(AP) The heavy metal cadmium, widely used in batteries and alloys, can affect rats in ways that mimic the female hormone estrogen, a new study has found.

Researchers say the study suggests the metal may be a risk factor for breast cancer. Scheduled for the August issue of the journal Nature Medicine, the study is being published online Monday.

"We never expected to see this strong a relationship, given how different the cadmium and estrogen compounds are," said Mary Beth Martin of Georgetown University. "Cadmium's ability to functionally mimic estrogen and its effect on cell growth is quite remarkable."

"What we saw suggests a direct link between low dose cadmium exposure and increased risk of breast cancer," she said.

Cadmium has long raised environmental concerns because chronic exposure can lead to kidney damage and bone disease.

But the study found that even relatively low doses of cadmium affected the mammary glands and sexual development of the animals.

The effects included an increase in weight of the uterus, changes in the lining of the uterus and increased density of the mammary glands. In rats exposed to cadmium while still in the womb, there were changes in their mammary glands and puberty began earlier than normal, reported Martin.

Previous studies in male rats showed changes in the prostate after administration of cadmium, Martin said.

She said it's too early to predict that the metal would affect human in the same way it does rats, but the findings suggest that it may be a hazard.

Early onset of puberty can increase a woman's risk of breast cancer, and increased breast density can also be an indication of the disease.

"The more we learn about how this works in rats, and eventually people, the better lifestyle choices women can make," she said.

The tests were designed to see the effect of cadmium at levels that might be encountered in real life.

The World Health Organization recommends a maximum exposure of 7 micrograms of cadmium per kilogram of weight per week. The rats were injected with the equivalent of 5 micrograms per kilogram of weight.

In the United States, dietary exposure to cadmium is estimated to range from 0.12 microgram to 0.49 microgram per kilogram daily.

The metal is common in pigments, alloys and batteries and can be encountered in soldering processes. It is also an air contaminant produced by burning fossil fuels and is present in foods, particularly shellfish, liver and kidney. Cigarette smoking can add 2 to 4 micrograms of cadmium per pack.

In their experiments, the researchers first removed the ovaries of female rats, eliminating the animals' main source of estrogen, Martin said.

After the animals recovered from the surgery, some were injected with cadmium while others were given estradiol, a common form of estrogen.

Four days later, there was a 3.8-fold increase in the uterus weight in rats given estradiol to replace their normal estrogen.

But the rats given cadmium also had a growth in uterine weight, which increased 1.9-fold, an indication that they were reacting to the metal as though it were a hormone.

In addition, some of the rats given cadmium were also given a chemical known to block the effects of estrogen. Those did not have an increase in uterus weight, Martin reported.

Rats treated with both estradiol and cadmium showed 50 percent increases in the density of the tissue in the mammary glands, the study found.

Martin said the relatively low dose of cadmium needed to produce the results was a surprise.

The researchers found no toxic effects on the animals' livers or kidneys.

Injection of cadmium into pregnant rats did not change the pregnancy, but the female offspring reached puberty earlier than rats not exposed to the metal and their mammary glands developed more fully.