By themselves, diets high in omega-3s are not proven to prevent or cure cancer.
This is the U.S. government position, based on an evidence review it commissioned eight years ago (MacLean CH et al. 2006).
However, a large number of studies, including many published since 2006, suggest that diets high in omega-3s tend to discourage cancer development and growth.
And the few clinical trials on the subject suggest that diets high in omega-3s may ease the wasting and fatigue associated with cancer and chemotherapy and boost the efficacy of conventional therapies.
Cancer prevention and treatment may also be aided by diets far lower in omega-6s fats than is the American norm … see our sidebar, “Omega-6 overload fuels cancer”.
Omega-6 overload fuels cancer
We've covered many studies that highlight the health risks of America's “omega imbalance”.
For example, see:
Most of the huge excess of omega-6 fats in the American diet comes from cheap vegetable oils (corn, soy, safflower, cottonseed, sunflower) and prepared/packaged foods containing them.
Now, an evidence review by American scientists suggests that diets rich in fatty fish may help prevent development of adenocarcinomas … a very common type of cancerous tumor
Omega-3s may curb common cancer type
The new study was led by James J. DiNicolantonio, PharmD, from Saint Luke's Mid America Heart Institute (DiNicolantonio JJ et al. 2014).
DiNicolantonio and his co-authors concluded that diets rich in fatty fish probably help prevent adenocarcinomas … a common type of cancerous tumor.
A large proportion of breast, prostate, pancreas, colon, and gastrointestinal cancers are adenocarcinomas.
Their paper starts by pointing out that daily low-dose aspirin reduces the risk for adenocarcinomas, due to its ability to modestly decrease the activity of COX-2 … a pro-inflammatory enzyme that promotes these tumors' development and growth.
The Saint Luke's team notes that, like aspirin, the omega-3 fats in seafood – which abound in fatty fish – also dampen COX-2 activity, and should thereby reduce risk for adenocarcinomas (see “Aspirin Mimics a Fishy Omega-3
However, epidemiological (diet-health) studies in some populations have failed to show a protective effect of fish consumption.
This is particularly true for “meta-analyses”, which examine the results of a number of studies to come to an overall conclusion.
The authors say that the failure of meta-analyses to detect a protective benefit from seafood-source omega-3s reflects two factors:
Fish consumption in many of the populations studied was too low to produce the body levels of omega-3s needed for cancer prevention.
The kind of fish consumed – and how it is preserved or cooked – can have a major impact on its potential to lower cancer risk.
Accordingly, the Saint Luke's team examined studies in which much of the surveyed population consumed fish oil or fatty, non-fried fish frequently, and had low intakes of competing omega-6 vegetable fats.
Encouragingly, their analysis supports the idea that the omega-3s in fatty fish and fish oil should help protect against common, COX-2-driven, adenocarcinoma tumors.
Why some studies see no cancer-preventing effect
The authors cite several reasons why many studies have failed to link diets rich in fatty fish or fish oil to reduced cancer risk.
These reasons fell into three broad categories:
Omega-3 intakes were too low
- Relatively Low Fish Intake: This was often true even among the “high-intake” part of the studied population. The average American's daily intake of the two key omega-3s in fish – DHA and EPA – is only about 100 mg. However, the authors say that it likely requires omega-3 intakes 10 times higher than that (1000mg daily) to impact COX-2 activity. In most studies, even the heavier fish consumers were unlikely to achieve those omega-3 intake levels.
- Lean, not Fatty Fish. Most participants ate lean fish low in omega-3s. Some studies examined by the authors linked intake of fatty fish with cancer protection, whereas no such link is seen with intake of lean fish. This is consistent with other evidence that the omega-3 fats in fish (DHA and EPA) are primarily responsible for the cancer protection afforded by fish consumption.
Omega-6 intakes were too high
- High Vegetable Oil Intake: In many studies, the participant's diets were high in omega-6-rich oils (soy, safflower, corn, sunflower, cottonseed). Omega-6 fats promote COX-2 activity and compete with omega-3s. Thus, a high intake of omega-6 fats raises the intake of omega-3 required for cancer protection. As the authors note, Mediterranean peoples who use olive oil (low in omega-6s) almost exclusively are more likely to show a protective effect from frequent fish consumption.
- Farmed Fish. Some study participants ate lots of farmed tilapia, catfish, or salmon. Because they are fed grains, farmed fish have much higher omega-6 levels, lower omega-3 levels, and a higher omega-6/omega-3 ratio than wild fish.
