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.
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.
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."
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.* Subjects consumed 1 gram EPA plus DHA a day. (720 mg EPA + 280 mg DHA)
Thus, it might be inappropriate for groups at risk of viral infection and some cancers to increase their intake of EPA."
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.