What we eat has a tremendous impact on our well-being: a healthy, balanced diet keeps us energized and happy. Unfortunately, consumption of palatable but unhealthy foods is rampant in Western civilizations and is a primary driver of the obesity epidemic. With rising obesity prevalence across the world, many of us understand that obesity is a major risk factor for heart disease and type 2 diabetes. But in addition, emerging research demonstrates that unhealthy eating and obesity can have a substantial impact on another critical organ — the brain. Indeed, experts are beginning to link obesity to mental health disorders such as depression. In this article, we explore the Alzheimer's diet, and how unhealthy foods and metabolic disorders might be risk factors for Alzheimer’s disease, one the most common types of neurodegenerative dementia.  

What is Alzheimer’s disease?

Alzheimer’s disease (AD) is a neurodegenerative disorder affecting more than 47 million people worldwide. Age is the most important known risk factor (but not the cause), with about one-third of people over 85 years of age suffering from AD. AD is a deterioration of cognitive function — particularly learning and memory. Common hallmarks for diagnosis of AD include:

• Mild cognitive impairment: This early stage condition may or may not later develop into AD and dementia. Symptoms include deficiencies in a person’s memory and problem solving that are uncommon for that person’s age and education. While an individual’s ability to function independently is retained, they may have difficulty with more complex tasks.
  
• Alzheimer’s dementia: This final stage of AD is characterized by marked impairments in memory and visual/spatial problems that prevent a person from functioning independently. Such cognitive impairments ultimately lead to significant deficits in behavior, speech, and visuospatial orientation — and eventually lead to death. 

Physiologically, AD has a significant impact on the structural and molecular integrity of the brain. The neuropathological diagnosis of this disease includes: 

• Accumulation of toxic proteins in the brain, in particular amyloid-β plaques and Tau neurofibrillary tangles. The presence of these toxic proteins in the brain are required for a definitive diagnosis of AD.
  
• The presence of amyloid-β and neurofibrillary tangles often affect brain regions that regulate learning and memory, such as the hippocampus.
  
• The microscopic detection of amyloid-β and neurofibrillary tangles is the gold standard for diagnosis, but macroscopic atrophy of the brain (particularly in the hippocampus) is also supportive of AD diagnosis. 

Is there a link between unhealthy eating and Alzheimer’s disease?

While many might consider obesity/unhealthy eating and Alzheimer’s disease as separate pathologies, experts have identified several links between metabolic disorders and Alzheimer’s disease. 

A wealth of emerging studies in animal models has revealed an association between consumption of high-fat, high-sugar foods and cognitive impairments:

• Rats fed a high-fat, high-sugar diet exhibit marked deficits in visuospatial learning tasks. Interestingly, these memory impairments arise well before obesity-related symptoms (e.g., weight gain, increased adiposity), suggesting that components of unhealthy foods (such as fat and sugar) — rather than later changes in metabolic profile — are driving these changes in the brain.
  
• Other scientists have demonstrated that adolescent consumption of sugar can have a long-term impact on memory. Rats fed sugar during early life exhibited memory deficits well into adulthood. 

Scientists have also shown in animal models that intake of high-fat diets can worsen AD pathology; however, the study results have been mixed: 

• Some researchers have demonstrated in mice that intake of high-fat diets can increase accumulation of amyloid-β and exacerbate cognitive deficits, an effect that can be reversed by removing high-fat food from the diet.
  
• Researchers have also found that high-fat diet intake in normal mice can impair memory to the levels of genetic mouse models of AD. These deficits were associated with structural brain changes in the hippocampus.
  
• However, other scientists have shown that high-fat diets can have protective effects. Such data can be difficult to interpret because, unlike in humans, rodents do not naturally accumulate amyloid-β with age — so the studies had to rely on the use genetically modified animals. 

Correlational studies performed in humans have suggested that obesity and associated co-morbidities can be a risk factor for Alzheimer’s disease:

• A study of 1,949 participants over the age of 65 demonstrated that overweight and obese participants had worse performance on neurocognitive tests compared to lean counterparts.
  
• In a longitudinal study, scientists found that midlife intake of saturated fat was associated with poorer cognitive function and an increased risk of mild cognitive impairment — symptoms potentially indicative of later Alzheimer’s disease development.
  
• Other longitudinal studies have found that having increased body weight or abdominal obesity is associated with an increased incidence of dementia. 

It is important to note that most causational studies linking diet and the brain have been performed in animal models. Moreover, many of these studies discovered associations between unhealthy foods and cognitive impairment, a pathology that is not exclusive to Alzheimer’s disease. 

How do unhealthy foods damage the brain? 

Metabolic-brain implications

Impaired glucose metabolism and insulin resistance are hallmarks of type 2 diabetes (T2D). Recent studies in animals have shown that poor glucose control, especially in the central nervous system, can also increase risk for AD. These findings, together with the association between unhealthy eating and Alzheimer’s disease (as indicated by both the mechanistic insights gained from animal studies and correlational links in human studies), have led many experts to coin the terms “type 3 diabetes” and “diabetes of the brain” for Alzheimer’s disease.

