A US study has found that the omega 3 fatty acid DHA found in fish oil does not slow the progress of Alzheimer's disease, despite conclusions drawn from previous research.
Researchers at the Oregon Health and Science University report in JAMA today on a study across 51 clinics on 400 patients with mild to moderate Alzheimer's.
Patients were divided into two groups. Half were given 2 g/day of DHA (docosahexaenoic acid) and the other half were given a control.
At the start of the study, patients were tested for their cognitive function and measurements were taken of their brain volume. Both are indicators of the progression of Alzheimer's disease.
After 18 months on the treatments, they were tested and measured again. Those who had taken DHA scored no better than the controls in cognitive tests and showed no difference in brain shrinkage over the period of the study.
The results are a puzzle, given that previous studies have linked a diet rich in fish with reduced dementia. Another study had shown that DHA can reduce the brain damage caused by proteins associated with Alzheimer's disease.
Genetic risk factor
So why didn't the DHA have any effect?
The researchers suggest that DHA might help if it were taken earlier in the progress of the disease. "Individuals intermediate between healthy ageing and dementia, such as those with mild cognitive impairment, might derive benefit from DHA supplementation," they say.
But the failure of DHA could also be related to genetic makeup, according to Professor Henry Brodaty, director of the Dementia Collaborative Research Centre at the University of New South Wales.
Professor Brodaty says there is a genetic risk factor for Alzheimer's, based around a gene called apolipoprotein E. This gene comes in several varieties, E2, E3 and E4. People who carry the E4 type variant have an increased risk for early onset of Alzheimer's disease, although it's not guaranteed.
When the Oregon researchers removed people carrying E4 from the analysis, they found that DHA did have a positive effect on slowing progression of the disease. However, they add that this aspect of the study would need replication to be confirmed.
"It may be [a possibility] that the effect of E4 is so strong, it swamps any protective effect of the fish oil," Professor Brodaty says.
There are several possibilities as to how fish oil might benefit the brain. For one, it is known to boost levels of a protein called LR11 which helps destroy Alzheimer's-like plaques in the brain.
The well-known anti-inflammatory properties of fish oil might also protect the brain in some way.