Friday, October 30, 2009

Can Omega-3 Fatty Acids (Fish Oil) Predispose Someone To Cancer?

Or to viral infection?

Omega-3 (n-3) fatty acids, which include longer-chain EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) and shorter-chain ALA (alpha-linolenic acid), have been shown to reduce inflammation and are beneficial for inflammatory conditions - those involving joints (rheumatoid arthritis), bowels (colitis, Crohn's), lungs (asthma, COPD), arteries (atherosclerosis), skin (psoriasis), and others.

Inflammation is thought to decrease via a change in activity or production of cells involved in the immune response. Our immune response is integral in keeping cancer (and infection) at bay.
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This study...
Leukocyte Numbers And Function In Subjects Eating N-3 Enriched Foods: Selective Depression Of Natural Killer Cell Levels, Arthritis Research and Therapy, 2008

... found that:
"Natural killer (NK) cell numbers were lower in n-3 supplemented subjects* than in controls, and were inversely related to the amount of EPA or DHA in erythrocytes."
So, the more omega-3 detected in red blood cells, the fewer natural killer cells the subjects had, and:
"NK cells are important in immune surveillance, particularly against viral infections and cancer. ... Individuals consuming these fatty acids may have greater susceptibility to viral infections. NK cells provide a first-line defense against these pathogens."
* Subjects consumed 1 gram EPA + DHA a day.
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This study described the role of immune-system cells in cancer:
The Innate Immune Response To Tumors And Its Role In The Induction Of T-Cell Immunity, Immunological Reviews, 2002
"Natural killer (NK) cells recognize many tumor cells but not normal self cells, and they are thought to aid in the elimination of nascent tumors."
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This study elucidated a type of n-3 that may be more immuno-suppressive:
Dietary Supplementation With Eicosapentaenoic Acid, But Not With Other Long-Chain N-3 Or N-6 Polyunsaturated Fatty Acids, Decreases Natural Killer Cell Activity In Healthy Subjects Aged >55 Y, American Journal of Clinical Nutrition, 2001
"Fish oil* caused a significant reduction (mean decline: 48%) in NK cell activity that was fully reversed by 4 wk after supplementation had ceased.

Thus, it might be inappropriate for groups at risk of viral infection and some cancers to increase their intake of EPA."
* Subjects consumed 1 gram EPA plus DHA a day. (720 mg EPA + 280 mg DHA)
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Other studies have shown that fish consumption or fish oil supplementation can be protective against cancer. It appears some antioxidant, like vitamin E, also needs be present - probably because it prevents oxidation of the easily-oxidized polyunsaturated omega-3 fats.

There is thinking that fish oil, and some forms of oil-based vitamin D, can be harmful to the liver.1 Anecdotally - some people experienced elevated liver enzymes while taking fish oil only to have them fall back into the normal range when they stopped. More study needed there.

Like most nutrients, there appears to be an ideal range for n-3 fatty acids in the body, or better, an ideal ratio of omega-6:omega-3, above and below which an individual can experience poor health.
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1 The Association Of Increasing Dietary Concentrations Of Fish Oil With Hepatotoxic Effects And A Higher Degree Of Aorta Atherosclerosis In The Ad Lib.-Fed Rabbit, Institute of Scientific and Technical Information (France), 1998

8 comments:

Jim Purdy said...

Today, in The Heart Scan Blog, Doctor William Davis has a post with the headline, "Fish oil makes you happy."

Happy cancer patients ... sounds unlikely.

Ruby said...

Interesting post, Bix. It's amazing how it's really about balance. Unfortunately, the way science is usually reported, people can get the impression that there's a magic bullet. But since we don't eat/live/breathe in a vacuum, we need to understand how everything we do affects everything else.

The more I learn, the more I get that message about balance: eating a variety of whole (non-processed) foods, leading an active life, and managing stress. Oh, and flossing.

Anonymous said...

The association of increasing dietary concentrations of fish oil with hepatotoxic effects and a higher degree of aorta atherosclerosis in the ad lib.-fed rabbit

I was browsing the rabbit abstract and thinking about my dogs home diet. Like people, their bodies don't manufacture n-6, essential to health. I finally settled on hemp seed oil for her. She eats primarily chicken, brown rice, and veggies. For me, it's fish oil.

Bix said...

Jim, I saw Dr. Davis' post. I agree, there does seem to be a relationship between omega-3 and depression. However, this relationship may have more to do with a high omega-6-to-omega-3 ratio that to absolute levels of omega-3.

Another way of achieving those higher relative amounts of omega-3, without taking fish oil, is to decrease the amount of omega-6 you eat. Americans' diets are awash in omega-6 ... from corn and soy and from livestock fed on corn and soy.

It may be that Inuits consuming a traditional diet had lower levels of omega-6 (the livestock they were eating were not fed grain). That would lower their n-6:n-3 ratio.

Ben P. DaSalt said...

"Another way of achieving those higher relative amounts of omega-3, without taking fish oil, is to decrease the amount of omega-6 you eat. Americans' diets are awash in omega-6 ... from corn and soy and from livestock fed on corn and soy." --Bix

"Another thing I learned doing this - When it comes to omega-3 content, there's hardly a difference between grass-fed beef and conventional beef. And neither of them provide more than a few milligrams in the raw state, probably negligible amounts after being cooked." --Bix

http://fanaticcook.blogspot.com/2009/04/omega-6-and-omega-3-in-foods.html

Bix said...

Not sure what you're trying to say there, Ben. But you'll notice that the livestock in that chart, both beef and chicken, have an omega-6 to omega-3 ratio of about 9:1, the beef that is pan-fried has an n-6:n-3 of 22:1, and the chicken nuggets an n-6:n-3 of 226-to-1! None of these foods are good choices if you're trying to get your n-6:n-3 down to about 2:1.

The foods that are good choices (if you're trying to get your n-6:n-3 down to about 2:1) would be anything that has a ratio less than 1. From this chart that would include seafood (tuna, shrimp, salmon), greens (spinach, broccoli, romaine, kale), beans (kidney beans), and flaxseeds.

Ben P. DaSalt said...

When you wrote "Americans' diets are awash in omega-6... from livestock fed on corn and soy" to my interpretation at least, it implies that livestock fed something else will alleviate the amount of omega-6 consumed in some dramatic manner.

I didn't think that is your position, but perhaps you had changed your mind since April, and there's nothing wrong with changing your mind, I just wanted to know if I missed something if you did.

Bix said...

I see. No, I did not change my mind. I still believe that what you feed an animal makes a difference in its lipid type and distribution.

From this same chart, conventional beef (which I understand to mean a cow that has eaten more grain) had an n-6:n-3 ratio that was more than 100% (more than double) the n-6:n-3 ratio of beef that was grass-fed.

Some of Cordain's work shows that wild ruminants have an even lower n-6:n-3 ratio than these grass-fed cattle, something on the order of 2.5:1 (much closer to a goal of 2:1) (grass-fed cattle from above had an n-6:n-3 of 4.8:1, almost double again what may have existed in meat before animal husbandry). See:

Fatty Acid Analysis Of Wild Ruminant Tissues: Evolutionary Implications For Reducing Diet-Related Chronic Disease:

http://www.nature.com/ejcn/journal/v56/n3/full/1601307a.html

As I have said a number of times, one way to increase the relative amount of anti-inflammatory omega-3 in your body (relative to pro-inflammatory omega-6), is to decrease the amount of omega-6.