The findings, which appear in the May issue of the American Journal of Clinical Nutrition, suggest that many of this nation's 4.8 million vegetarians who choose fortified soy milk as their primary source of calcium may need to drink more to ensure they are getting their daily supply of this essential mineral.
Depending on age, recommended calcium intake falls between 1,000 mg and 1,300 mg a day, according to the National Osteoporosis Foundation. Calcium is needed to help build healthy bone and prevent the brittle bone disease osteoporosis.
In the new study, 16 men aged 22 to 51 drank either 2%-fat cow milk or calcium-fortified 2%-fat soy milk as part of breakfast. After performing tests to see how well calcium was absorbed, the researchers found that calcium from the fortified soy milk was absorbed only 75% as efficiently as the calcium found in the cow milk.
"The take-home message is that soy beverages can be a good source of calcium, but manufacturers are going to have to put more calcium in the fortified soy milk to match the calcium in cow milk," lead researcher Robert P. Heaney, MD, the John A. Creighton University Professor at Creighton University in Omaha, Neb., tells WebMD.
The calcium in soy milk is not as available as the calcium in cow milk, he says. In fact, it takes about 500 mg of calcium in the fortified soy milk to equal 300 mg of calcium in cow milk, according to the new study. Unfortified soy milk contains about 10 mg of calcium per serving. Depending on the brand, manufacturers have been fortifying soy milk with 80 mg to 500 mg of calcium per serving.
Aside from dairy foods like milk, cheese, and yogurt, calcium is found in dark green veggies, fortified orange juice, rice, and beans.
Calcium in soy can still be a good source of calcium, says Maurice Bennink, PhD, a professor of food science and nutrition at Michigan State University in East Lansing. And soy has benefits even without calcium fortification, he says. "Soy is very nutritious. It may help lower risk for heart disease, and it looks like prostate cancer, colon cancer, and breast cancer can be reduced by consumption of soy products, but research is still going on," he tells WebMD.
According to the FDA, eating about 25 g (about an ounce) of soy protein daily may reduce total cholesterol levels as well as "bad" cholesterol, or low density lipoprotein (LDL), levels -- and thus lessen the risk of heart disease. High blood levels of total and LDL cholesterol are risk factors for heart disease.
For more information on how much soy is in common foods, visit the U.S. Soyfoods Directory. More information about calcium is available at the web site of the National Dairy Council.