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I read some articles and heard from people that eating fish and walnuts is very useful for brain, because it contains omega-3 acid. However I haven't ever understood completely what is meant by 'useful' here - most articles claim that it may prevent you from having mental/brain illnesses only:

Clinical studies have shown that healthy people with normal brain function did not see improvements in brain function after taking fish oil supplements.

So my question is - can brain capabilities (memory, learning ability) be improved by eating omega-3 enriched food?

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    Somewhat anecdotical, but this claim is very widespread in advertising for butter, oil or yogurts products enriched in omega 3 (at least in France) – Aserre Apr 5 '18 at 15:38
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    See also Are fish oil capsules beneficial? – ChrisW Apr 5 '18 at 21:21
  • I could be that "omega-3" in general shows no difference, but DHA (one of the omega-3 acids) does. The warning I have heard is: a child below the age of 2 should definitely not be on a low-fat diet, presumably because omega-3 acids are essential for brain development. – GEdgar Jan 4 at 15:20
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0192909

After the 16-week treatment period no statistically significant differences were found between treatment groups post-intervention.

...

376 children were randomized. Reading, working memory, and behavior change scores showed no consistent differences between intervention and placebo group.

https://www.ncbi.nlm.nih.gov/pubmed/29240924

Results:Current evidence indicates that n-3 LC-PUFAs administered during pregnancy or breastfeeding have no effect on the skills or cognitive development of children in later stages of development. Evidence regarding the improvement of cognitive function during childhood and youth or in attention deficit/hyperactivity disorder is inconclusive. Moreover, it is still unclear if n-3 LC-PUFAs can improve cognitive development or prevent cognitive decline in young or older adults.

https://www.ncbi.nlm.nih.gov/pubmed/22696350

AUTHORS' CONCLUSIONS:Direct evidence on the effect of omega-3 PUFA on incident dementia is lacking. The available trials showed no benefit of omega-3 PUFA supplementation on cognitive function in cognitively healthy older people. Omega-3 PUFA supplementation is generally well tolerated with the most commonly reported side-effect being mild gastrointestinal problems.Further studies of longer duration are required.

Summary: the trials/studies done do not support it.

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    One has to be skeptical whether the 16-week trial period of the first reference is sufficient to see any effect. The trial periods in the second reference are not specified. The third reference does indicate mostly longer trial periods, but is limited to dementia-related symptoms. – Daniel R Hicks Apr 7 '18 at 18:35
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There have been at least some studies indicating positive effects on cognition:

https://www.ncbi.nlm.nih.gov/pubmed/22018509

Supplementation with DHA-rich fish oil, in comparison with placebo, resulted in a significant increase in the concentrations of oxy-Hb and total levels of Hb, indicative of increased cerebral blood flow.

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0046832

pre-supplementation RBC DHA levels was predictive of baseline performance (i.e., adjusted hit rate, AHR on 3-back) on the n-back task (y = 0.19+0.07, r2 = 0.55, p = 0.009). In addition, subjects AHR performance improved on 3-back post-supplementation (pre 0.65±0.27, post 0.80±0.15, p = 0.04). The correlation between n-back performance, and DHA levels are consistent with reports in which higher DHA levels is related to improved cognitive performance.

https://www.ncbi.nlm.nih.gov/pubmed/16269019

Blood analyses showed that after Omega-3 supplementation the arachidonic acid/eicosapentaenoic acid ratio (AA/EPA) was strongly reduced. The mood profile was improved after Omega-3 with increased vigour and reduced anger, anxiety and depression states. This was associated with an effect on reactivity with a reduction of reaction time in the Go/No-Go and Sustained Attention tests.

Summary:

  • may increase cognitive performance
  • may increase cerebral blood flow
  • may increase mood & reaction time

Conclusion: conflicting results (see other answer) indicate more studies should be made

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