So you can reduce your dementia incidence risk from Q1 -> Q5 by a whopping 0.08%-points.
But in media you surely will read about a 40% reduction.
*edited: %-points instead of %
*edited: %-points instead of %
EDIT: don't assume the causality
That 0.08% reduction would mean approximately 28,000 fewer EOD cases - not to be sniffed at!
What would it mean for salmon stocks and increased environmental damage?
To be clear my preference would be to source n3s from algal supplements and, once food safety testing for humans is complete, n3s from GM rapeseed.
In time I hope we end up with lab meat/plant-based meat alternatives that use these n3s so we can get the benefits of fish without the environmental and ethical concerns of getting n3s from fish.
I don't know enough about any of this to have an informed opinion, but I do understand that menhaden put Reedville, VA on the map.
And to be clear, many things that people worry about is less likely than that. Homicides (over an 8 year period about about 0.04 per 1000 people), terrorism (vanishingly small), and on and on.
None of this means that people should stock up on omega-3s, and as likely the study is actually finding a correlation with something else (e.g. wealthier people enjoy more fish rich diets and are less exposed to toxins, or something else), but halving something terrifying that isn't that uncommon is legitimately newsworthy.
One should weigh the cost of the proposed intervention in time/money/other_expense against the potential benefit. The potential benefit is the total reduction in risk * the magnitude of the unwanted outcome.
40% is less relevant.
For example you see this oft-quoted stat about "statins only increase lifespan by 3 days" based off relatively short RCTs, but this doesn't capture the effect of statin use over decades, which is where we see much, much bigger gains.
It seems to me that both RR and AR are things to take into consideration and we have to be mindful of the shortcomings of each.