A new study, published in a special supplement of the Journal of Parkinson's Disease, reviews what we know so far about the link between the gut and Parkinson's disease. Homing in on the gut may help diagnose the condition much sooner and slow down its progression.

According to the National Institutes of Health (NIH), around 50,000 people in the US receive a diagnosis of Parkinson's disease each year. Currently, around 500,000 people have the condition. By the time physicians diagnose it, most brain cells that are affected in Parkinson's have already died. For this reason, it is more difficult to slow down the progressive disease. Therefore, researchers have recently been looking into ways to detect the condition much earlier, going beyond the movement-related neurons and neurotransmitters in their search for a culprit.

In their exploration of the causes behind Parkinson's disease, researchers have zeroed in on the gut. More and more studies are suggesting that the condition starts in the gastrointestinal system—at least for some people who have digestive symptoms years before any motor symptoms develop. Some studies have even shown that the alpha-synuclein protein, which is abnormal in Parkinson's disease, travels from the brain to the stomach via the vagus nerve, a major component of the parasympathetic nervous system.

So, what is the current state of existing research on the gut-brain connection in Parkinson's? A new review, entitled "The gut and Parkinson's disease: Hype or hope?" set out to investigate. Dr. Filip Scheperjans, PhD—of the Department of Neurology at the Helsinki University Hospital in Finland—is the first and corresponding author of the review.

How the gut can help diagnose Parkinson's

Dr. Scheperjans explains the motivation for the study, saying, "Better understanding the role of the gut in [Parkinson's disease] will help us to understand the origin of the disease and to improve treatments."

"There is accumulating evidence that at least in some [...] patients, the origin of the disease may lie in the gut with possible involvement of abnormal protein aggregates, local inflammation, and the gut microbiome."

"Therefore, further studies into the role of the gut in [Parkinson's] are important and may reveal new possibilities for diagnosis and treatment," he explains.

In their review, Dr. Scheperjans and colleagues identified four main takeaways:

• While scientists have found deposits of alpha-synuclein in the enteric nervous system of people with Parkinson's, more research is needed to determine whether these protein aggregates are "biochemically similar to the ones found in the brain." The authors continue, "[T]his might be critical in our understanding of the role of the gut in [Parkinson's disease] pathogenesis"
• Intestinal hyperpermeability might be what triggers alpha-synuclein aggregation in the enteric nerves. More research is now required to find out whether people with Parkinson's also have higher intestinal permeability
• Studies that have utilized immunohistochemistry to study alpha-synuclein aggregates in the enteric nervous system yielded mixed results, so scientists must develop newer, alternative ways of detecting alpha-synuclein deposits in the gut
• Large multicenter studies involving people with Parkinson's, as well as animal studies, are necessary to identify the mechanisms that underlie the connection between the gut and Parkinson's. Human studies should look at the composition of the gut microbiota both before and after Parkinson's diagnosis

Furthermore, the study authors appreciate that in the next few decades, the gut microbiota will play a special role in the development of new therapies for Parkinson's. Such therapies can include dietary changes, the use of pro- and prebiotics, and fecal transplants. "Our understanding and appreciation of the importance of the gut-brain connection in [Parkinson's have] grown rapidly in recent years," says Dr. Scheperjans.

"We are confident that the coming 2 decades of microbiome-gut-brain-axis research will see an even accelerated development in this area that will reshape our understanding of the pathogenesis of [Parkinson's]," he adds.

Dr. Patrik Brundin, PhD—editor-in-chief of the Journal of Parkinson's Disease—also comments on the findings. "The gut has emerged as one of the new frontiers in [Parkinson's] research," he says. "We predict there will be several advances regarding the gut in the coming 20 years."

"Changes in the gut might be utilized to diagnose [Parkinson's] earlier; new therapies targeting these changes might slow disease progression, reduce constipation, and improve gut function in patients who have already been diagnosed."

—Dr. Patrik Brundin, PhD