Last month, Swedish researchers reported positive results from the first-ever clinical trial designed to determine whether supplemental omega-3s can help prevent or ameliorate Alzheimer's disease: the most common form of dementia.

Fortunately, they found that fish oil capsules reduced progression of Alzheimer's disease in people with mild, early-stage cases of the dreaded disorder. (See "Fish Oil May Halt Memory Decline in Alzheimer's”.)

Now, a mere month after release of the Swedes' landmark results, American researchers present us with yet more evidence of the dementia-discouraging effects of fish and fish oil, and add a scientifically significant detail (Schaefer EJ et al 2006).

The new findings suggest that one of the two key omega-3s in fish fat—called DHA—may deserve the most credit for the brain-preserving benefits of fish fat.

USDA-Tufts team probes DHA's role in dementia-prevention
In 2003, after analyzing data from participants in the world-famous Framingham Heart Study, a team led by Ernst Schaefer, M.D. report a major finding with regard to fish oil and dementia. (Dr. Schaefer performs research at the USDA-funded Human Nutrition Research Center on Aging at Tufts University in Boston.)

The Tufts team divided the study population into four groups (quartiles) reflecting the participants' differing blood levels of omega-3 DHA.

After accounting for other risk factors, they found that the volunteers with the highest blood levels of DHA were 47 percent less likely to have developed dementia and 39 percent less likely to have developed Alzheimer's disease,

compared with people in the three quartiles with lower blood levels.

The participants with the highest blood levels of DHA reported eating an average of 2.9 fish servings (about 10 ounces) a week: an intake level that would deliver an average of 0.18 grams of DHA a day.

The participants in the lower-DHA-level quartiles reported eating 1.3 to 2.3 servings of fish per week, on average.

This nine-year-long branch of the Framingham Heart Study included 899 men and women (average age 76) who were participating in the Framingham Heart Study.

The researchers took blood samples from and administered mental tests to all 899, while subgroup of 488 also filled out a food-frequency questionnaire.

After nine years, there were 99 new diagnoses of dementia, including 71 cases of Alzheimer's disease, among the participants.

We should note that the study had two limitations:
  • DHA measurements were taken only once
  • Food intake data was only available from about half of the participants
Still, as Dr. Martha Clare Morris of Chicago's Rush University Medical Center said in an accompanying editorial, "The study… is an important contribution to a young field of study on diet and neurodegenerative diseases.… [it] provides the first evidence that… [higher levels of] DHA in human [blood are] … related to lower Alzheimer disease risk.”

Why was DHA brain-protective but not fish or EPA?
It seems odd and therefore significant that reduced risk of dementia in this study was linked to participants' blood DHA levels, but not with their fish intake or with their blood levels of EPA, the other key omega-3 fatty acid in fish oil.

The failure to find a link with fish seems strange for two reasons:
  • Virtually all of the DHA measured in the participants' blood came from the fish they ate
  • Several earlier studies found that dementia risks fall as fish consumption rises
However, as Dr. Morris said in her editorial, "… it is possible that the analysis was [too statistically] underpowered to observe a protective association of fish consumption, as the inverse relative risk was based on a relatively small sub-set."

Death before dementia called increasingly uncommon
Many people in developed countries live well into their 80s or 90s, and a new study suggests that dementia may become more common among them than believed.

British scientists enrolled people aged 65 or older and examined them periodically for 10 years, with sobering results recorded among the 2,500 or so participants who'd died by the end of the study:
  • Overall, 30 percent had dementia at the time of death
  • Six percent of those who died between 65 and 69 had dementia
  • 58 percent of those who died above age 95 had dementia
  • Almost 80 percent of those who died above age 95 suffered from moderate-to-severe cognitive impairment
  • Women were at higher risk
However, as the study authors noted, preventive measures could delay the onset of dementia, with enormous personal and societal benefits

What about the lack of a link between dementia risk and EPA? This finding echoes the widely reported results of the Chicago Health and Aging Project study led by Dr. Morris, which found a strong preventive effect from eating fish: an outcome related to blood levels of DHA but not to blood levels of EPA (Morris MC et al 2003).

It wouldn't be unreasonable to suppose that EPA might be more important than DHA for dementia-prevention, since inflammation is a hallmark of Alzheimer's disease and EPA is the stronger anti-inflammatory agent.

But there are good reasons why omega-3 DHA displays superior brain-protecting power.

First, recent findings indicate that fish oils possess brain-protective properties unrelated to the anti-inflammatory powers of EPA:
And, compared with EPA, DHA is far more critical to optimal brain function:
  • The most abundant fatty acid in the brain's gray matter is DHA
  • DHA is most concentrated in the most active areas of the brain (cerebral cortex, nerve synapses, and mitochondria)
  • DHA is abundant in the membranes of "ion channels” in the brain, making it easier for them to change shape and transmit electrical signals
Regardless of the precise reason for the fish-driven benefits it records, this newest branch of the venerable Framingham Heart Study awards fish another accolade in the fight to delay and diminish Alzheimer's disease and other forms of senile dementia.

Delaying dementia as long as possible seems a strong incentive to up our fish and turmeric intake. The minds we save will thank us … or themselves, depending on one's point of view.

  • Schaefer EJ, Bongard V, Beiser AS, Lamon-Fava S, Robins SJ, Au R, Tucker KL, Kyle DJ, Wilson PW, Wolf PA. Plasma phosphatidylcholine docosahexaenoic Acid content and risk of dementia and Alzheimer disease: the framingham heart study. Arch Neurol. 2006 Nov;63(11):1545-50.
  • Morris MC, Evans DA, Bienias JL, et al. Consumption of fish and n-3 fatty acids and risk of incident Alzheimer disease. Arch Neurol. 2003;60:940-946.
  • Gamoh S, Hashimoto M, Sugioka K, et al. Chronic administration of docosahexaenoic acid improves reference memory-related learning ability in young rats. Neuroscience. 1999;93:237-241.
  • McGahon BM, Martin DS, Horrobin DF, Lynch MA. Age-related changes in synaptic function: analysis of the effect of dietary supplementation with omega-3 fatty acids. Neuroscience. 1999;94:305-314.
  • Cole GM, Lim GP, Yang F, et al. Prevention of Alzheimer's disease: omega-3 fatty acid and phenolic anti-oxidant interventions. Neurobiol Aging. 2005;26:133-136.
  • Lim GP, Calon F, Morihara T, et al. A diet enriched with the omega-3 fatty acid docosahexaenoic acid reduces amyloid burden in an aged Alzheimer mouse model. J Neurosci. 2005;25:3032-3040.
  • Barberger-Gateau P, Letenneur L, Deschamps V, Peres K, Dartigues JF, Renaud S. Fish, meat, and risk of dementia: cohort study. BMJ. 2002;325:932-933.
  • Kalmijn S, Launer LJ, Ott A, Witteman JC, Hofman A, Breteler MM. Dietary fat intake and the risk of incident dementia in the Rotterdam Study. Ann Neurol. 1997;42:776-782.
  • Engelhart MJ, Geerlings MI, Ruitenberg A, et al. Diet and risk of dementia: does fat matter? Neurology. 2002;59:1915-1921.
  • Heude B, Ducimetiere P, Berr C. Cognitive decline and fatty acid composition of erythrocyte membranes: The EVA Study. Am J Clin Nutr. 2003;77:803-808.