If you needed another reason to eat nuts, mice that eat an abundance of walnuts may be less likely to develop breast cancer, according to new study presented Tuesday at the meeting of the American Association for Cancer Research in Denver.

Earlier research had suggested that daily doses of walnuts slowed down breast cancer once the rodents had it, but now researchers are saying the nuts might also lower the odds of developing the disease in the first place.

Researchers led by Elaine Hardman, a cell biologist at Marshall University School of Medicine in Huntington, West Virginia, compared breast cancer incidence in mice eating a diet that included lots of walnuts (the equivalent for humans of two ounces, or about 14 whole walnuts per day), and those eating a similar diet with no walnuts. They discovered that the walnut munchers were less likely to develop breast tumors, and if they did, their tumors were smaller and affected fewer mammary glands. The mice, which are genetically engineered to develop cancer, normally get tumors within five months, Hardman says. But, she adds, "At 145 days, 100 percent of the mice on the normal diet had cancer, and 50 percent of the mice on the walnut diet had cancer."

Hardman and her colleagues suspect the nuts' apparent anti-cancer powers may stem from the activity of several nutrients within them: omega-3 fatty acids, which are also found in fish and promote heart and brain health, antioxidants, which shield cells from damage by free radicals (unstable atoms produced by the body's own chemical reactions or through environmental exposures, such as cigarette smoke), and phytosterols, plant fats that have been shown to reduce levels of low density lipoproteins, or "bad cholesterol," in the body. All of these nutrients have been shown to slow cancer growth in their own right, the researchers report in their study abstract.

These results have yet to be replicated in animal studies, let alone in humans, but there are plenty of other reasons to eat walnuts, says Diane McKay, a nutrition researcher at Tufts University in Boston. Eating a handful a day reduces the levels of both cholesterol and triglycerides (fatty molecules in the blood that contribute to hardening of the arteries).  For example, a study on 18 healthy men found that eating walnuts (about 20 percent of their daily calorie intake) for just four weeks might reduce total cholesterol by about 12 percent.

But even if you’re ready to eat more walnuts based on limited animal research data, be careful about the effects of walnuts on your waistline: Walnuts are also brimming with calories, warns Karen Collins, a nutrition advisor to the American Institute for Cancer Research. The mice in the study were getting 18 percent of their daily calories from walnuts – the equivalent of about 360 calories for someone who eats a standard 2000-calorie diet. "[Eating] excess calories, if that that leads you become overweight, is one of the greatest risk factors in cancer," Collins says.

And walnuts are not the only foods packing nutrients that may ward off cancer; fruits, vegetables, and other nuts, many of which are lower-cal, contain thousands of antioxidants, which protect cells from DNA damage that might trigger cancer, she says. "The benefit really comes when you have this wide range of antioxidants [from a variety of food sources]."

Hardman agrees that walnuts should be just one component of a healthy diet to prevent cancer. Depending on what expert you ask, she says, "30 to 70 percent of cancers are probably preventable with lifestyle changes," and diet is a key factor.

Image © iStockphoto/MistikaS