• The apolipoprotein E4 (apoE4) gene is the strongest genetic risk factor for AD. In genetic mice designed to carry apoE4, researchers found impaired insulin signaling within neurons of the brain as the mice age, and these effects are exacerbated when these mice were fed a high-fat diet.
  
• Researchers have found that the hallmark toxic proteins found in AD, amyloid-β and tau neurofibrillary tangles, negatively impact the function of insulin within the brain.
  
• Similar to insulin resistance found in beta cells of the pancreas during T2D, we see a similar type of insulin resistance in neurons. This includes decreased levels of insulin, the insulin receptor, and insulin signaling in AD brains.
  
• In rats, high-fat, high-sugar diets induced insulin resistance that correlated with impaired learning and memory, as well as damage in the hippocampus. 

Vascular-brain implications

Scientists have also described Alzheimer’s disease as a vascular disorder, where the blood-brain barrier (BBB) that protects the central nervous system becomes compromised as a result of amyloid-β. Similar damage has also been observed with unhealthy eating and obesity:

• A longitudinal study examining mid-life adiposity and BBB integrity found a correlation between increased adiposity and worse BBB integrity, suggesting that obesity could serve as a trigger for vascular disorders that later damage the blood brain barrier.
  
• In rats fed a high-fat, high-sugar diet, scientists found increased BBB permeability and decreased levels of molecules that maintain BBB integrity.
  
• Since consuming foods that are high in saturated fatty acids can increase amyloid-β in the bloodstream, some scientists hypothesize that this circulating amyloid-β travels to the brain to disrupt the structure of the BBB. This assertion, coupled with studies showing neuroinflammation following unhealthy eating, suggests that poor diets compromise the BBB, ultimately leaving the brain vulnerable to circulating amyloid-β and other toxic compounds. 

It seems clear that your gut microbiome is strongly impacted by diet. In this emerging field of research, some have suggested that diet-induced changes in the microbiome can have a strong impact on brain function, including implications in neurodegenerative disorders such as AD: 

• In one examination of gut microbiota within AD patients, researchers not only found differences in bacterial abundance and diversity compared to control patients, but also saw correlations between these variations and biomarkers of Alzheimer’s disease.
  
• In animal studies, scientists are also finding links between diet-induced changes in the gut microbiome and cognitive impairment. For example, intake of unhealthy foods increases bacterial endotoxins that promote neuroinflammation and cognitive dysfunction. Other researchers have demonstrated that diet-induced alterations in gut microbiota can impair insulin sensitivity, which we described earlier as a risk factor for cognitive deficits. 

Granted, the physiological mechanisms underlying unhealthy diets and the brain merit much more research. Many scientists utilize a variety of genetic animal models to replicate Alzheimer’s disease — each with their own pros and cons — and causational links between dietary patterns and humans with Alzheimer’s disease still need to be clarified. However, the preliminary associations and correlations that have been demonstrated are compelling. 

Can healthy eating prevent Alzheimer’s disease?

Research examining the mechanisms through which unhealthy diets can impact our brain health continues to develop. However, can we apply the same concepts found in the research above to healthy diets and lifestyles? Are the brain and cognitive deficits observed with unhealthy diets reversible with healthy foods? 

Two popular diets with significant health benefits are the Mediterranean diet and its variation known as MIND (Mediterranean-DASH intervention for Neurodegenerative Delay). Indeed, some researchers have found some evidence for MIND diets as a way to prevent Alzheimer’s disease and improve cognition: 

• The MIND diet includes natural plant-based foods and limits intake of animal products and foods high in saturated fats. Notably, this dietary pattern emphasizes the intake of berries and green leafy vegetables. Interestingly, high adherence to a MIND diet may reduce risk for AD development.
  
• Other research has shown that the MIND diet can significantly slow cognitive decline associated with aging and is associated with better cognitive functioning. Together, these results suggest that the MIND diet can contribute to greater cognitive resilience in elderly populations. 

While the neurocognitive benefits of Mediterranean and MIND diets are still under investigation, the overall health benefits of these diets are significant: both have been endorsed by leading medical organizations, such as the American Diabetes Association, because of the well-researched correlation with reduced risk of T2D as well as the protection against total cardiovascular disease (CVD), ischemic stroke, and coronary heart disease. Cognitive benefits, and even the potential of reduced Alzheimer’s disease risk (if scientists can eventually fully demonstrate this link), would be another huge reason to adopt this style of eating.

Key Takeaways

It has already been well documented that unhealthy eating drives obesity, a major risk factor for heart disease and type 2 diabetes. Emerging research also demonstrates that unhealthy eating can have a substantial impact on another critical organ — the brain. Alzheimer’s disease (AD) is an extraordinarily complex disease with no known cure. While large and well-designed studies looking into the direct links between dietary patterns and the development of AD in humans are few in number, and mostly assess cognitive decline generally, research in animal models has established a link between high-sugar, high-fat diets and Alzheimer’s disease. Meanwhile, the Mediterranean and MIND diets — already associated with so many cardiometabolic and other health benefits — could hold the promise of improved brain health and function based on early, albeit compelling research pointing to promising impacts on cognitive health, thus becoming a solid Alzheimer's diet choice for those looking to follow the latest research.

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