- Regular consumption of fish, especially oily fish, is associated with a reduced risk of Alzheimer's and better cognitive performance in old age.
- High levels of omega-3 oils are associated with less brain shrinkage, and less brain damage in old age.
- The benefits of supplements (as opposed to deriving these oils from food) is less clear, with inconsistent findings. It seems likely that their benefits depend on the individual's health and genetic profile, as well as on the nature and type of supplement.
There have been quite a few studies looking into the possible benefits of omega-3 fatty acids and fish (a good source of the oils), particularly for older adults. Several large studies have found that regular intake of oily fish is associated with lower rates of dementia, and some evidence that eating fish regularly slows the rate of 'normal' age-related cognitive decline.
However, findings into the effects of omega-3 oils have not been consistent, and one reason for that may lie in its interactions with other factors. For example, B vitamins have been found to improved cognition in older adults, but only when omega-3 levels are high. A number of studies have found regular consumption of fish is associated with reduced Alzheimer's risk, but one study found that, although it was associated with less Alzheimer's pathology, this was only in those with the 'Alzheimer's gene' (APOE4).
The same study found that fish oil supplementation was not associated with any differences in neuropathology — but higher levels of alpha-linolenic acid (an omega-3 fatty acid found in flaxseed, chia seeds, walnuts, etc) were associated with a reduced chance of brain infarctions. Another large study found that omega-3 supplements (fish-based, not plant), together with vitamins C, E, beta carotene, and zinc, had no effect on cognitive decline among older adults with age-related macular degeneration. But it was suggested that this may be because the participants were too old to benefit from them.
Among other groups, the question of whether supplements of omega-3 fatty acids can help memory and cognition has been even more contentious, with some studies showing a positive effect and others failing to find an effect.
My own take on this issue is that, like so many other things, it all depends on what you’re working with. The most important of these factors is surely diet — it would be unsurprising if supplements are only of benefit when the individual's diet is seriously deficient. It may also well be the case that genes are a factor. (I'm ignoring, but we shouldn't forget, that many of the studies are probably poorly done, for this is a problematic area and it is tricky to do well.) There does seem to be more evidence that a diet high in omega-3 oils is good, than that supplements are going to help. And the weight of the evidence certainly favors the importance of high omega-3 levels for older adults.
Although it's not yet clear which fatty acids are most important, one is definitely docosahexaenoic acid, or DHA. Salmon, mackerel, herring, sardines, and anchovies are all good sources (not, I am sorry to say, your standard fried fish from the chippie). Other sources include almonds, walnuts, soy, flaxseed oil, and eggs laid by chickens that eat DHA-supplemented feed.