Moderate alcohol consumption, or about two drinks a day, has often been touted as heart healthy in recent years, but a new study finds the same quantity causes cancer.

Mice given the human equivalent of two drinks daily developed breast tumors that were nearly double the weight of those in their "dry" relatives.

Nearly 179,000 U.S. women will develop breast cancer this year, according to the National Cancer Institute. Even so, scientists lack a strong grasp on why one woman develops the disease and another remains cancer free.

Presented here at the American Physiological Society (APS) annual meeting, the research not only shows the link between alcohol consumption and breast cancer, but it proposes how that glass of wine or bottle of beer works to stimulate tumor growth.

"Alcohol [consumption] is the most important avoidable risk factor for women getting breast cancer," lead scientist Jian-Wei Gu of the University of Mississippi Medical Center told LiveScience. Genetic factors would be considered "unavoidable," since people inherit DNA from their parents.

The APS meeting is being held as part of the larger, annual Experimental Biology meeting.

Drinkin' mice

Gu and his colleagues, also from the University of Mississippi Medical Center, fed six-week-old female mice moderate levels of alcohol for four weeks which would be the equivalent of two drinks each day in humans. In the second week, they injected mouse breast-cancer cells into each mouse's mammary glands.

They found that tumors in the alcohol-fed mice weighed on average about 1.4 grams, nearly two times the weight of tumors in the control mice. Images of the alcohol-fed mice show a visible lump in the upper portion of the animal's chest.

The scientists suggest the difference in tumor weight is a result of jacked-up growth in blood vessels due to alcohol consumption. That's because when a rodent, or a human, gulps down a drink, the cells in their bodies go into overdrive to get rid of the "toxins."

The stressed-out cells send out a hormone dubbed VEGF that stimulates the growth of blood vessels - a tumor's means of getting oxygen and nutrients to survive. Supporting that idea, the alcohol-fed mice showed a significant increase in VEGF as well as more blood vessels than the other mice.

Animal models

This is the first study to use an animal model that accurately mimics human breast cancer, Gu said.

Previous studies injected human breast-cancer cells into "nude" mice, or those lacking an immune system. Without a line of defense, the mice's bodies would let the foreign cells grow and scientists could run experiments.

"In regular mice, if you implant a human tumor, it won't grow because it's foreign [so the immune system would stage an attack]," said study team member Emily Young. That also meant any results would apply only to an organism lacking an immune system.

Instead, "we used regular mice and we implanted the mouse breast-cancer form into it," Young said.

As well, many studies have used toxic levels of alcohol, leading to results that were less applicable to humans who typically don't consume that much booze.

The researchers say their results, unlike the others, can be squarely extrapolated to humans and have implications to both prevent and help treat breast cancer.

"Normal people produce cancer cells every day," Gu said, "but at the beginning the cancer doesn't have blood vessels." So it's easier for the body's immune system to fend them off. Once the cancerous cells acquire a blood-vessel lifeline, which this study suggests is fostered by alcohol, the tumor growth takes off.