DHA and Non-Fish-Eating Cultures
I just got back from AR2011 where someone asked me why vegans would need to take DHA if there are cultures that have lived without eating fish. I told him I don’t know the details about the health or DHA levels of such cultures.
What we do know is that vegans tend to have very low blood levels of DHA, even if they supplement with ALA. I pointed out that blood levels of DHA do not necessarily mean that vegans have unhealthfully low levels of DHA in other tissues, such as the brain, but until we know more, most vegans should take a moderate DHA supplement just to be safe.
One thing that could explain any phenomena of ancient cultures not eating fish, yet having adequate DHA status, is that our ratio of omega-6 to omega-3 is much higher today. This higher ratio hampers conversion of ALA to DHA; people in ancient cultures were probably able to convert ALA to DHA more efficiently.
People who can get their omega-6 to omega-3 ratio down to 4:1 or less, might be able to convert DHA efficiently. But I would recommend doing this without raising ALA intake much higher than 1.5 g per day (a level that has been associated with damage to the eyes). In today’s world, getting the ratio down to this level without adding large amounts of ALA to the diet would likely mean a very low fat diet which would be difficult to maintain for many people.