DEAR DR. BLONZ: If I purchase tuna that is packed in water, will I get the same level of omega-3 fat that I would if I got the tuna packed in oil? -- M.M., Logan, Utah

DEAR M.M.: As a general rule, the oil used for canned tuna is either soybean or canola oil, not fish oil. Tuna in oil will contain extra fat, but little in the way of extra amounts of the healthful omega-3 fats. A 3.5-ounce serving of light tuna in water (drained) contains 0.8 grams of fat and 0.3 grams of omega-3 fats. Contrast this with light tuna in oil (drained), which contains 8.2 grams of fat and 0.2 milligrams of omega-3 fats.

For higher levels of omega-3 fats, use the albacore (white) tuna. A 3.5-ounce serving of water-packed albacore (drained) contains about 3 grams of fat, of which 1 gram is omega-3 fats.

Light tuna is made from the yellow fin and skipjack varieties, which contain less of the omega-3 fatty acids than albacore. One additional note about tuna: Because tuna is relatively high on the food chain, it can contain higher levels of contaminants such as mercury. It remains a flavorful, healthful fish, but it also makes sense to take steps to make good choices. When buying tuna, consider opting for tuna caught via troll, or pole-and-line. This will tend to be stated on the label. Check with the Seafood Watch (tinyurl.com/d464hdm) for more on the different types of canned tuna.

DEAR DR. BLONZ: What are the things in plant-based foods that prevent minerals from being absorbed? I'm concerned because my main source of calcium comes from vegetables and grains. -- R.R., Oakland, Calif.

DEAR R.R.: The compounds that limit absorption are known as chelators (KEY-late-erz) in that they bind, or chelate, nutrients -- in this case, minerals such as calcium and zinc. The two most common chelators are phytate, found in some whole grains, and oxalate, found in many fruits and vegetables, but most notably spinach and rhubarb.

Zinc deficiency was first described in people consuming a high-phytate cereal grain diet. It does not represent a problem when the grains are a part of a yeast-leavened bread, because yeast contains an enzyme that breaks the bond between the phytate and the mineral. That is not the case with an unleavened bread such as pita bread. Indeed, zinc deficiency tends to be prevalent in parts of the Middle East where pita bread accounts for about 85 percent of calories consumed.

There are plenty of reasons to enjoy spinach, such as its valuable phytonutrients, but its oxalate content means it should not be considered a good dietary source of iron and calcium (although it is high in both). Usually about 10 to 25 percent of the calcium in foods can be absorbed, but a study in the April 1988 issue of the American Journal of Clinical Nutrition reported that only about 5 percent of the calcium in spinach was actually absorbed. A second study two years later in the same journal reported that the calcium absorption in a low-oxalate vegetable, such as kale, was comparable to that found with dairy products.

If vegetables and grains are the core of your diet, make sure you include a wide variety of different food sources. Fruit and vegetable sources of calcium without significant quantities of oxalate or phytate include broccoli, turnip greens, collards, kale, mustard, figs and almonds.

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: Could you tell me if roasted or raw would make any difference with nuts, health-wise? What about the difference between oil roasting and dry roasting? One proponent of raw peanuts states that one half-cup of raw peanuts has 430 calories, but processed into peanut butter, a half-cup contains 780 calories. How can this be? -- S.S., Malibu, Calif.

DEAR S.S.: You are mixing volume with weight. There is space between those peanuts in the half-cup, which weighs about 73 grams. Contrast this with a half-cup of peanut butter, which weights 129 grams. If you were to look at similar weights of these two foods, the caloric content would be comparable.

When you roast nuts, or any food for that matter, it can bring about flavor-enhancing chemical reactions. With nuts, this can be accomplished with or without oil. Dry roasting requires specially designed ovens and utilizes hot air. Salt, paprika or other flavorings can then be added if desired.

