What do the trillions of microorganisms living in our intestinal tract have to do with Parkinson’s disease? Scientists are starting to unveil clues about the Parkinson’s microbiome connection.

Even though Parkinson’s research has come a long way, scientists still don’t completely understand where, when or how the disease begins inside our bodies.

Researchers have studied repeated head trauma as a potential factor. They’ve identified a significant link between PD and exposure to herbicides and pesticides. Scientists have also explored genetic risk factors that might make certain people more predisposed to Parkinson’s than others. Credible theories abound but no concrete answers exist—yet.

The good news is that scientists are uncovering clues that may bring us closer than ever to a cure. New research has revealed that Parkinson’s may be linked to the microbiome, the trillions of microorganisms and their genetic material that live inside the intestinal tract.

Actually, this research isn’t all that new

The earliest evidence that the gut could be connected to Parkinson’s disease emerged more than two centuries ago from the English surgeon James Parkinson, after whom the disease was named.

In 1817, Parkinson reported that patients living with a condition he called “shaking palsy” all had one thing in common: constipation. He found that treating their gastrointestinal issues also seemed to improve their motor symptoms. (Read our nutritional recommendations to learn more about foods that can help with constipation.)

Even today, doctors have identified constipation as one of the most common symptoms of Parkinson’s, affecting up to 80 percent of people living with the disease. In fact, constipation often precedes the onset of mobility issues.

And yet, for decades, most of the research into this disease has centered on the brain. This is how scientists were able to figure out that Parkinson’s patients tend to lose dopamine-producing neurons involved in many important brain functions including movement.

More recently, they’ve focused their research on the accumulation of alpha synuclein, a protein that twists into an unusual shape in Parkinson’s patients.

The discovery heard ‘round the world

One of the most significant “ah-ha” moments in Parkinson’s research to date occurred in 2003, when German neuroanatomist Heiko Braak and his colleagues at the University of Ulm proposed that Parkinson’s may actually begin in the gut instead of the brain.

Braak and his colleagues studied Parkinson’s patients post-mortem and found clumps of alpha synuclein not only in the brain, but also in the gastrointestinal nervous system that controls gut function. They began to see predictable patterns of the disease that started in the gut and ended in the brain.

But how did it get from point A to point B? Braak and crew theorized that the vagus nerve might be involved. The vagus nerve connects major bodily organs (specifically the heart, lungs and digestive tract) to the brainstem, which joins the spinal cord to the brain. In other words, it enables communication between the brain and gut.

Other studies have found that gut microbiome is altered in people with Parkinson’s disease. In 2016, scientists at the California Institute of Technology conducted an experiment where they removed microbiome found in the intestines of mice with Parkinson’s-like symptoms.

After these microorganisms were removed, motor symptoms improved. When they transplanted bacteria from Parkinson’s patients back into the mice, the symptoms returned.

Scientists realize they’re onto something

Now this notion that the earliest stages of Parkinson’s disease may occur in the gastrointestinal tract is taking the medical community by storm. It’s a huge scientific breakthrough considering how imperative it is to detect and diagnose PD as early as possible.

And researchers are just revving up. In 2018, the U.S. Army Medical Research and Materiel Command awarded a four-year, $2.5 million grant to researchers at the University of Alabama at Birmingham (UAB) to investigate the role of Parkinson’s microbiome.

Scientists already know that microbiome is easily modifiable and could play a fundamental role in the treatment or prevention of many diseases. UAB’s goal is to figure out whether changes made to bacteria in the intestinal tract might alter Parkinson’s disease progression.

So far, the UAB scientists have analyzed fecal samples from hundreds of Parkinson’s patients across the country. They found that the microbiome found in the guts of people with PD is drastically different from disease-free individuals.

The microbiome also seemed to vary according to the person’s geographical location, which may explain how environment, lifestyle and diet factor into a individual’s biological system, including exposure to pesticides.

The UAB researchers also reported that Parkinson’s disease medications had a direct effect on changes in microbiome. This made sense considering the microbiome in our gut are partially responsible for metabolizing drugs.

But scientists aren’t quite sure whether medications change the microbiome in a specific way or how intestinal bacteria influences a patient’s response to treatment. And the question of how changes in the intestines drive neurodegeneration in the brain still remains a mystery.

– Kathryn Jones