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 firstname.lastname@example.org. Due to the volume of mail, personal replies cannot be provided.