I think it's based on one population, and not a very strong correlation:onlyifchased wrote: Related nutrition question since this thread is discussing it: I've heard it's not good to eat TOO much ALA (walnuts, flax, etc.) since it's bad for the eyes. I can't find a good source for this info though. Anyone heard of this?
http://veganhealth.org/articles/omega3#Eyesight
We have no mechanistic evidence (I tend to hold mechanistic evidence in much higher regard than correlations in population studies, the latter just give us ideas of what to look into), so it's very likely a coincidence or from some kind of confounding variable. It may arise from damaged fats due to high temperature cooking of ALA, Norris seems to agree those are probably dangerous.
The best advise would be to not cook your oils for a long time or at high temperature, especially if exposed to oxygen, and particularly canola. I try to use them uncooked, or just in baking, and usually cook with monounsaturated fats if I'm frying anything.
None of this is really relevant to raw flax or walnuts.
All of that said, even if it is true, it takes a very long time to show up if at all, and macular degeneration is non-lethal. I'd be more worried about stroke, heart attack, and cancer.