A Norwegian study has found a link between eating fish and improved mental skills in older people, a Dutch study found a link between higher omega 3 in the blood and lower mental decline, while a New Zealand study found a link between omega 3 levels in the blood and better physical health, although the link to better mental health was less convincing.

All three studies are published in the November issue of the American Journal of Clinical Nutrition.

For the Norwegian study, which was conducted by researchers from the University of Oslo and other colleagues, more than 2,000 people aged 70 - 74, roughly half men and half women, were recruited from the general population in Western Norway. The researchers found that those participants who ate more than 10 grams (a third of an ounce) of seafood a day scored significantly higher in cognitive performance tests than those who ate less.

The participants completed a battery of 6 cognitive, verbal, memory and learning tests. The results showed that those participants whose mean daily intake of fish or seafood was equal to or higher than 10 grams a day (the vast majority of the cohort) had significantly better mean scores and a lower rate of poor cognitive performance (defined as scoring in the worst ten per cent of each test) than those who ate less than 10 grams of seafood a day (a tiny minority of under 100).

They found the link became stronger the more seafood the participants consumed, with the highest test scores occurring at 75 grams of seafood a day. The effect was strongest for non-processed lean fish and fatty fish, said the researchers.

They concluded that:

"In the elderly, a diet high in fish and fish products is associated with better cognitive performance in a dose-dependent manner."

For the Dutch study, which was conducted by researchers from Wageningen University and other colleagues, over 800 people aged 50 to 70 were examined. The researchers found that the cognitive skills of those participants with higher blood levels of omega 3 fatty acids at the start of the study declined more slowly over three years. The link was strongest for tests where participants had to respond quickly, but there was no link found in more general tests of mental ability.

Omega 3 fatty acids, also called n-3 PUFAs (long chain polyunsaturated fatty acids with 3 double bonds) are found in a range of seafoods and plants. The main types are DHA, EPA and ALA. DHA and EPA are found in fatty fish like salmon and mackerel; ALAs, are found in vegetables, seeds and nuts, for instance spinach, flax seeds and walnuts.

The researchers used data from the FACIT trial, which was primarily designed to look at the effect of folic acid on mental skills. The participants took folic acid or placebos for three years, but the researchers were also able to use the blood samples and cognitive test results to look at the link between other nutrients and mental skills.

The results showed that higher blood levels of n-3 PUFAs were linked with slower decline in sensorimotor speed and complex speed over three years. They were not linked with changes in memory, information processing speed or word fluency, said the researchers. Also, a cross-sectional analysis showed no links between blood levels of n-3 PUFAs and performance in 5 areas of cognitive ability. This is an interesting contrast to the Norwegian study.

The researchers concluded that:

"In this population, plasma n-3 PUFA proportions were associated with less decline in the speed-related cognitive domains over 3 y."

They said these results need to be confirmed with controlled clinical trials.

The New Zealand study, which was carried out by researchers from the University of Otago and other colleagues, involved about 2,400 participants aged 15 and over who gave blood samples and filled in questionnaires about their mental and physical health. The study found strong links between the omega 3 fatty acid EPA and self-reported physical health but the evidence linking this nutrient and mental health was "less compelling" said the researchers.

Finally, in an editorial in the same edition of the journal, Dr Irwin Rosenberg, Jean Mayer Professor of Nutrition, from the Friedman School of Nutrition Science and Policy at Tufts University in Boston, USA, said that while the studies are interesting, they don't prove that consuming seafood and omega 3 fatty acids caused the observed improvements or lack of deterioration. Only randomized clinical trials can do this, he wrote.

"Cognitive performance among the elderly and dietary fish intake: the Hordaland Health Study."
Eha Nurk, Christian A Drevon, Helga Refsum, Kari Solvoll, Stein E Vollset, Ottar Nygård, Harald A Nygaard, Knut Engedal, Grethe S Tell, and A David Smith.
American Journal of Clinical Nutrition, Vol. 86, No. 5, 1470-1478, November 2007.
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"n-3 Fatty acid proportions in plasma and cognitive performance in older adults.
Carla Dullemeijer, Jane Durga, Ingeborg A Brouwer, Ondine van de Rest, Frans J Kok, Robert-Jan M Brummer, Martin PJ van Boxtel, and Petra Verhoef.
American Journal of Clinical Nutrition, Vol. 86, No. 5, 1479-1485, November 2007.
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"Serum phospholipid n-3 long-chain polyunsaturated fatty acids and physical and mental health in a population-based survey of New Zealand adolescents and adults."
Francesca L Crowe, C Murray Skeaff, Timothy J Green, and Andrew R Gray
American Journal of Clinical Nutrition, Vol. 86, No. 5, 1278-1285, November 2007.
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Written by: Catharine Paddock