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 email@example.com. Due to the volume of mail, personal replies cannot be provided.