrot. Other healthful fat-soluble phytochemicals would also remain, as would all the fiber.

Carrots remain a healthful food, even when cooked. Best to be less concerned about minor nutrient losses and more focused on having a variety of healthful ingredients in all your dishes -- prepared in a way that helps you enjoy every bite.

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.