People who rely on fortified orange juice for calcium may in some cases get less than they paid for, a new study suggests.

It’s not that the juices contain less calcium than the label states; the issue is the quality of the fortification, according to researchers.

In a comparison of two brands of calcium-fortified orange juice, they found that the calcium in one juice was far better absorbed by the body than that in the other product.

The findings confirm past research showing that calcium added to foods can vary widely in its absorbability. Consumers should be aware that even though a product may contain a certain level of calcium, that’s not necessarily what their bodies are getting, according to Dr. Robert P. Heaney, the study’s lead author.

It’s not clear why the calcium in different foods varies in its “bioavailability,” or absorbability, said Heaney, a professor of medicine at Creighton University in Omaha, Nebraska.

The nutrient comes in about a dozen forms that can be added to foods, he told, but even knowing the form cannot assure consumers of the calcium’s bioavailability. Calcium carbonate, for example, has been found to vary in absorbability depending on the product.

“I’m for fortification of foods,” Heaney stressed. But, he added, “In general, I’m concerned about the quality of fortification.”

For the current study, published in the May issue of the Journal of the American Dietetic Association, Heaney and his colleagues had 25 healthy young women drink the two test juices, on separate days, with breakfast. The researchers then took blood samples to measure how well the calcium from each juice was absorbed.

Servings of each juice provided 500 milligrams of calcium - one in the form of calcium citrate malate, the other through a combination of tricalcium phosphate and calcium lactate.

The researchers found that, on average, the women’s absorption of the calcium citrate malate was 48 percent greater compared with the calcium in the other product.

The bioavailability issue, according to Heaney, is not limited to orange juice or to calcium. In general, consumers have no way of knowing the absorbability of any nutrient they ingest through food or supplements.

As it stands, Heaney said, manufacturers have little incentive to test the bioavailability of the fortificants they put in foods or pills. Since nutrition labels state only the amount of a nutrient in a product, consumers likely assume that foods with the same level of calcium or other nutrients are equivalent, he noted.

Manufacturers would have an incentive to test for bioavailability - a simple, inexpensive procedure - if consumer advocates turned on the pressure, Heaney said. And it would be in a company’s interest, he added, to tout the high absorbability of a product’s added nutrients.

For now, according to Heaney, the best way for consumers to ensure that their calcium is optimally absorbed is to get it from natural sources. Naturally occurring calcium also varies in its absorbability, but some good sources include dairy products like milk, cheese and yogurt, and green vegetables such as kale, turnip greens and broccoli.

The study was partially funded by Tropicana, maker of the orange juice containing the better-absorbed calcium citrate malate.

SOURCE: Journal of the American Dietetic Association, May 2005.