- Participants often ate breaded, deep-fried fish. Frying breaded fish in omega-6-rich vegetable oils sharply raises its omega-6 content and omega-6/omega-3 ratio, and the oil can contain cancer-promoting compounds created by high frying temperatures. This also explains why diets high in fried fish are linked to higher stroke risk, versus diets high in non-fried fish (e.g., poached, sautéed, baked, broiled, roasted) … see “Fried Fish Seen to Raise Stroke Risk”.)
Presence of cancer-promoting contaminants
- Salt-Preserved Fish: In some studies – particularly Asian and Nordic ones – salt-preserved fish was the predominant type consumed, and it commonly contains mutagens that promote cancer.
“An easy way to see the benefit of omega-3s is to look at Italy,” Dr. DiNicolantonio said. “The staple oil used in cooking and as a salad dressing in Italy is olive oil, which is quite low in omega-6. Meanwhile, fish – high in omega-3 – is a staple food in the Italian diet, and this fish is rarely salt-preserved or fried.” (SL 2014)
“In Italian studies, subjects who consumed fish at least twice weekly as compared to those who ate fish less than once a week, were found to be at a significantly lower risk for a number of cancers, including ovarian, endometrial, pharyngeal, esophageal, gastric, colonic, rectal, and pancreatic.” (SL 2014)
The authors also focus on several recent studies in which regular consumption of fish oil is correlated with lower subsequent cancer risk.
These studies have detected lower risks for colorectal, breast, and advanced prostate cancer in people taking omega-3 fish oil supplements.
And a recent study from the University of Washington – which estimated its participants' omega-3 intakes from fish and fish oils – linked high omega-3 intakes with a 23 percent reduction in total cancer mortality (Bell GA et al. 2014).
Indeed, death from all causes in that study was significantly lower in those with higher omega-3 intakes.
The authors also noted that the cancer-promoting COX-2 enzyme is significantly active in pre-malignant and early stage adenocarcinomas, but often reduces as these cancers mature.
They believe that this may be why damping of COX-2 activity via increased omega-3 intake seems to have greater potential for cancer prevention than for cancer therapy.
Agency for Healthcare Research and Quality (AHRQ). Diet High in Omega-3 Fatty Acids Unlikely to Reduce Risk of Cancer. January 24, 2006. Accessed at http://archive.ahrq.gov/news/press/pr2006/o3cancerpr.htm
Agency for Healthcare Research and Quality (AHRQ). Effects of Omega-3 Fatty Acids on Cancer Summary Evidence Report/Technology Assessment: Number 113. Accessed at http://archive.ahrq.gov/clinic/epcsums/o3cansum.htm
Bell GA, Kantor ED, Lampe JW, Kristal AR, Heckbert SR, White E. Intake of long-chain ω-3 fatty acids from diet and supplements in relation to mortality. Am J Epidemiol. 2014 Mar 15;179(6):710-20. doi: 10.1093/aje/kwt326. Epub 2014 Feb 3.
DiNicolantonio JJ, McCarty MF, Chatterjee S, Lavie CJ, O'Keefe JH. A Higher Dietary Ratio of Long-Chain Omega-3 to Total Omega-6 Fatty Acids for Prevention of COX-2-Dependent Adenocarcinomas. Nutr Cancer. 2014 Oct 30:1-6. [Epub ahead of print]
MacLean CH, Newberry SJ, Mojica WA, Khanna P, Issa AM, Suttorp MJ, Lim YW, Traina SB, Hilton L, Garland R, Morton SC. Effects of omega-3 fatty acids on cancer risk: a systematic review. JAMA. 2006 Jan 25;295(4):403-15. Review. Erratum in: JAMA. 2006 Apr 26;295(16):1900.
McCarty MF. Minimizing the cancer-promotional activity of cox-2 as a central strategy in cancer prevention. Med Hypotheses. 2012 Jan;78(1):45-57. doi: 10.1016/j.mehy.2011.09.039. Epub 2011 Oct 15.
Papanikolaou Y, Brooks J, Reider C, Fulgoni VL 3rd. U.S. adults are not meeting recommended levels for fish and omega-3 fatty acid intake: results of an analysis using observational data from NHANES 2003-2008. Nutr J. 2014 Apr 2;13:31. doi: 10.1186/1475-2891-13-31. Erratum in: Nutr J. 2014;13:64.
Saint Luke's Mid America Heart Institute (SLMAHI). Diets High in Long-chain Omega-3 Fatty Acids Shown to Help Prevent Cox-2 Dependent Adenocarcinomas. November 06, 2014. Accessed at http://www.prweb.com/releases/2014/11/prweb12301879.htm