Shaper noted an absence of coronary heart disease in Kenyans. Burkitt pondered the rarityof bowel cancer among Nigerians. And Bang wondered why there wasn't much heart disease among Eskimo in Greenland, who live on a diet of meat and fat. Thus started three great stories in dietary epidemiology: cholesterol, fibre and fish oil.
Here is the latest review of the fish story. Many points of interest but don't try to read it all in one sitting or your brain will explode. There is a clear explanation of exactly what an omega-three oil is; a good review of trials of omega-3 oils in both primary and secondary prevention; and a new use of the Number Needed to Treat, or perhaps in this case the Number Needed to Catch: how many fish do you have to eat to prevent one heart attack? (Answer: pretty much all the fish in an average-sized ocean.)
The review also points out the need for exact detail in dietary trials - not just 'fish oil supplements' but what dose? And also detail on the subjects - what do they normally eat? In Japan everyone eats a lot of fish anyway, so offering yet more fish oil as a pill isn't likely to make much difference.