A High Ratio Of Dietary N-6/N-3 Polyunsaturated Fatty Acids Is Associated With Increased Risk Of Prostate Cancer, Nutrition Research, January 2011
Our results showed no significant associations between specific n-3 or n-6 PUFA intakes and prostate cancer risk.The odds ratios were startlingly high.
Our findings suggest that a high dietary ratio of n-6/n-3 fatty acids may increase the risk of overall prostate cancer among white men and possibly increase the risk of high-grade prostate cancer among all men.
Here's a table I made last year that shows omega-6 and omega-3 content of some common foods:
Click to enlarge.
The best way to increase your relative omega-3 is to decrease your omega-6 intake. If you eat a sizable amount of omega-6, then eating more fish or other foods high in omega-3, or taking fish oil, may just increase your overall fat intake without improving your ratio.
Vegetable oils are high in omega-6. As you can see from the chart, nuts, seeds and oils made from them (e.g. corn oil and soy oil) are leading sources of omega-6 in Americans' diets. (The grains themselves, like corn or rice, are not as much of a problem since they are low in fat and provide little omega-6 per serving.)
Note that greens (romaine, broccoli, kale, spinach) generally have more omega-3 than omega-6 and so are excellent ways to improve your ratio. Walnuts on the other hand have more omega-6 than omega-3 and so are not a good way to improve your ratio.
Both omega-3 and omega-6 are polyunsaturated fats. If you take extra omega-3, in pill or food, you are intaking more polyunsaturated fat. These fats have highly-reactive double bonds and oxidize quickly, especially when they meet acids in your stomach. Oxidized fats, and their metabolites, have been shown to increase risk for atherosclerosis and some cancers.
A good ratio to shoot for is 4:1 or less. That's 4 units (or less) of omega-6 for every one unit of omega-3.