Bottomline: Eating high levels of omega-6 fat (linoleic acid) increases the risk of developing breast cancer nearly 2-fold in genetically susceptible women.  Last month Swedish researchers found a similar risk.  Linoleic acid is the most common polyunsaturated fat in the American diet.
 
Background: Omega-6 fatty acids have a strong promoting effect on breast cancer development in animal studies. The omega-6 fatty acids exert their cancer-promoting effects, when they are turned into inflammatory compounds by enzymes commonly found in the body, COX and LOX.  

The LOX enzyme converts the potent omega-6 fat, arachidonic acid, into leukotrienes, which are inflammatory mediators in diseases such as asthma, cardiovascular diseases, and have been implicated in several types of cancers in different tissues and organs (including lung, colon, kidney, bladder and esophagus). Therefore, researchers explored the association of dietary omega-6 fat and the role of genetics related to the LOX enzyme and breast cancer. 

Summary: In a case-control study on nearly 1700 women, researchers found that women with a genetic aberration effecting the LOX enzyme, combined with eating a high level of linoleic acid (>17.4 grams/day), had a significant increase in breast cancer risk, nearly 2-fold. Yet, when women with the same genetic profile ate a lower linoleic acid diet, the genotype had no influence on breast cancer risk.  Notably, an unrelated study on the LOX gene, found an increased risk of atherosclerosis with eating a high omega-6 fat diet.  (See Clogged Arteries--Increased Risk with Dietary Omega-6 Fat).  This study demonstrates that a diet-gene interaction increases the risk of developing breast cancer, which may explain why previous human studies have been inconsistent. This is the first study to consider the impact of genetics and dietary omega-6 fat with breast cancer incidence.

Study Quote: “…our results suggest a role of the omega-6 polyunsaturated fatty acid pathway in breast cancer etiology.”

Comment: This study adds to the growing evidence of excessive omega-6 fat increasing the risk of breast cancer.  Just last  month Swedish researchers found a similar risk (See Omega-6 Fat May Promote Breast Cancer Development). Americans eat an average of 13 grams of linoleic acid a day, which is close to the high intake associated with breast cancer (>17.4 grams/day).  Yet when genetically susceptible women ate a lower omega-6 fat diet, there was no increase in cancer risk, which may indicate that the “cancer-gene-machine” did not get switched on.  This appears to be a modifiable risk factor.  It may also be the reason that previous studies show an association with diets low in omega-6 fat with a reduction of all-cause mortality, which includes cancer (See What Everyone Ought to Know About the Mediterranean Diet.   The highest contributors of linoleic acid in the American diet are soybean oil, corn oil and cottonseed oil.  (See Foods High in Omega-6 Fat for more diet info).

Link to Study:
Wang J et al. 5-Lipoxygenase and 5-Lipoxygenase-Activating Protein Gene Polymorphisms, Dietary Linoleic Acid, and Risk for Breast Cancer. Cancer Epidemiol Biomarkers Prev October 1, 2008(17): 2748-2754

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