Fatty Acid Composition Of Plasma Phospholipids And Risk Of Prostate Cancer In A Case-Control Analysis Nested Within The European Prospective Investigation Into Cancer And Nutrition, American Journal of Clinical Nutrition, 2008
It shows that high blood levels of the omega-3 fatty acid EPA was associated with increased risk for high-grade prostate cancer. As well, it shows an increased risk for high-grade prostate cancer with high levels of the omega-3 alpha-linolenic acid (the type of omega-3 found in plant sources such as flax and walnuts). (Some saturated fats also show increased risk for localized and advanced prostate cancers but I was more interested in the omega-3s right now.)
Just this week a study reported an increased risk for aggressive prostate cancer from high blood levels of the omega-3 fatty acid DHA.
There does seem to be something going on with omega-3 fats and more aggressive forms of prostate cancer. It's hard to tell just from these studies if the relationship involves intake. These are epidemiological studies so cause and effect is not clear. When it comes to cancer, we don't have the benefit of human intervention trials (which is why we don't know for certain if smoking causes lung cancer).
Other Known Drawbacks To Omega-3 Fatty Acids
Omega-3 fatty acids can be immunosuppressive:
- Omega-3 intake results in fewer immune cells such as natural killer (NK) cells. NK cells recognize tumors and are important against cancers and viral infections.
- Leukocyte Numbers And Function In Subjects Eating N-3 Enriched Foods: Selective Depression Of Natural Killer Cell Levels, Arthritis Research and Therapy, 2008
- Omega-3 intake reduces immune cell activity.
"Fish oil caused a significant reduction (mean decline: 48%) in NK cell activity that was fully reversed by 4 wk after supplementation had ceased."
- 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
- A significant increase in sudden cardiac death was associated with a group given fish oil capsules in this study.
- Secondary Prevention Of CHD In UK Men: The Diet And Reinfarction Trial And Its Sequel, Proceedings of the Nutrition Society, 2007
- This cardiac death was also seen in men with angina in the DART-2 trial, primarily those taking fish oil supplements.
- Lack Of Benefit Of Dietary Advice To Men With Angina: Results Of A Controlled Trial, European Journal of Clinical Nutrition, 2003
- DART-2 was included in a meta-analysis which stated:
"UK guidelines encourage the general public to eat more oily fish, and higher amounts are advised after myocardial infarction. This advice should continue at present but the evidence should be reviewed regularly. It is probably not appropriate to recommend a high intake of omega 3 fats for people who have angina but have not had a myocardial infarction."
- Risks And Benefits Of Omega 3 Fats For Mortality, Cardiovascular Disease, And Cancer: Systematic Review, British Medical Journal, 2006
How Much Fish Oil Is Good?
The Japanese in Okinawa in 1950, some of the longest lived people in the world, were consuming fish at about 1% of their total calories - half an ounce a day. That's about one 3-ounce serving of fish a week.
Mainland Japanese in the 1950s were consuming fish at about 4% of their total calories - 62 grams or 2 ounces a day. Recent data1 show Japanese to be eating about the same as they did then, ~60 grams a day. Depending on the fish (e.g. catfish, tuna, mackerel), that's 100-200 mg omega-3 a day.
So, a gram of omega-3s a day is more than 5 times the amount the Japanese eat, and 20 times the amount older Okinawans ate. Given the evidence accruing for harm linked to higher intakes of omega-3, it seems prudent to limit consumption - both of fish and fish oil.