DEAR DR. BLONZ: We had great-tasting grilled frozen swordfish steaks at a friend’s place, prepared according to the package instructions. I would appreciate your comment on whether this is a good fish to have, given concerns about mercury content. My main question comes from my learning that the leftovers had inadvertently been left in a sealed container on the counter overnight, rather than being put in the freezer as planned. My friend told me she put them in the freezer the next morning. I questioned the wisdom of this, and she told me that the original grilling would have killed the existing bacteria on the uncooked fish, and that freezing (even if begun the next morning?) would kill any bacteria that might have grown during the night. Then, when it came time to have the leftovers, reheating would finish the job. I questioned this, and she said it was a common practice, and no one had ever gotten sick. Is this a safe? Does freezing kill bacteria? Not to mention what it might taste like, and the general safety of swordfish.

-- W.N., Portland, Oregon

DEAR W.N.: You certainly are a good friend, and I hope you will pass this reply to your neighbor before they consider having those leftovers. Freezing would not kill bacteria in this situation; all that freezing does is to suspend the growth of microorganisms. It is essential to realize that freezing does not make food fresher than it was before it was frozen. Whenever a food is defrosted, bacterial growth resumes exactly where it left off. Eating foods left out on the counter is an unnecessary roll of the food-safety dice. There could be no guarantee of safety, even after reheating the next day. The idea of them being in a sealed container on the counter provides no reassurance, as cooling foods can draw the bacterial wild card of external air into the container while the food is temperature-sensitive. In my considered opinion, the best course of action would be to discard the items.

As for the healthfulness of eating swordfish, it provides an excellent source of protein, vitamins D and E, most of the B vitamins, along with omega-3 fatty acids. There are, however, real concerns about its mercury content. Fish are the main dietary source of this toxin for humans. As healthful as fish can be, estimates in the scientific literature are that about 90% of the mercury in our bodies comes from fish, typically from varieties high up on the marine food chain. The most commonly consumed higher-mercury fish include swordfish, shark, orange roughy, tilefish and certain types of tuna. (Side note: Salmon is not a fish of concern in this regard.)

Mercury is distributed throughout a fish, so cleaning and the method of preparation can’t solve the problem. It’s best to have no more than one serving per month of any high-mercury fish. Pregnant women and children should be at the lower end of that range, if you get my meaning. For more on mercury in fish, see b.link/pb677p5. Low- or no-mercury fish can be consumed more frequently. Check seafoodwatch.org for fish recommendations.

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 have been eating more nuts and seeds, and also adding them to our children's lunches. Walnuts are a favorite. I would like to know their effect on blood lipids and triglycerides. In your reply, I hope you will provide a simple explanation of the difference between blood lipids and triglycerides. -- R.J., Raleigh, North Carolina

DEAR R.J.: "Lipids" is an umbrella term for a group of chemicals that includes fats (solid at room temperature), oils (liquid at room temperature), fatty acids, phospholipids, waxes and steroids. They are all composed primarily of carbon atoms in the form of chains or ring structures. Each type of lipid has a structure that sets it apart from the others. All lipids share one characteristic: They cannot dissolve in water, but instead dissolve in organic solvents such as mineral spirits or cleaning fluids. Some lipids can have water-soluble elements attached. Triglycerides also belong to the lipid family, but they're the form in which fats and oils are packaged in the body. And to complete the picture, cholesterol, a type of steroid, is another member of the lipid family.

About 80% of the calories in walnuts come from fat. Eating walnuts could have a variable effect on one's blood triglyceride level, depending on the amount eaten, other items in the meal and any predisposition one might have toward elevated blood lipid levels.

An interesting side note is that nuts, when eaten as a pre-meal snack, can contribute to satiety and decrease caloric intake. A classic study involving university students found that consuming one ounce of walnuts as a pre-meal snack was associated with a reduction in total calories consumed (nuts plus the meal). One ounce of walnuts (14 halves) provides 185 calories, 18 grams of fat and 2 grams of dietary fiber.

There can be differences in health effects between the various types of nuts and seeds, but the general impacts are comparable. There are similar results with peanuts, often regarded as nuts but actually classified as legumes. On a personal note, I rotate between individual nuts and blends of almonds, cashews, pistachios, walnuts, peanuts and pecans, often making a trail mix with added dried fruit and a few dark chocolate chips.

DEAR DR. BLONZ: What is vitamin A palmitate? I know palmitic acid is one of the saturated fats. Is it harmful in diets for those of us who have a cholesterol problem? Why is this form of vitamin A in so many foods? -- V.H., San Diego, California

DEAR V.H.: Vitamin A can exist in several related chemical forms. One of these compounds is made from vitamin A and palmitic acid, a saturated fatty acid. This form of vitamin A is stable and relatively inexpensive, making it the compound of choice for fortifying foods. The small amount of palmitic acid you would receive from vitamin A palmitate (also called retinyl palmitate) would be of little consequence to your cholesterol problem.

To put some numbers here, the daily value of vitamin A is 900 μg RAE (micrograms of retinol activity equivalents), also referred to as 3,000 IU. This level will be in 1.65 mg of vitamin A palmitate, which contains a paltry 0.8 mg of palmitic acid.

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 have a question concerning recommended servings of foods, how they have been determined over the years and the confusion with food labels. It can be unclear when checking serving sizes on labels and those used to make recommendations, such as the USDA food pyramid. Why can't these use the same standards? -- J.L., San Francisco, California

DEAR J.L.: Food serving sizes on the food pyramid and Nutrition Facts labels have evolved over the years, always attempting, at least in theory, to be consistent with those used in USDA dietary recommendations (see myplate.gov).

In recent years, products have had their individual container size reduced. Many in the food and beverage industry have opted for unannounced downsizing, allowing, at least in theory, for price maintenance. (If one company does it, others tend to follow, so their products are not priced above their competition.) However, the influences of the pandemic, inflation and tariffs have overwhelmed any ability of minor volume reductions to stabilize food prices.

Nutrition Facts should align with the USDA serving sizes whenever possible; this typically occurs when a container contains multiple servings. But be aware that products often focus on the container size. For example, checking the Nutrition Facts label of a 64-ounce container of vegetable juice, such as V8, would display nutrients in one serving (as per the USDA) along with the number of servings per container. However, if you were to check smaller containers of the same juice, such as their 5.5-ounce and 11.5-ounce cans, you would find that both their Nutrition Facts labels reflect the nutrients in a single-can serving.

Best to use Nutrition Facts to help compare brands and options. The amount of "added sugars" should always be on your radar for healthy eating.

DEAR DR. BLONZ: I am seeking your advice on a comparison between psyllium husk (I take 4 teaspoons daily) and a new product made from guar gum. This new option is pleasant to take, non-thickening and grit-free, with a flavor-free option. But is it equally effective? Please reply and let me know which one to choose. Thank you so very much. -- J.S., Raleigh, North Carolina

DEAR J.S.: Many whole-grain foods, fruits and vegetables are rich in nutrients and can provide the body with the dietary fiber it needs. If you have been instructed by your health professional to take a fiber supplement, either of these products will accomplish that goal. Pick whichever one you feel most comfortable with, and be sure to follow the label directions. For a good discussion of dietary fiber, check the MedlinePlus article at b.link/j43qu.

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.