People who eat a healthy Mediterranean diet may be less likely to develop Alzheimer’s and depression, writes Dr Samantha Gardener, post-doctoral research fellow.
Depression and Alzheimer’s disease, the most common mental and neurological disorders among older adults, cause immense societal impact and financial burden.
One in six women and one in eight men will experience depression at some point in their lives. It can occur at any time and can lead to severe distress and disruption of life.
Alzheimer’s disease is the most common form of dementia. In 2021, the Australian Institute of Health and Welfare estimated there were between 386,200 and 472,000 Australians living with Alzheimer’s at that time, equivalent to 83 people per 1000 Australians aged 65 and over.
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Alzheimer’s disease and depression are frequently comorbid; depression increases future risk of Alzheimer’s, and depression often accompanies an Alzheimer’s disease diagnosis.
A recent meta-analysis observed a 14.8% prevalence of major depression in Alzheimer’s disease. Approximately one-third of adults with depression have concomitant mild cognitive impairment, which often precedes Alzheimer’s disease.
Furthermore, depression, has been shown to promote the conversion of mild cognitive impairment into Alzheimer’s disease. The impact of depression on the clinical course of Alzheimer’s is as yet unclear.
You are what you eat
A 2024 Lancet Commission into ‘Dementia prevention, intervention, and care’ identified 14 modifiable risk factors that are proposed to account for around 45% of dementia cases, worldwide.
Of these 14 modifiable risk factors, six can be positively impacted by a healthy diet – they are hypertension, obesity, alcohol intake, type 2 diabetes, depression, and high low-density-lipoprotein cholesterol.
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Moreover, certain dietary factors and dietary patterns have been proposed to modulate Alzheimer’s disease risk through direct effects on Alzheimer’s disease pathogenesis such as oxidative stress, inflammation, and cerebral amyloid-beta production and/or clearance – accumulation in the brain is a hallmark of Alzheimer’s.
Studies show that certain nutrients may play an important role in the onset, severity and duration of depression, as well as cognitive decline associated with Alzheimer’s disease.
The impact of diet on Alzheimer’s disease, depression and the relationship between these conditions may be explained by a complex set of pathways in which dietary components interact with each other.
These interactions could reduce Alzheimer’s risk directly, via neuroprotective influences, or indirectly as protective factors against depression and cardiovascular disease, which are established risk factors for Alzheimer’s.
However, the specific dietary components that confer the greatest protection against depression and Alzheimer’s disease and impact the complex relationship between these conditions, requires further investigation.
Current evidence suggests that polyphenolic compounds, folate, B vitamins, polyunsaturated fatty acids, and adherence to dietary patterns like the Mediterranean diet are important.
The Mediterranean diet is a way of eating that is inspired by the traditional eating habits of people living in countries bordering the Mediterranean Sea. It’s known for being heart-healthy and is rich in fruits, vegetables, whole grains, legumes, nuts, and seeds.
Recent research has advanced our understanding of the changes in levels of blood-based biomarkers over the course of Alzheimer’s disease.
Higher risk of other disease
Longitudinal studies have demonstrated that individuals with Alzheimer’s disease have elevated levels of Glial Fibrillary Acidic Protein, which increases as a response to injury or disease in the brain, and neurofilament light chain, a biomarker for neurodegeneration, compared to those who are cognitively unimpaired.
In addition, a lower ratio of Aβ42:Aβ40 and elevated GFAP, and NfL in cognitively unimpaired individuals are associated with future cognitive decline.
New research using a sample of 89 cognitively unimpaired participants from the Australian Imaging, Biomarkers and Lifestyle study has shown that in males with a less healthy diet – average and below average Mediterranean diet adherence – depressive symptoms were associated with higher NfL levels.
In those with a healthier diet, depressive symptoms did not induce higher NfL levels.
Apolipoprotein E ε4 allele carriage is the strongest known genetic risk factor for Alzheimer’s disease, and in Apolipoprotein E ε4 non-carriers (those at lower risk of the disease) with a less healthy diet, depressive symptoms were associated with higher NfL and Aβ40 levels.
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Elevated NfL levels, indicative of neurodegeneration, have been found in various neurological disorders like multiple sclerosis and Parkinson’s disease.
This suggests that increased NfL levels in the results might not exclusively represent Alzheimer’s disease pathology but could also be influenced by other neurodegenerative processes.
The research shows that the Mediterranean diet is potentially a moderator of the relationship between depressive symptoms and Alzheimer’s disease-related blood-based biomarkers.
Overall, higher Mediterranean diet adherence potentially mitigates some of the negative impact depressive symptoms have on Alzheimer’s disease-related blood-based biomarker alterations.
Based on these insights, further research into personalised dietary interventions, considering genotypes and sex-specific needs, is needed. This approach could lead to effective strategies in preventing or mitigating symptoms of depression and Alzheimer’s disease.
ED: Dr Gardener is a researcher at ECU and co-lead author on the study: The moderating effect of diet on the relationship between depressive symptoms and Alzheimer’s disease-related blood-based biomarkers.
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