segunda-feira, 28 de novembro de 2016

Is it true that plant foods are a better source of calcium than dairy foods?


David Schardt

Are dairy foods a “mediocre source of calcium” compared to other foods, as a reader claims?  The reader objects to our posts that “paint dairy in a beneficial light.”

digestive balance

The short answer is no, dairy foods are a good source of absorbable calcium, as are many other foods.

While no adult needs cow’s milk and some people avoid dairy foods for a variety of reasons, many others enjoy the taste of milk, cheese, and yogurt and benefit from the nutrients in these foods. And calcium is one of those nutrients.

Calcium absorption from foods

About 30 percent of the calcium in milk, cheese, and yogurt is absorbed. That’s a higher rate than from beans, spinach, and sweet potatoes, and a lower rate than from broccoli, kale, and bok choy.Broccoli with Balsamic Dressing

Calcium bioavailability from plant foods is affected by their contents of oxalate and phytate, two naturally occurring compounds which interfere with the absorption of calcium. (Dairy foods contain neither.)

In general, the higher the oxalic acid content of a food, the lower the rate of calcium absorption. So calcium bioavailability is low from both American and Chinese varieties of spinach and rhubarb, intermediate from sweet potatoes, and high from low-oxalate vegetables such as kale, broccoli, and bok choy.

Dried beans, on the other hand, have a substantially lower calcium bioavailability because they’re rich in phytate. An exception to this generalization is soybeans, which are rich in both oxalate and phytate, yet have a relatively high calcium bioavailability.

Calcium content

But what also matters is the calcium content of the food to start with and that’s where dairy foods have an advantage. They contain a lot of calcium and this combined with a good absorption rate means that a serving of milk, cheese, or yogurt provides more absorbable calcium – about 100 mg —  than any other natural food.

Certainly, you can get the same amount of absorbable calcium from plant foods, but you’ll have to eat more than one serving of those foods. Four and a half servings of broccoli, two and a half of bok choy, three of kale, sixteen of spinach, for example.milk Clearly doable, but that doesn’t make dairy foods a mediocre source of calcium. (Of course, there are plenty of other reasons to include more plant foods in our diets, but we’re talking about just calcium absorption here.)

A new U.S. Department of Agriculture study confirms the nutritional value of the calcium in dairy foods. The researchers gave 12 women aged 50 to 65 extra calcium — 1,200 mg to 1,300 mg — as dairy foods or dietary supplements for six weeks each. Result: both sources of calcium were equally effective at halting normal calcium loss in the women.

: Am J Clin Nutr 70 (Suppl): 543S, 1999; Bone Reports 5: 117, 2016.

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