D'OH! DHA Supplements Don't Reduce Alzheimer's Risks

Another promising dietary supplement fails to deliver protection against a target disease, this time Alzheimer's. Image courtesy of outcast104.

Another promising dietary supplement fails to deliver protection against a target disease, this time Alzheimer's.

DHA (or docosahexaenoic acid for the geekier among you) is an omega-3 fatty acid that is abundant in the brain. Epidemiological studies have suggested that people who consume more DHA from fish have a lower incidence of Alzheimer's disease. Further, DHA supplementation has improved markers of cognitive impairment in mouse models of Alzheimer's disease.

Scientists speculated that DHA supplementation may be beneficial in treating cognitive decline because previous research has suggested that among all omega-3 fatty acids, DHA was the only one associated with a reduced incidence of impairment. Also, the other major omega-3 fatty acid found in fish, eicosapentaenoic acid (EPA), is not present in the human brain, whereas DHA is abundant.

The study, published in JAMA, was a collaborative effort by scientists from the Oregon Health and Science University, UC San Diego, Yale, UC San Francisco, NYU and others. It was a randomized, double-blind, placebo-controlled trial of DHA supplementation in patients with mild to moderate Alzheimer's disease.

The researchers found no benefit of 2 g/day DHA supplementation on cognitive performance on the Alzheimer's Disease Assessment Scale (ADAS) or Clinical Dementia Rating (CDR) compared to placebo. There was also no measurable benefit of DHA on brain volume, which typically declines with Alzheimer's progression.

Though this research does not rule out a benefit of DHA on cognitive health, it does not bode well for regular supplementation. The treatment lasted for 18 months and cognitive changes were detected in both groups. So if DHA had any effect on the rate of cognitive decline it should have been apparent.

It is possible that beginning DHA treatment after early signs of Alzheimer's have already been detected is too late for any meaningful protection offered by DHA. Maybe some benefit would have been found if the treatment began in healthy adults before symptoms of cognitive decline developed.

It may also be that DHA is beneficial, but is not effective in supplement form. DHA is very vulnerable to oxidative damage, and some research has shown that it provides more cognitive benefit when co-administered with an antioxidant (lutein) to protect it. DHA ingested in the form of food (fish) would not be subject to the same level of oxidative degradation, which may explain the results seen in epidemiological data.

It is not uncommon for supplements to fail to replicate epidemiological benefits seem from foods, and more careful studies are needed to determine the nutritional benefit, if any, of DHA on cognitive aging.

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