Could Parkinson's and Alzheimer's Start in the Gut? Decoding the Gut-Brain Link

Gut health is known to impact mental health, but new evidence suggests it may be linked to some brain diseases too.

The gut-brain connection: Could the origins of some neurological diseases lie in the gut?

(Photo: iStock/altered by the Quint)

You would remember some of your peers running to the toilet to ease themselves whenever faced with a difficult question in an exam or just before an interview. Idioms like, ‘nervous gut’, ‘shitting in his pants’ or ‘butterflies in the stomach’ underscore the link between nerves and the gut.

This connection is mainly driven by the gut-brain axis, a two-way street through which the digestive tract and the central nervous system communicate with each other.

It is well established that gut health can influence mood and mental health conditions like anxiety, depression, response to stress and even cognition, and conversely, the mood and the mental state can also affect the gut function.

Increasing evidence now shows not only mental health but several brain diseases and organic brain disorders like Parkinson's disease may have origins in the digestive tract, particularly through the gut-brain axis.

Of Hormones, Bacteria and Neurotransmitters

Over the last few years, the research done in this area has increased our understanding of the mechanisms through which the gut affects mood and vice versa.

This axis is a complex of the vagus nerve, immune system, hormones, and very importantly, the gut microbiome – the trillions of microbes comprising various bacteria, viruses and fungi in our gut – an ecosystem in itself.

Certain chemicals (neurotransmitters) like serotonin, dopamine and gamma-aminobutyric acid (GABA) modulate the mood, anxiety and stress response.

In fact, serotonin, a major mood-regulating transmitter, is produced largely (up to 90 percent) in the gut by the microbes.

Gut bacteria can also trigger inflammation through the release of cytokines which is linked to mood disorders like depression and anxiety. Conversely, certain neuroactive compounds like a short-chain fatty acid called Butyrate, which have a mood-elevating effect by directly affecting the brain function, are produced by these bacteria in the gut.

On the other hand, mood changes like anxiety, fear etc., affect the gut motility by releasing hormones like cortisol, which lead to either diarrhoea or constipation.

Further, these hormones also alter the gut permeability causing bloating, cramping etc. In fact, they can also cause certain changes in the delicately balanced microbiome.

Similar actions can be mediated through Vagus nerve also. Changes in the mental state affecting digestion and causing either increase or decrease in appetite is a common experience.

Do Some Organic Brain Diseases Originate in the Gut?

Emerging evidence suggests that Parkinson’s disease may actually begin in the gut. A protein, alpha-synuclein possibly accumulates in the intestine and may then travel from the gut to the brain via the vagus nerve, leading to neurodegeneration.

Constipation, which actually precedes the neurological symptoms in several patients, may hint at the origin of the disease in the gut.

There is emerging evidence, although not very strong at the moment, that Alzheimer’s disease and certain other disorders like Multiple Sclerosis (MS) and even epilepsy and schizophrenia may potentially have their origin in the gut because of dysbiosis (alteration in the gut microbiota).

Certain gut bacteria are linked to the production of Brain-Derived Neurotrophic Factor (BDNF), a protein that supports brain health and promotes resilience against mental disorders like depression.

Gut microbiota are a diverse population of bacteria, certain viruses and fungi. When this diversity for some reason becomes less, BDNF levels decrease, potentially increasing vulnerability to mental health issues.

Scientists now understand that each person’s unique collection of traits are actually the result of complex interactions between human and microbiota processes.

Research shows that individuals with depression and anxiety often have gut microbiome compositions different from those without these conditions.

The effectiveness of Probiotics and prebiotics in treating anxiety and depression, at least in some patients, points towards dysbiosis as one of the potential causative factors of such maladies.

It must, however, be borne in mind that the gut connection is only one of the potential factors in the multifactorial causation of such diseases.

Understanding The ‘Second’ Brain

So, in a way, the gut does have a "brain" of its own, often called the enteric nervous system (ENS) which consists of around half a billion neurons embedded in the walls of the digestive tract.

It is sometimes referred to as the “second brain” because it operates independently of the brain and spinal cord and can regulate many digestive processes on its own.

The ENS is highly autonomous, but still cannot ‘think’.

These neurons act in tandem with certain other humoral factors to control muscle contractions, enzyme secretion, and can even manage local blood flow to carry out complex tasks like peristalsis (the wave-like muscle movements that push food through the digestive system) and absorption of nutrients.

Probiotics as Treatment

Certain probiotics, known as psychobiotics, which act by working on gut-brain axis have shown promise in improving mood and reducing symptoms of depression and anxiety.

Consumption of highly processed foods and excessive sugar have shown to affect mental well-being adversely by causing inflammation and altering the microbiota.

On the other hand, vegetarian diets rich in fibre, omega-3 fatty acids, and fermented foods associated with a healthier microbiome have shown to improve mood.

Though not as a standalone treatment, probiotics as a part of a comprehensive treatment plan, have been shown to be helpful in treating Parkinson's disease, Multiple sclerosis, epilepsy and Alzheimer's disease.

(Dr Ashwini Setya is Adjunct Professor in Gastroenterology at the ESIC Medical College in Faridabad, and Senior Consultant with Medanta Institute of Digestive & Hepatobiliary Sciences in New Delhi. The views expressed are the author's own. The Quint neither endorses nor is responsible for them.)