China Study Shows Dietary Arachidonic Acid Ups Risk for Colorectal Cancer
Of particular interest is the prostaglandin E2 (PGE2), a pro-inflammatory compound, which is associated with a high risk of colorectal cancer. Notably, PGE2 is derived from arachidonic acid, which is facilitated by the cyclooxygenase (COX) enzymes. But the omega-3 polyunsaturated fatty acid, EPA, also uses COX enzymes, which form other prostaglandins.
Study: Scientists wanted to explore if diets lower in arachidonic acid (and other omega-6 fats), in conjunction with higher EPA and other omega-3 fats, are associated with a lower production of PGE2, and ultimately, a lower risk of colorectal cancer.
Data from a large prospective cohort study on nearly 75,000 Chinese women (from the Shanghai Women’s Health Study) were evaluated for a urinary metabolite of PGE2, dietary fatty acids, diet, lifestyle and medication use. They found a strong association of colorectal cancer risk with the:
• Ratio of dietary omega-6 to omega-3 polyunsaturated fatty acids
• Absolute intake of dietary arachidonic acid
The research team also found a possible reason for conflicting results from other studies, which showed no relationship between dietary omega-3 and omega-6 and colorectal cancer risk.
Previous studies show that taking fish oil supplements suppress PGE2, only if people eat a diet with a low ratio of omega-6 to omega-3 fats of 2.5:1. The fish oil benefit was not achieved with higher dietary ratios. Therefore, it’s possible that benefits of eating a low ratio of omega-6 to omega-3 fats is only detectable below an absolute threshold.
Notablely, the Chinese women in this study ate a median daily linoleic acid intake of 5.9 grams, with a ratio of omega-6 to omega-3 fats of about 6:1. Whereas, American women eat nearly double that amount of omega-6 fat (11 grams per day), with a ratio of omega-6 to omega-3 fats of about 16:1. Therefore, the lack of apparent association in previous studies may be due to consumption omega-6 fats too high above the potential toxic threshold level to see an effect.
Quote: “These findings are intriguing and suggest that dietary fatty acid intake could alter the production of inflammatory prostanoids and consequently the risk of colorectal cancer but that this protective effect may only be at earlier stages of colon carcinogenesis.”
Comment: These findings are especially notable, given that a study published this week in the Journal of the American Medical Association (August 12) showed that taking aspirin improves survival of patients diagnosed with colorectal cancer. Aspirin inhibits COX-2 enzymes, which are over-expressed in 80-85% of colorectal cancers. Recall that COX enzymes convert arachidonic acid into (PGE2), the pro-inflammatory compound, which is associated with a high risk of colorectal cancer.
Imagine the possibilities of an intervention study on colorectal cancer patients, evaluating the efficacy of a low-omega-6 fat diet (with adequate omega-3 fatty acids). Unlike aspirin, there are no side-effects from eating a lower omega-6 diet (which in essence, is the classic Mediterranean diet, the components of which, were replicated in the Lyon Diet Heart trial).
For information on dietary sources of omega-6 fats, see:
- How Much Arachidonic Acid in Meats and Poultry? The Free Omega-6 Fat Tracker Widget
- How Much Omega-6 in Vegetable Oils? The Omega-6 Fat Tracker Widget
Murff HJ, Shu XO, Li H, Dai Q, Kallianpur A, Yang G, Cai H, Wen W, Gao YT, and Zheng W.
A Prospective Study of Dietary Polyunsaturated Fatty Acids and Colorectal Cancer Risk in Chinese Women.
Cancer Epidemiol. Biomarkers Prev. 2009 18: 2283-2291.
Andrew T. Chan; Shuji Ogino; Charles S. Fuchs
Aspirin Use and Survival After Diagnosis of Colorectal Cancer.
JAMA. 2009;302(6):649-658.
Posted by Evelyn Tribole MSRD at August 13, 2009 11:02 AM 
Categories: Arachidonic Acid,Linoleic Acid,Diet,commentary,Cancer,omega-6/omega-3 Ratio,2009 Studies,Cox,polyunsaturated fatty acids,fish oil,Biomarkers,EPA,Abstract
Categories: Arachidonic Acid,Linoleic Acid,Diet,commentary,Cancer,omega-6/omega-3 Ratio,2009 Studies,Cox,polyunsaturated fatty acids,fish oil,Biomarkers,EPA,Abstract
http://www.prevention.com/cda/article/the-vanishing-youth-nutrient/6dec72fe5deb2210VgnVCM10000030281eac____/news.voices/in.the.magazine/september.2009.issue/0/0/1
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Susan--thanks for sharing the full-text link to your story on the imbalance between dietary omega-3 fats (too little) and omega-6 polyunsaturated fatty acids (too much).
[Note: Science writer, Susan Allport, is the author of the Queen of Fats: Why Omega-3s Were Removed from the Western Diet and What We Can Do to Replace Them.]
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