DEAR DR. BLONZ: My husband and I eat fresh Atlantic salmon once weekly. The particular salmon we usually eat is farm-raised and purchased from Whole Foods Market. Recently I have read that the mercury levels in salmon differ depending on type of salmon and where it was farmed or caught. This makes sense to me. Is this correct? To what authority should we refer for safe consumption guidelines of different fish species caught or farmed in different areas of the world? -- E.S., Concord, Calif.

DEAR E.S.: A good start is to visit the site of the Monterey Bay Aquarium (tinyurl.com/23ze8sj), which has an excellent explanation of different fishing methods and can introduce you to the language of the industry. Fish farms are enterprises that can vary in their practices, and they should not be condemned outright. We need to avoid any that do a lousy job cleaning up after themselves, and those that pollute and ruin the environment for other fish, animals and people affected by their output. Once you understand the issues and the potential negatives, you can then consult a site like cleanfish.com to see examples of fish farms that have taken steps to distinguish themselves and provide healthful food with minimal environmental impact.

I always encourage customers to speak with the store where they buy their fish to find out what the sellers know about their suppliers, especially any farmed-fish suppliers. This information should be available from supermarket chains, especially one such as Whole Foods, but you may have to ask, or even insist. Let them know that if they want your business, they need to be upfront with this information. If you are given the name of a supplier, you can do your own research online to find out more. I won't buy farmed fish unless it says where it is from, or unless I have confidence that my fishmonger has done this type of screening for me. This practice also makes sense for your meat and poultry purchases.

You mention salmon, which is definitely a favorite in our house. We enjoy salmon from Loch Duart (lochduart.com), a sustainable fish farm in Scotland. You can find out more about this particular type of salmon at the cleanfish.com site. My local market, Kensington Fine Foods, carries the Loch Duart salmon; you can encourage your store to carry this or another type that meets your needs. Stores want to carry quality items that their customers want, but you have to let them know.

Your question included a concern about mercury and this is certainly a valid concern regarding fish, as mercury is a dangerous toxin. A study in the April 2013 issue of the journal Environmental Science and Technology reported that over 70 percent of the mercury in the human body comes from the mercury present as a contaminant in fish. The good news is that salmon is not one of the fish of serious concern in this regard. The most commonly consumed fish found to have higher levels of mercury contamination include swordfish, shark and certain -- but not all -- types of tuna. For information about the mercury content of fish, gotmercury.org is a site that allows you to assess the risk of different types of fish, regions and fishing methods, and the impact of various levels of mercury intake.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 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, In our house, broccoli is a go-to vegetable due to its availability and relative low cost. I prepare the stems as well as the florets, cutting the stems in diagonal disks so they can get tender when steamed. I am guessing there is less nutritional value in the stems than in the darker florets. How significant is that difference? I.K. Gilroy, CA

Dear I.K.: Broccoli is a member of the crucifer family. Other members of this healthful group include cauliflower, Brussels sprouts, kale, bok choi, turnips, rutabagas and cabbage. All of these contain a number of healthful phytochemicals, but broccoli is high in vitamins A and C and is also a source of vitamin K and dietary fiber.

It also contains sulforaphane, which has been reported to have cancer-fighting properties. Young broccoli sprouts and seedlings are the richest source of the sulforaphane, but the florets are next best. The fibrous stems contain less of the nutrients and phytochemicals, but are a great source of dietary fiber. Your method of preparation allows you to have it all. An item of interest regarding broccoli is that there is an enzyme in the broccoli that helps the sulforaphane form. When the cells are ruptured, such as during cutting, mashing or chewing, the enzyme goes to work, but it can be inactivated by heat. This means uncooked or quickly cooked broccoli will offer more sulforaphane than broccoli that has been overcooked.

