Most women know that calcium is critical in preventing osteoporosis, the disease of progressive bone loss and fractures that affects millions of Americans.

But which source is better — calcium-rich foods or supplements? A preliminary study by researchers at Washington University School of Medicine suggests dietary calcium may be better at protecting bone health.

Though not definitive, the study found that women who get most of their daily calcium from food have healthier bones and higher bone density than women whose calcium comes mainly from supplemental tablets.

That was true even though the supplement-takers had higher average levels of calcium.

Calcium from dietary sources is generally better absorbed than that from supplements, which could help explain the difference, said the study's lead author, Dr. Reina Armamento-Villareal.

Those getting calcium from foods also had more estrogen in their bodies; the hormone is needed to maintain bone mineral density. Researchers can't yet explain the food-estrogen connection.

The research is preliminary and offers "a springboard to do something more, a hypothesis to test," said Armamento-Villareal, a bone specialist and assistant professor in the School of Medicine's division of bone and mineral diseases. It was published in the May issue of the American Journal of Clinical Nutrition.

Researchers asked 183 postmenopausal women to meticulously document their diet and their calcium supplement intake for seven days. They tested their bone mineral density and their urine for levels of estrogen.

The women then were divided into three groups: those who got at least 70 percent of their daily calcium from supplements, those who got the same amount from dairy products and other food, and those whose calcium-source percentages fell somewhere in between.

The "diet group" took in the least calcium, an average of 830 milligrams per day. Yet, the group had higher bone density in their spines and hip bones than women in the "supplement group," who consumed 1,030 milligrams per day.

Women in the "diet plus supplement group" tended to have the highest bone mineral density as well as the highest calcium intake at 1,620 milligrams per day.

An analysis showed that women in the "diet group" and the "diet plus supplement group" had higher levels of estrogen, needed for bone mineral density.

Dr. Robert Recker, who heads osteoporosis research at the Creighton University School of Medicine in Omaha, noted weaknesses in the study, which he said "is certainly not definitive."

Those who got calcium from their diet might have also taken in more vitamin D from milk, which would aid in calcium absorption. As for the estrogen connection, they might have eaten plant sources containing more of the hormone, he said.

"Nevertheless it's not to be ignored," Recker said. "Observation studies are very good for generating a hypothesis to be tested later in an outright experiment."

Dairy foods and calcium-fortified orange juice are excellent sources of calcium. Dark green, leafy vegetables also contain it, though it is not as readily absorbed as calcium from dairy, researchers said.

Armamento said she'd like to do a long-term study of teenagers whose bones are still developing to see what, if any, differences might emerge among young women taking calcium from diet versus supplements.

"It's a lifestyle issue," she said, noting that some teenagers avoid dairy products.