DEAR DR. BLONZ: I have been eating vegetarian for over a year now, and I keep reading about compounds in vegetables that prevent minerals from being absorbed. I’m concerned, because my main sources of calcium are vegetables and grains. -- G.R., Portland, Oregon

DEAR G.R.: Some vegetables do contain compounds that bind, or “chelate” (KEY-late), minerals such as calcium, magnesium, iron and zinc, preventing their absorption. Two examples of these compounds are phytates, found in oatmeal and other whole grains, and oxalates, found in rhubarb and spinach (among other foods).

Spinach, for example, appears to be a good source of calcium and magnesium: A 100-gram (3 1/2-ounce) serving has 99 mg of calcium (10 percent of the Daily Value) and 79 mg of magnesium (20 percent of the Daily Value). However, by virtue of its oxalate content, spinach should not be considered a good dietary source of these minerals.

Usually, about 10 to 25 percent of the calcium in foods tends to be absorbed, but a study in the April 1988 American Journal of Clinical Nutrition reported that only about 5 percent of the calcium in spinach is 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 of dairy products. It is important to appreciate that spinach is a wonderful food and a rich source of phytonutrients, aside from any impact of oxalates on the bioavailability of its minerals.

An interesting story along these lines relates to zinc deficiency, which was first described in men consuming a high-phytate cereal-grain diet. The phytates in whole grains are not a problem in yeast-leavened breads: Enzymes in yeast can break the bond between the phytate and the mineral, making the mineral bioavailable. With pita bread and similar items, however, which undergo a relatively short fermentation and baking, there isn’t sufficient time for the yeast enzyme to work its magic. As such, zinc deficiency tends to be more prevalent in parts of the Middle East where pita bread can account for as much as 85 percent of calories. Zinc deficiency has a number of effects, one of which is stunted growth if present during childhood. For more on zinc, see tinyurl.com/owql53a.

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

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 enjoy making tuna salad sandwiches, but wanted to know about the different types of tuna. Does canned tuna packed in oil have higher levels of omega-3 fat than tuna packed in water? -- M.M., Hayward, California

DEAR M.M.: With few exceptions, the oil used for canned tuna is either soybean or canola oil, not fish oil. As a result, the oil-pack contains 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 grams of omega-3 fats.

For more of the omega-3 fats, consider using the albacore (white) tuna. A 3.5-ounce serving of water-packed albacore contains about 3 grams of fat, of which 1 gram is omega-3. This is due to the fact that light tuna is made from the yellowfin and skipjack varieties, and these contain less of the omega-3 fatty acids than albacore.

For more issues relating to tuna, including a brief discussion of mercury, sodium and ecological concerns relating to how the fish are caught, check out the article at goo.gl/RyzX8u.

DEAR DR. BLONZ: I am all about avoiding partially hydrogenated oils and trans fats, but what about “fractionated vegetable oils”? What does “fractionated” mean? I’ve come across the ingredient several times, most recently in a “soy chocolate peanut” protein bar. -- L.S., San Diego

DEAR L.S.: Fractionation is a process by which fats and oils are separated into various fractions, often based on melting characteristics. It is a preferable alternative to partial hydrogenation because it does not lead to the formation of trans fatty acids. One example of the use of fractionated vegetable oil might be as an ingredient in a coating or frosting. In this case, manufacturers have an interest in selecting the “fraction” of the oil that works best to help the product remain solid at room temperature.

DEAR DR. BLONZ: I have been taking Coumadin for about a year now, as prescribed by a Kaiser clinic. I recently had an episode of internal bleeding and needed surgery. I am recovering, and they want me to begin taking the Coumadin again. To your knowledge, are there any studies that differentiate Coumadin from other blood thinners? Are there any natural dietary supplements I might take to act as blood-thinner agents? -- K., Richmond, California

DEAR K.: As I am not a medical doctor, I cannot offer medical advice, and that is what you need. The health professionals at Kaiser should be able to discuss the advantages/disadvantages of various anticoagulant medications. I would strongly advise against any reliance on dietary supplements for this purpose. That being said, it is essential that you inform your health care team of any dietary supplements you take.

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 some questions about some steaks I recently purchased. They looked really good in the store: a fresh, bright red. The date on the package also indicated that they were fresh. When I got home and opened the package and trimmed the steaks a bit, it became obvious that the middle of the steaks were brownish; only the outside surfaces were red and fresh-looking. The steak didn’t have a bad smell. I have never seen anything like this before. I returned the steaks and got my money back, no questions asked, but also no answers. What could have caused this? Was the meat “stressed”? Was it spoiled, and then treated somehow to look fresh? -- W.B., San Bruno, California

DEAR W.B.: Color and general appearance help determine the appeal of any food product, including meats. Meat is muscle tissue, and when working in the body, it is normally dark-red to purple in color. When exposed to oxygen, a chemical reaction takes place that causes it to turn bright red. This phenomenon is referred to as “bloom,” and involves a reaction with myoglobin, a muscle protein. Below the surface, or even on the bottom of the package, the color can be darker because the reaction hasn’t yet taken place there.

If a cut of meat that has bloomed is allowed to sit out, exposed to the air, a second reaction can occur, causing the red color compound of the meat to oxidize and turn brown. This is not necessarily a sign that the meat is of poor quality, but in time, the flavor will deteriorate.

Interestingly enough, once meat has bloomed, the length of time it stays red can depend on the antioxidant content of the meat. Studies have found that upping the vitamin E content of the animal’s diet can increase the time in bloom.

In general, butchers cut enough to satisfy turnover. They can use an overwrap film to limit air exposure, or even flush packages with a mixture of oxygen and carbon dioxide. All this causes the meat to bloom more slowly. The cutting and packaging processes are coordinated with the time it takes to get the meat into the display case and sold.

Products that turn brown on the surface can be trimmed, allowing a new layer to bloom. But in your case, the meat was red on the surface, but dark all the way through. This would seem to indicate that something was awry. It could very well have been due to stress, as you suggest, but there may have been other factors as well. In any event, it made perfect sense to take it back.

DEAR DR. BLONZ: What are good foods for magnesium? -- C.B., York, Pennsylvania

DEAR C.B.: Magnesium is an essential element needed for normal bones -- in fact, half of our bodies’ magnesium is in our bones -- but also required for normal nerve transmission, muscle relaxation and normal heart rhythm. Good dietary sources include avocado, nuts, bananas, legumes, whole grains, dark leafy greens, milk and oysters.

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.