Not that this study proves anything, and it's certainly a fish out of water where fish oil studies are concerned. But ...
Dietary Omega-3 Polyunsaturated Fatty Acids Promote Colon Carcinoma Metastasis In Rat Liver, Cancer Research, 1998.
Rats were fed either a:
- Low-fat diet
- Fish oil diet (source of omega-3 fatty acids)
- Safflower oil diet (source of omega-6 fatty acids)
Findings: (The figure at right shows livers of rats on the low-fat diet (A - Top), the safflower oil diet (B - Middle), and the fish oil diet (C - Bottom). The arrows show tumors. Click for larger.)
"At 1 week after transplantation, the fish-oil diet had induced 7-fold more metastases (in terms of number and size) than had the low-fat diet, whereas the safflower oil diet had not affected the number and total volume of metastases."Both the omega-3 diet and the omega-6 diet (both polyunsaturated fats) promoted the growth of liver tumors when compared to the low-fat diet -- with the omega-3 (fish-oil diet) more than twice as risk-promoting as the omega-6 (safflower oil) diet, in terms of both number of tumors formed, and size.
"At 3 weeks after tumor transplantation, the fish oil diet and the safflower oil diet had induced, respectively, 10- and 4-fold more metastases (number) and over 1000- and 500-fold more metastases (size) than were found in the livers of rats on the low-fat diet."
"The effects of omega-3 PUFAs on the proliferation of colon cancer cells and on the growth of metastases in the liver are stronger than the effects of omega-6 PUFAs."
Omega-3 fatty acids can suppress the immune system. That's one supposed mechanism for their anti-inflammatory effect. A suppressed immune system could hinder natural cancer-fighting pathways. However, these researchers' analysis "revealed that the immune system in the liver (Kupffer cells, pit cells, T cells, newly recruited macrophages, and the activation state of macrophages) did not play a significant role in this diet-dependent outgrowth of tumors," causing them to conclude that "the enormous effect of omega-3 PUFA on colon cancer metastasis in the liver is not mediated via alterations of the immune system."
There goes that theory.
Another study I saw, "Does Dietary Alpha-Linolenic Acid Promote Liver Metastases In Pancreatic Carcinoma Initiated By BOP In Syrian Hamster?" from Annals of Nutrition and Metabolism, 1999, found something similar. Omega-3 fat (alpha-linolenic acid: ALA, the type of omega-3 found in flax seeds) increased liver metastases, this time from pancreatic cancer, and did so dose-dependently, meaning the more omega-3 they gave the hamsters, the more liver cancer they got.
Maybe omega-3 helps in cancer prevention, but once you have cancer, these studies show ingesting omega-3 can hasten its progression.