What Is Alzheimer’s Disease?

Alzheimer’s disease is the most common type of dementia in adults and is usually diagnosed in people aged 60 years and older, although it can develop in younger people. People with dementia often experience declines in cognitive function that affect their memory and other thinking skills like language, problem-solving, attention, and reasoning. Their behavior, feelings, and relationships can also be affected, with significant effects on their daily lives.

Alzheimer’s disease develops in three stages:

1. The first “preclinical stage” is characterized by changes in the brain without the person experiencing any obvious symptoms.
2. In the second stage, known as “mild cognitive impairment”, the person experiences problems with their memory and thinking skills, although they are not severe enough to affect their independence.
3. In the third stage, which is when a diagnosis of Alzheimer’s disease is usually made, the person may experience memory loss and have difficulties with, or an inability to, recognize and identify familiar people, objects, and words.

What Causes Alzheimer’s Disease

Alzheimer’s disease is known to be caused, at least in part, by the abnormal functioning of two proteins called beta-amyloid and tau. In people with Alzheimer’s disease, beta-amyloid forms clumps called “plaques” on neurons (central nervous system cells that transmit messages between different parts of the brain and around the body) that make it hard for them to stay healthy and communicate with each other. Abnormal forms of tau cling to other tau proteins inside neurons and form “tau tangles”. However, this is not the whole story, and Alzheimer’s disease is now believed to be a complex condition that is caused by a variety of factors – including genetic, environmental, and lifestyle factors – that affect the brain over time.

There is no cure for Alzheimer’s disease. Current treatment approaches focus on managing its symptoms and the changes in behavior that happen as the disease progresses. As a result, researchers are investigating factors that may slow or prevent the development of the disease in the future, and improve the lives of patients and their carers.

What Is the Gut–Brain Axis?

The gut–brain axis is the term given to the connections between the central nervous system (which includes the brain and the spinal cord) and the gut (also known as the gastrointestinal system; it refers to the parts of our bodies that are involved in digestion, including the stomach and intestines), and the ways that they communicate with each other. A key component of the gut that enables it to function effectively is its microbiota, which is the name given to the community of microorganisms that live in it. Together, these microorganisms play essential roles in breaking down food, producing nutrients that the body needs, protecting against infection, and regulating the immune system.

How May the Gut–Brain Axis Be Linked to Alzheimer’s Disease?

Some of the microorganisms that make up the microbiota release signaling molecules that can influence brain function, as well as having anti-inflammatory and potentially neuroprotective properties. This means that the different microorganisms present in a person’s microbiota may either contribute to the development of Alzheimer’s disease or slow its development.

In addition, changes to the normal composition of the community of microorganisms that make up the microbiota – caused by factors such as infections, poor diet, and stress – can lead to long-term (chronic) inflammation in the gut. This inflammation is known to activate immune cells in the central nervous system called microglia, and chronic inflammation is a known characteristic of Alzheimer’s disease that is thought to contribute to the neurons becoming damaged.

In turn, the brain influences the gut and its microbiota in what can become a feedback loop. By releasing stress hormones like cortisol, the brain is able to alter the function of the gut and the composition of its microbiota. As a result, stress negatively affects the gut, which can in turn result in the gut having a negative effect on some neural conditions and contribute to the development of neurodegenerative diseases like Alzheimer’s disease.

The Microbiota and Alzheimer’s Disease

Recent research has suggested that the gut microbiota may directly influence the development and progression of Alzheimer’s disease. Some bacteria found in the gut produce amyloid proteins that are similar to those found in the brain, and which may add to the buildup of plaques. Researchers have also reported that the different microorganisms that make up the gut microbiota are present in different proportions in people with Alzheimer’s disease compared with people without the condition, with some types of bacteria potentially being useful as biomarkers for the development of Alzheimer’s disease or targets for treatment in the future.

Potential Future Treatments

A number of treatment approaches are being investigated that are related to the microbiota to see whether or not they have any potential in treating people with Alzheimer’s disease. Among them, fecal microbiota transplantation (FMT, also known as poo or stool transplantation) has already been suggested by some researchers to have a benefit on cognition in patients with dementia. FMT works by the microbiome from a healthy donor being transferred to the intestines of a recipient, usually in capsule or liquid form. However, the lack of knowledge regarding potential long-term side effects (any unintended effects of a treatment) means that larger studies are needed before FMT can be judged to be clinically useful in the treatment of patients with Alzheimer’s disease.

Other approaches that are being investigated include the use of probiotics (live yeasts and bacteria that are found in some foods and supplements), although there have been conflicting results to date, and dietary changes. Diet is thought to be the single most influential factor on the composition of the gut microbiota. A diet that is low in red meat and processed foods, but high in fruits and vegetables with moderate consumption of dairy products, poultry, eggs, and fish, has been reported to be potentially protective against chronic inflammation and diseases that are caused by it.

All of these approaches are affected by each person’s individual gut microbiota composition. As research to better understand the mechanisms that link the gut–brain axis continues, future treatments will need to be personalized to be most effective in preventing a person’s cognitive decline and reduce the risk of side effects.