The researchers found no differences in declines in brain sharpness among the women who had high levels of omega 3s in their blood compared with those with low levels of omega 3s.

So why the negative finding? Well, the researchers didn't know what the women's eating habits were before or after the study — or how their habits may have changed during the study.

So, for instance, if the women's fish consumption or supplement regimen changed after the time of the first blood test, this study wouldn't have picked it up. That's one explanation.

It's also possible that the study wasn't conducted for a long enough period of time, or that the age of the women made it hard to suss out potential long-term benefits.

The other possibility? Maybe a steady intake of omega-3s doesn't really lead to any measurable benefits in staving off cognitive decline.

One other note about the study design: It didn't track how participants got their omega-3s — whether it was through food or through fish oil supplements.

As study author Eric Ammann, of the University of Iowa, points out in an email, "most randomized trials of omega-3 supplements have not found an effect on cognitive function."

He cites this meta-analysis, which concluded that taking omega-3 supplements does not seem to help healthy, older people fend off cognitive decline.

So, this raises a question: If you eat fish, rather than take a fish-oil supplement, is there more likely to be a benefit? There's more than a suggestion that this is indeed the case.

For instance, a study of older folks (65 and older) enrolled in the Chicago Health and Aging Project found that people who were eating two or more fish meals per week had a slower rate of cognitive decline — about 13 percent slower — compared with those who ate fish less than once a week.

"When you eat fish, there are other nutrients such as vitamin E or vitamin D" that you're getting at the same time, says researcher Rosebud Roberts of the Mayo Clinic. In other words, it's the whole food, as a package, that may be beneficial.

And this seems to be the general picture emerging in human health: getting nutrients and healthful fats from the foods we eat as part of a healthful diet, rather than from supplements, may be the way to go.

In fact, there's increasing evidence, as outlined by Paul Offit, that we do our bodies no favors by taking a daily regimen of vitamins or supplements.

Copyright 2013 NPR.

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Why Can't Fish Oil Supplements Keep Our Brains Sharp?