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More omega-3, less omega-6 for colorectal protection

By Stephen Daniells , 12-Aug-2009

Increasing the intake of omega-3 fatty acids, and decreasing intakes of omega-6, could reduce the risk of colorectal cancer, suggests a new study from China.

The highest dietary ratio of omega-6 to omega-3 was associated with a 95 per cent increase in the risk of women developing colorectal cancer, according to results of a study with 73,242 Chinese women participating in the Shanghai Women's Health Study.

The study, published in this month’s issue of Cancer Epidemiology Biomarkers & Prevention, adds to a small but growing body of evidence supporting the importance of balance between omega-3 omega-6 fatty acids.

Previously, the ratio of omega-3 to -6 has been linked to prostate cancer risk (Clinical Cancer Research, Vol. 12, Issue 15, Journal of Clinical Investigation, July 2007).

According to Harvey Murff from Vanderbilt University and his co-workers, data on how polyunsaturated fatty acids (PUFA) may impact on the risk of colorectal cancer have been “inconsistent”.

Using data derived from two food frequency questionnaires, Murff and his co-workers investigated if PUFA intake could impact on colorectal cancer risk in Chinese women.

Their findings suggested that “the dietary total omega-6 to omega-3 PUFA ratio was strongly associated with colorectal cancer risk”. Indeed, increasing ratios of omega-6 to omega-3 were associated with increased risks of colorectal cancer. Compared to women with the lowest ratio, women with the highest ratio of omega-6 to -3 had a relative risk 95 per cent higher.

The omega-6 fatty acid arachidonic acid (AA) was also linked to an increased risk of colorectal cancer. Specifically, women with the highest average intakes had an associated risk 40 per cent higher than women with the lowest average intakes.

Previously, researchers from other groups have proposed the role of metabolites of omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and the omega-6 acid, arachidonic acid as playing a important role in carcinogenesis. These three fatty acids compete to be converted by cyclooxgenase enzymes (COX-1 and COX-2) into prostaglandins, which can become either pro-inflammatory and increase tumour growth, or anti-inflammatory and reduce growth.

Indeed, in a subset of 150 cancer cases and 150 healthy controls, the researchers noted that an increasing omega-6 to omega-3 ratio as linked to increased levels of the pro-inflammatory prostaglandin E2 (PGE2).

“These results suggest that dietary PUFA and the ratio of omega-6 to omega-3 PUFA intake may be positively associated with colorectal cancer risk, and this association may be mediated in part through PGE2 production,” concluded Murff and his co-workers.

There are 363,000 new cases of colorectal cancer every year in Europe, with an estimated 945,000 globally. There are about 492,000 deaths from the cancer each year. Only about five per cent of colorectal adenomas are thought to become malignant, and this process could take between five and ten years.

Source: Cancer Epidemiology Biomarkers & Prevention
2009 18: 2283-2291 doi: 10.1158/1055-9965.EPI-08-1196
“A Prospective Study of Dietary Polyunsaturated Fatty Acids and Colorectal Cancer Risk in Chinese Women”
Authors: H.J. Murff, X.-O. Shu, H. Li, Q. Dai, A. Kallianpur, G. Yang, H. Cai, W. Wen, Y-T. Gao, W. Zheng

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