The new study shows that there is no link between seafood consumption, mercury levels in the brain and dementia.
They publish their findings in JAMA.
Previous studies have uncovered protective links between eating seafood and dementia, but because mercury is a known neurotoxin, concern has been growing about how it could be detrimental to the brain and potentially contribute to Alzheimer's disease.
Alzheimer's is a progressive disease that starts with mild memory loss, but it can potentially lead to loss of being able to carry on a conversation or respond to the environment. It affects parts of the brain related to thought, memory and language.
Although the health care community is learning more about the disease, what causes it is still largely unknown.
Seafood is high in the long-chain n-3 fatty acid DHA (docosahexaenoic acid), which has established health properties that are specifically related to brain function.
To further investigate the murky waters of seafood consumption, mercury levels in the brain and brain neuropathologies, the researchers - led by Martha Clare Morris of Rush University Medical Center in Chicago, IL - conducted an analysis of deceased patients who were part of the Memory and Aging Project clinical neuropathological cohort study from 2004-2013.
First study to examine mercury levels and brain neuropathology
In total, there were 544 participants, 286 of whose brains were autopsied after death. The average age of death was 90 years, and 67% of the subjects were women.
The team measured seafood intake through a food frequency questionnaire an average of 4.5 years before death.
Results showed that consuming one or more meals that incorporated seafood per week was significantly correlated with less Alzheimer's disease pathology, which includes lower neuritic plaque density, less severe neurofibrillary tangles and lower "neuropathologically defined Alzheimer's disease."
The researchers note that this finding was only among apolipoprotein E (APOE ε4) carriers, which is a gene variant linked to an increased risk of developing Alzheimer's disease.
Furthermore, although consumption of seafood was linked with higher brain levels of mercury, the researchers say these higher mercury levels were not significantly linked with increased brain neuropathology levels.
And fish oil supplementation did not have any statistically significant link with any markers of neuropathology, the team adds.
The researchers say theirs is the first study to examine the link between levels of mercury in the brain and brain neuropathology or diet. They add:
"The finding of no deleterious correlations of mercury on the brain is supported by a number of case-control studies that found no difference between Alzheimer disease patients and controls in mercury concentrations in the brain, serum or whole blood."
'Evidence that increased mercury is not correlated with dementia'
Although their study has many strengths, the researchers point to a few limitations. Firstly, the observational nature of the study design precludes causal explanations of the data.
The subjective measures of dietary intake also present a limitation, but the team notes the dietary tool was validated in older adults. Furthermore, because seafood intake in the study population was moderate, the researchers say their findings cannot be generalized to populations with either higher seafood consumption or higher mercury levels.
Finally, their study population was a very old, mostly non-Hispanic white group, so the study findings may also not be generalizable to younger adults or other ethnic groups.
Morris spoke with Medical News Today about the implications of their findings for the general public:
"A major concern in public health was whether the increased mercury exposure that comes from consuming seafood might have harmful effects on the brain as we age. This study provides evidence that the increased mercury exposure is not correlated with increased brain pathologies associated with dementia."
In the future, she and her team hope to look into whether seafood consumption and mercury levels affect other measures of brain health.
"We are also making plans to investigate whether the accumulation of other metals in the brain lead to neurodegenerative diseases linked to dementia, like Alzheimer's disease," she told us.
MNT recently reported on a study that suggested Alzheimer's-related brain changes occur 2 decades before symptom onset.