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Your Gut Biome and Neurodegenerative Disease

How disruptions in the gut biome may contribute to degenerative disease.

Key points

  • As we age, the composition of our gut biome changes, and we become more prone to diseases related to our gut or gut bacteria.
  • Researchers are investigating how disruptions in the gut biome may contribute to the onset of Alzheimer’s and other degenerative diseases.
  • The evidence is mounting that gut health has an influence on brain health.
  • Observational studies of more than 900 adults following the MIND diet found that it was associated with a reduced risk of Alzheimer’s disease.

Last week, I posted an article describing the role of the gut biome in immune health. Seventy percent of your immune system resides in your gut. Through a connection called the gut-brain axis, your brain and gut communicate regularly. This pathway is mediated by your vagus nerve, along with your involuntary nervous system (the autonomic nervous system), hormones, and neurochemicals.

As we age, the composition of our gut biome changes, and we become more prone to diseases related to our gut or gut bacteria. For example, the overall diversity of our gut microbiome decreases along with the amount of beneficial immune-supporting bacteria. In addition, the amount of pathogenic, inflammatory-producing bacteria increases the older we become.

Inflammaging is defined as an age-related increase in the levels of inflammatory markers in our blood and tissues. This is a strong risk factor for multiple diseases. Age-related changes in the makeup of our biome can influence the level of inflammation in our body.

Alzheimer’s is a degenerative disease of the central nervous system that generally occurs in older individuals, and accounts for 60 to 80 percent of all dementias. It is characterized by deposits of amyloid, along with tau proteins, in the brain which ultimately leads to cell death through a process called neuroinflammation.

Researchers are investigating how disruptions in the gut biome may contribute to the onset of Alzheimer’s and other degenerative diseases. Findings so far include:

  • A clinical trial conducted on patients with Alzheimer’s found that those with amyloid deposits in their brains had lower levels of immune-supporting bacteria and higher levels of inflammatory bacteria in their stool. They hypothesized that some of the gut components of these patients could secrete large amounts of amyloid proteins which cross the gut barrier and travel through the blood to the brain. This in turn could lead to amyloid deposits in the brains of affected patients. A potential therapeutic option, therefore, could involve manipulating the gut environment to decrease inflammatory processes.
  • In animal studies, researchers performed fecal transfers from healthy individuals to mice affected with Alzheimer’s and found a decrease in amyloid as well as improved cognitive performance in those animals.
  • Dr. Edna Silajdžić, a scientist working at King’s College London, analyzed blood samples from people with Alzheimer’s and compared them to those without the disease. The results revealed a distinct gut bacteria makeup in people with Alzheimer’s as well as more inflammatory markers in their stool and blood samples. She stated, “Most people are surprised that their gut bacteria could have any bearing on the health of their brain, but the evidence is mounting, and we are building an understanding of how this comes about. Our gut bacteria can influence the level of inflammation in our bodies, and we know that inflammation is a key contributor to Alzheimer’s disease.”
  • Professor Yvonne Nolan, a collaborator in the same research institute, studied rats with gut bacteria transplanted from people with Alzheimer’s disease. She found they performed worse in memory tests, didn’t grow as many new nerve cells in areas of the brain associated with memory, and had higher levels of inflammation in their brains.

Dr. Nolan suggested that symptoms of Alzheimer’s may be partially caused by abnormalities in our gastrointestinal tract. She stated that although it is currently difficult to directly tackle Alzheimer’s processes in the brain, the gut potentially represents an alternative target that may be easier to influence with medications or dietary changes.

What Does This Mean For Possible Treatment Strategies?

Changes in the human brain can occur years before the first symptoms of Alzheimer's appear, which suggests a possible window of opportunity to prevent or delay cognitive decline. Scientists are looking at many ways to do this, including medication, lifestyle changes, and combinations of these interventions.

Certain risk factors for Alzheimer’s, such as genetic loading, cannot be changed. However, you can control your lifestyle choices such as diet, exercise, and cognitive training.