Although dry-roasted nuts contain no added oils, there will only be a nominal difference in overall fat content as compared to oil-roasted nuts. This is because half the weight of the nut, and over 70 percent of its calories, come from its fat content. A honey-roasted nut will contain less fat per serving because some of the weight is taken up by the sugary coating. In terms of nutritional value, the high heat process will destroy some of the folate and thiamin (vitamin B1), but the other nutrients are unharmed.

Raw peanuts will have a different taste from roasted ones, but a comparable fat content. One item of interest is a study in the October 2000 issue of the Journal of Allergy and Clinical Immunology, which reported that raw peanuts provoked less of an allergic response in sensitive individuals than roasted peanuts. However, as there raw peanuts carry a slightly higher risk of microbiological contamination, they are not recommended for individuals with compromised immune systems. Raw peanuts should be stored in the refrigerator. Only buy them from sources that sell a lot of them, and avoid places where the raw peanuts might have been sitting around for a long time in a bulk bin.

DEAR DR. BLONZ: Can you explain why the vitamins and minerals in a dietary supplement expire? What is the best way to store these products? -- S.H., Arlington Heights, Ill.

DEAR S.H.: Vitamins are reactive compounds that play an integral role in the chemical reactions that comprise our metabolism. Although mineral supplements tend to be quite stable, vitamins can slowly lose their potency over time. The factors that encourage breakdown are humidity, temperature and light. Formulas that contain fat-soluble nutrients tend to break down the fastest. The best way to store products is in a tightly sealed container in a cool, dark place. Many supplements include moisture-absorbing capsules or packets to help maintain product quality. Product expiration dates indicate when the vitamins can be assured of full potency -- assuming that the product has been appropriately stored.

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: You have written before about the nutrients in nuts and their value as a snack. For the last several months, I have been researching the fat content in peanuts still in the shell. From everything I have read, peanuts contain lots of monounsaturated fat, but there is no mention of saturated fat. But when I check packaged unshelled peanuts, the Nutrition Facts labels reveal a different story. The packages list 2 grams of saturated fat per serving, which is about half a cup (with shells). There is no mention of monounsaturated fat. This difference is important, since saturated fat can form cholesterol. Could you shed some light on this mystery? -- J.B., New Orleans

DEAR J.B.: Peanuts are predominantly monounsaturated fat (48 percent), but there are a bunch of polyunsaturated fatty acids (33 percent) and some saturated fats (19 percent) as well. This is usually how it is: Foods often contain a mixture of all the different types of fat, but tend to have more of one type than the others. The food then becomes known as a source of that type of fat. There is a chart providing the breakdown of typical food fats at tinyurl.com/cbwrox.

I would not be concerned with the little saturated fat found in peanuts. Saturated fat does not turn into cholesterol, and small amounts of it should not be an issue. It can contribute to problems when it becomes the predominant fat in your diet, AND when the rest of your diet is missing the variety of healthful foods that empower your body to handle the fats you eat. The reason that monounsaturated fats are not always present on food labels is that there is no requirement to list them. The only items mandated are total fat and saturated fat. Stay well.

DEAR DR. BLONZ: I wanted your take on xanthan gum and guar gum. They may not be harmful or toxic, but they do cause constipation in me, which has been proven time and time again. Since their purpose seems to be to bind things together, this would make sense. I avoid them like the plague. -- I.J., via email

DEAR I.J.: Not much of a plague here. Although they are called gums, they do not really "gum up" the digestive system. These additives are used in relatively small amounts, as evidenced by the fact that they are toward the end of many food ingredient lists. They can act as thickeners, texturizers, emulsifiers and/or stabilizers. Xanthan gum, for example, is often found in salad dressings and sauces for these purposes. It is also finding more uses these days in gluten-free foods.

There is no evidence, or reason to believe, that the small amounts of xanthan or guar gum would contribute to constipation. At levels higher than their normal usages, they might actually do the opposite, because they would add more bulk to the foods traveling through the digestive tract. That being said, we are all individuals; regardless of what might be a typical response, if you are convinced that these substances cause problems for your body, then by all means avoid them. There is certainly nothing essential that you would be missing.

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.