Data from the famed Physicians’ Health Study supports prior experimental findings; Omega-3s appear protective, omega-6s may raise risks
by Craig Weatherby
Late last month, we reported the results of a study in mice, which, like prior experiments in human cancer cells and animals, found that a high intake of omega-6 fatty acids, combined with a low intake of omega-3s, increased risk of prostate tumors.
Now, researchers at the Harvard School of Public Health say that their analysis of data from a large epidemiological investigation—the famed Physicians' Health Study—support the cancer-preventive potential of omega-3s and the risks of diets high in omega-6 fatty acids from common vegetable (seed) oils.
In 1982, researchers drew blood from 14,916 apparently healthy male physicians participating in the Physicians' Health Study, and froze it.
In 1995, new blood samples were taken from the 476 participants who had been diagnosed with prostate cancer during the intervening 13 years, and from healthy controls with similar characteristics.
When the scientists analyzed the 1982 and 1995 blood samples from the cancer patients and the healthy controls, they found markedly different fatty acid profiles (Chavarro JE et al 2007).
During the 13-year period, the men with lower average blood levels of omega-3s, and higher average blood levels of two omega-6 fatty acids were about 50 percent more likely to be in the group with cancer.
The two omega-6 fats associated with higher cancer risk are gamma-linolenic acid (GLA) and dihomo-gamma-linolenic acid (DGLA).
These omega-6 fatty acids are breakdown products (i.e., metabolites) of a dietary omega-6 called LA (linoleic acid).
As it happens, LA is the fatty acid most abundant in the American diet, because it is the dominant fat in our most common vegetable oils (corn, cottonseed, soy, canola, sunflower, safflower), and therefore in the processed and prepared foods made with these oils.
Conversely, during the 13-year study period, the healthy controls had higher average blood levels of omega-3s and lower average levels of the two omega-6 metabolites associated with increased prostate risk, known as gamma-linolenic acid (GLA) and dihomo-gamma-linolenic acid (DGLA).
Chronic inflammation is known to promote cancer.
Interestingly, one of the two metabolic breakdown products of omega-6 associated with increased prostate cancer risk—omega-6 GLA—is generally anti-inflammatory.
And despite a well-established link between inflammation and cancer, the men with higher blood levels of a long-chain omega-6 called arachidonic acid (AA)—which is used to make pro-inflammatory messenger chemicals—did not have higher cancer risk.
Finally, like men with higher blood levels of omega-3s, men with higher blood levels of omega-6 LA itself—as opposed to its two omega-6 metabolites—were also less likely to have cancer.
This, it appears possible that the ability of dietary omega-6 LA to promote cancer via it metabolites GLA and DGLA may be influenced by genetic or other dietary factors, including omega-3 intake.
This analysis adds to a fast-growing pile of evidence suggesting the vital significance of the omega-3/omega-6 ratio.
It looks smart to strive for a “Japanese-style” intake ratio of about four parts omega-6s to one part omega-3s, in contrast to the average American’s grossly imbalanced intake ratio… about 30 parts omega-6 to one part omega-3s.
- Chavarro JE, Stampfer MJ, Li H, Campos H, Kurth T, Ma J. A prospective study of polyunsaturated fatty acid levels in blood and prostate cancer risk. Cancer Epidemiol Biomarkers Prev. 2007 Jul;16(7):1364-70. Epub 2007 Jun 21.
- Gann PH, Hennekens CH, Sacks FM, Grodstein F, Giovannucci EL, Stampfer MJ. Prospective study of plasma fatty acids and risk of prostate cancer. J Natl Cancer Inst. 1994 Feb 16;86(4):281-6. Erratum in: J Natl Cancer Inst 1994 May 4;86(9):728.
- Harvei S, Bjerve KS, Tretli S, Jellum E, Robsahm TE, Vatten L. Prediagnostic level of fatty acids in serum phospholipids: omega-3 and omega-6 fatty acids and the risk of prostate cancer. Int J Cancer. 1997 May 16;71(4):545-51.
- Hall MN, Campos H, Li H, Sesso HD, Stampfer MJ, Willett WC, Ma J. Blood levels of long-chain polyunsaturated fatty acids, aspirin, and the risk of colorectal cancer. Cancer Epidemiol Biomarkers Prev. 2007 Feb;16(2):314-21.