The Mediterranean diet shows some promise in lowering risk factors for Alzheimer’s disease. It emphasizes consuming fruits, vegetables, whole grains, legumes, seafood, and unsaturated fats. It encourages lower consumption of red meat, eggs, and sugar.

A variation of this diet, denoted by the acronym MIND (short for Mediterranean-DASH Intervention for Neurodegenerative Delay), may also show promise. MIND combines the DASH diet, designed to control high blood pressure, with the Mediterranean diet. As a side benefit, this diet can lower blood pressure which is an independent risk factor for Alzheimer’s disease.

Ingredients of the MIND Diet

The MIND diet focuses on plant-based foods linked to dementia prevention. It encourages eating from 10 healthy food groups:

  • Leafy green vegetables, at least 6 servings/week
  • Other vegetables, at least 1 serving/day
  • Berries, at least 2 servings/week
  • Whole grains, at least 3 servings/day
  • Fish, 1 serving/week
  • Poultry, 2 servings/week
  • Beans, 3 servings/week
  • Nuts, 5 servings/week
  • Wine, 1 glass/day*
  • Olive oil

In addition, the MIND diet limits servings of red meat, sugar, cheese, butter, margarine, and fast/fried food. It also encourages participants to limit their daily consumption of alcohol. Observational studies of more than 900 dementia-free older adults, found that closely following the MIND diet was associated with a reduced risk of Alzheimer’s disease and a slower rate of cognitive decline.

While scientists aren’t sure yet why the Mediterranean diet might help the brain, its effect on improving cardiovascular health might reduce dementia risk. Two recent studies suggest that eating fish may be the strongest factor influencing higher cognitive function and slower cognitive decline. In contrast, the typical Western diet increases cardiovascular disease risk, which may contribute to more rapid aging of your brain.

What Is The Evidence?

  • In an observational study of one hundred and sixteen cognitively normal adults, those who followed a Mediterranean diet had thicker cortical brain regions than those who did not. These brain regions shrink in people with Alzheimer’s, so having thicker regions could mean cognitive benefit.
  • A follow-up observational study documented lower glucose metabolism and higher levels of amyloid protein in people who did not follow the Mediterranean diet closely, compared to those who did.
  • After an average of four and a half years, people who most closely adhered to the MIND diet had a fifty-three percent reduced rate of Alzheimer’s disease compared to those who did not.

Changes in the gut microbiome as we age have been linked to disruptions in our immune system, as well as persistent inflammation and chronic diseases. Researchers are exploring how these changes are related to each other and to brain changes related to Alzheimer’s.

Studies in humans and animals indicate the composition of the gut microbiome in Alzheimer’s and mild cognitive impairment is different from that in cognitively normal beings. Identifying the good and bad gut microbes associated with Alzheimer’s could help scientists learn more about the biology of the disease and develop new ways to predict and potentially treat this emotionally and financially devastating disorder.

References

Xueling ZhuBo Li Pengcheng Lou Tingting Dai, Yang Chen, Aoxiang Zhuge, Yin Yuan,Lanjuan Li.(2021). The Relationship Between the Gut Microbiome and Neurodegenerative Diseases.Neurosci. Bull. October, 37(10):1510–1522

Lisa Mosconi, Michelle Walters , Joanna Sterling , Crystal Quinn , Pauline McHugh , Randolph E Andrews , Dawn C Matthews , Christine Ganzer Ricardo S Osorio , Richard S Isaacson, Mony J De Leon, Antonio Convit. (2018). Lifestyle and vascular risk effects on MRI-based biomarkers of Alzheimer's disease: a cross-sectional study of middle-aged adults from the broader New York City area. British Medical Journal Mar 23;8(3).

Martha Clare Morris , Christy C Tangney , Yamin Wang Frank M Sacks Lisa L Barnes, David A Bennett , Neelum T Aggarwal. (2015). MIND diet slows cognitive decline with aging.Alzheimer’s Dementia 2015 Sep;11(9):1015-22.

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