Dear Dr. Blonz, I read your column every week and also love all the advice that you give on your website. I read the column about the cooking methods and destruction of vitamins, especially the water-based vitamins. My question is this: I love mixing all kinds of vegetables and making mixed veggie soup. The vitamins of the vegetables that have been ruined -- are they in the broth of the soup, or they are completely gone? Does any percentage of them stay in the broth? Though I love raw vegetables, my digestive system cannot handle the raw fiber, but if I cook them and eat the soup as well, then I am OK. So I hope that I do get some nutrients. Thank you very much. A.S. Oakland, CA

Dear A.S.: There can be losses when there is destruction by heat. In such cases there will less of the heat-labile nutrients once a key temperature has been exceeded for a period of time. (As with the broccoli answer above, longer times equal greater destruction.) There can also be nutrient losses through dilution if the nutrients are water-soluble and the portion that ends up in the cooking water is not a part of the meal. Minerals tend to be heat-stable, but depending on the form they are in and how the food is prepared, some can be lost with the cooking water as well.

It is difficult to come up with specific percentages that remain, but if you eat a healthful, varied diet, I don’t recommend you be overly concerned about these losses. There are usually other foods that provide these nutrients, and oftentimes these other foods represent the primary sources. For example, cooking vegetables may destroy their vitamin C, but it is fruits, not vegetables, that tend to be the major source in most diets. Your vegetables and soup combination sounds wonderful, so don’t sweat the small stuff.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 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 have been advised to bring down my cholesterol, or else I'm going to need to take medication. My wife insists that I cut the cholesterol that I eat, and she makes direct mention of my love of eggs. I am thinking of using cholesterol-free egg substitutes, but find them a bit pricey. As an alternative, I have been buying eggs by the dozen, separating the yolks and saving only the whites. Is this a reasonable way to avoid cholesterol? -- R.R. Palo Alto, Calif.

DEAR R.R.: It is definitely an option. The white of an egg is a source of high-quality protein; it is the standard to which all other proteins are compared. There are approximately 3.5 grams of protein in the white of a large egg. There are small amounts of other nutrients, including selenium, potassium and sodium. The yolk contains lesser amounts of protein, together with small amounts of vitamin A, folate, phosphorous, potassium and selenium.

The eggshell is a great container. As you can tell by the date stamped on an egg carton, whole eggs can usually last in a refrigerator for several weeks before they need to be eaten. Once you open the shell, however, the clock runs at a more rapid pace. Egg whites, out of the shell, have a recommended shelf life of only four days. This is something to consider when opening up a dozen at a time. You might consider freezing the whites in an ice-cube tray. Egg whites can last up to a year when frozen.

You state your desire to watch your cholesterol intake. The yolk of one large egg contains approximately 213 milligrams of cholesterol. For those with an otherwise balanced diet, the periodic use of whole eggs has been shown to have only a minimal impact on one's blood cholesterol level. If you truly have a passion for eggs, having one a day can fit within the American Heart Association's guidelines. A compromise might be to use one yolk for every two egg whites.

It might also be helpful to discuss the connection between dietary cholesterol and the level of cholesterol in the blood. Please bear in mind that this might not apply to your specific case, but in general, the American public has been persuaded to wear blinders that let them see only the fat and cholesterol in their diets as the factors responsible for elevated blood cholesterol and heart disease. This has led to thinking that reducing dietary fat and dietary cholesterol are the best ways to lower blood cholesterol and the risk of heart disease. Things, however, are not that straightforward.

First, consider that lowering dietary fat might lead to eating more carbohydrates, and if that meant a higher intake of sugars, you wouldn't be doing yourself any favors. There's little question that the levels and types of dietary fat and cholesterol can play a role, but they are not the unqualified culprits we've been led to believe. More important is a diet that's balanced and rich in fruits, vegetables, greens, whole grains, nuts, seeds, dietary fiber, etc. These supply the food factors to enhance our health. A body that is healthfully nourished is equipped to handle the fats and cholesterol in the foods it eats.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 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.