Dr. Ana Nobis reports:

It's a supplement that millions of Americans take each day, hoping to reduce their risk of heart disease. But a new review of research suggests that fish oil might protect against another killer: breast cancer.

Chinese researchers looked at 21 studies and found that a higher intake of fish oil, but not necessarily fish itself, appears to be linked to a lower risk of breast cancer later in life. Specifically, they found that a high intake of fatty acids found in fish oil was associated with a 14 percent reduced risk of breast cancer in postmenopausal women.

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If it's real, the link could have big implications for women and their health. Breast cancer is the most common cancer diagnosed in U.S. women other than non-melanoma skin cancers, and the second deadliest cancer in women, following only lung cancer, according to the U.S. Centers for Disease Control and Prevention.

"These findings could have public health implications with regard to prevention of breast cancer through dietary and lifestyle implications," the authors wrote in their study, published Thursday in the journal BMJ.

But while it is known that a healthy diet and lifestyle decrease the risk of breast cancer, past studies have reached different conclusions when it comes to the consumption of fish oil and breast cancer risk.

Science Still Slippery on Fish Oil Health Connection

One thing we do know is that including oily fish in your diet is good for you, a reason that it is recommended by many nutritionists. The benefits of fish oil supplements are less clear, although this has not stopped fish oil from becoming big business.

Americans spent $739 million on fish oil supplements in 2008, according to the trade publication Nutrition Business Journal. Proponents have primarily touted them as heart-healthy, and past research has also pointed to the effects of fish and fish oil on breast cancer risk.

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But this research has been less than conclusive. On one hand, two large prospective studies and several case-control studies have suggested a protective effect on breast cancer risk. On the other, a number of studies have found no such association.

Dr. Kathy Helzlsouer, breast cancer expert and director of the Center for Prevention and Research at Mercy Health Services in Baltimore, Md., said the reasons behind the finding that fish oil supplements were linked to a lower risk of breast cancer, but that fish consumption was not, are unclear.

She also called the link between fish oil consumption and breast cancer prevention "modest," and noted that it is still hard to say that these supplements deserve all the credit.

"Whether this is cause and effect is not certain," Helzlsouer said, adding that the authors themselves admit in the paper that more research is needed to better understand the reasons for their findings.

What Women Should Do

The good news is that there is little out there to suggest increasing your intake of fish oil is harmful, and you might even be doing yourself some good.

But Helzlsouer says she believes the best, and perhaps tastiest, option to achieve the benefits of fatty acids found in fish is to eat more oily fish.

"I usually recommend consumption of fish rather than supplements," she said. "I believe fish consumption is a healthy part of the diet and I have recommended it."

Here are a few tips women should consider to reap the possible benefits of fish oil:

• Nutritionists suggest one to two servings per week of oily fish like salmon, sardines or tuna;
• If you're not a fan of fish, taking a daily fish oil supplement might not be a bad idea;
• The two important omega-3 polyunsaturated fatty acids found in fish are: docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Try to look for these if you decide to go with a fish oil supplement.