Current estimates indicate that over 10% of the world’s population in 2030 will be at least 65 years old, which will inevitably increase the prevalence of Alzheimer’s disease (AD) and other dementias. In the United States, researchers predict that over eight and 14 million people will have AD by 2030 and 2060, respectively.
No effective therapies are currently available for treating AD. However, various preventive measures are being investigated, among which are lifestyle interventions, including dietary patterns.
A new paper in Nutrients discusses the impact of diet on the risk of AD, in which researchers describe the interrelationship between specific nutrients, dietary patterns, other risk factors for AD, and psychosocial factors driving these patterns.
Study: Dietary patterns and Alzheimer’s Disease: an updated review linking nutrition to neuroscience. Image Credit: Elenadesign / Shutterstock.com
What is AD?
Dementia is due to brain trauma and disease processes that ultimately cause neuronal death and impaired brain function over time. Dementia primarily affects cognitive processes by causing poor memory, lapses in thinking, and reduced brain processing, which ultimately prevents the affected individual from performing activities of daily living (ADL).
AD eventually causes loss of neuronal synapses and pyramidal neurons, with early signs including memory loss and difficulty in thought and language. The hippocampus and neocortex are the first areas of the brain to exhibit signs of disease in AD, often two decades before its clinical symptoms appear. Changes in glucose metabolism also characterize AD pathophysiology and result in brain atrophy.
How does diet affect AD risk?
The rise in AD and related dementias (ADRD) incidence in the West is linked to the Western diet, which is rich in refined carbohydrates, including sugar, saturated fats, and salt, with a low intake of fruits and vegetables. These dietary patterns have historically been linked to metabolic aberrations causing obesity, type 2 diabetes, autoimmune disease, cancer, and cardiovascular disease. Alterations in the gut microbiome due to the high consumption of ultra-processed foods and soft drinks also contribute to a state of chronic inflammation.
Altered or lost insulin sensitivity and high blood lipid levels also cause abnormal endothelial structure and function, poor hemodynamic function, and poor vascular tone, which can alter blood flow to the brain. Each of these conditions contributes to the onset of dementia.
Up to one-third of AD cases involve risk factors that may be prevented by proper lifestyle and dietary interventions. Such preventive interventions likely act by limiting brain aging due to direct and indirect actions on metabolic health.
Prior research shows that certain dietary patterns, including the Mediterranean and Dietary Approaches to Stop Hypertension (DASH) diets, are linked to a lower incidence of ADRD. Such diets are in accordance with the American Heart Association and U.S. Dietary Guidelines that recommend a low intake of red meat, sodium, sugar, saturated fats, and highly processed foods, while favoring plant-based foods, along with fish, dairy, and lean meat.
These diets offer nutrients that combat oxidative damage and inflammation while promoting mitochondrial and endothelial function. In addition, these nutrients mitigate the harmful effects of diabetes, obesity, and other lifestyle diseases that act as risk factors for AD.
The Mediterranean diet
Based on plant foods, dairy, wine, and fish, the Mediterranean diet offers nutrient-rich foods that promote overall health and brain function. Earlier research indicates the favorable impact of this diet on dementia onset and incidence, less cognitive decline, and reduced β-amyloid and tau tangle levels.
The greatest benefits of the Mediterranean diet have been observed among those who follow this dietary pattern for ten years or more, especially those who live in the Mediterranean region.
In addition to its direct effects on ADRD, the Mediterranean diet appears to act through multiple pathways, including anti-inflammatory, antioxidant, and cholesterol-lowering actions, while also stabilizing and reducing blood pressure, improving mitochondrial integrity and function, improving blood flow to the cerebrum, and increasing cortical thickness and brain volume.
The DASH diet
The DASH diet is also based on plant foods but discourages sodium, sugar, sweet drinks, and alcohol. Like the Mediterranean diet, the DASH diet has favorable effects on high blood pressure and cholesterol, as well as metabolic health.
Multiple studies have reported an association with better cognitive function at older ages in those consuming the DASH diet. However, simultaneous weight loss efforts have been linked to superior cognitive outcomes as compared to the diet alone. Further research is needed to confirm the utility of this diet on ADRD risk and progression.
The Mediterranean-DASH Intervention for Neurogenerative Delay (MIND) diet focuses on including berries and green leafy vegetables in the diet in lieu of fruits. Several researchers have demonstrated the benefits of this diet on cognitive function and slowing cognitive decline in a dose-dependent manner. These associations have been reported even among those already suffering from AD.
Ketogenic diets
The Ketogenic Diet (KD) is based on a high fat, low carbohydrate intake pattern, with ketones being generated for use in energy production. Originally reported to successfully improve outcomes among refractory epilepsy patients, the impact of KD for cognitive functioning is currently being investigated.
However, the restrictive nature, potential metabolic impacts, and resulting alterations in cardiovascular health of this diet necessitate additional research in larger trials.
The modified Mediterranean-ketogenic diet (MMKD) also appears to improve metabolic health and brain function while reducing the adverse impacts of restricting certain food categories.
Vegetarianism and veganism
Vegetarian diets are associated with a reduced risk of dementia, as well as better long-term compliance with dietary patterns. Improved memory and language skills have also been reported. Notably, one limitation of the vegetarian diet is the lack of sufficient protein in older people from vegetable products alone.
Vegan diets may also protect against AD due to the rich presence of fiber, polyunsaturated fatty acids, and vitamins. However, this diet poses the risk of vitamin B12 and D deficiencies. Larger studies are needed to delineate the mechanisms of action and the degree of benefit of these diets.
What are the implications?
Many studies have reported that specific nutrients found in mostly plant-based diets are linked to better cognitive and overall brain functioning. Multiple mechanisms are implicated in this benefit, including improved cerebral perfusion, less oxidative damage and neuroinflammation, the formation of more neurons, and better neuroplasticity. Enhanced gut microbiome health could also contribute to this effect, to which dietary fiber from plant foods contributes significantly.
Polyunsaturated fatty acids of the omega-3 category have anti-inflammatory, antihypertensive, and endothelium-protective effects. Likewise, vitamins such as tocoferol and B12 also protect cerebral neurons from oxidative stress.
These diets also promote metabolic health through multiple pathways, including insulin sensitivity, lipid-lowering actions, and improved glucose control.
Loss of taste, poor oral health, and difficulty with chewy foods may present barriers to food consumption that could impact adherence to a healthy diet. Medications and other therapies could also contribute to dietary deficiencies in this group.
In addition to dietary patterns, weight loss and physical exercise, as well as social engagement and brain activity, favor cognitive improvement. Sleep hygiene and stress relief, in addition to avoiding risky substances, also support a healthy lifestyle.
Ultimately, it has been found that multidomain programs yield better outcomes in slowing down disease progression compared to single-domain interventions.”
How a ketogenic diet could reduce autoimmune disease severity through host-microbiome interactions