These three digestive diseases are linked to a higher risk of Alzheimer's and Parkinson's.

People diagnosed with certain bowel diseases, such as colitis , gastritis , esophagitis, or functional bowel disorders, are also more likely to develop Alzheimer's and Parkinson's, according to a new 15-year analysis based on clinical and genomic data from the UK Biobank and two other public sources, published in the journal Science Advances.

Taking advantage of these extensive databases, the study explores the relationship between intestinal disorders and neurodegeneration, incorporating genetics and proteomics into its characterizations.

Gut pathologies can influence brain health and vice versa. This is known as the gut-brain axis. Unraveling the intricacies of this connection, especially the relationship between neurodegenerative diseases such as Alzheimer's and Parkinson's, and digestive disorders, could help scientists identify predictors of disease and improve early detection and treatment techniques.

Using data from the UK Biobank, the SAIL database, and the FinnGen project, Mohammad Shafieinouri and colleagues investigated correlations between 155 diagnoses of nutritional, metabolic, digestive, and endocrine disorders. They used statistical models that examined the connection between the timing of diagnosis of gut-brain disorders and that of Alzheimer's or Parkinson's diagnosis (ranging from 1 to 5, 5 to 10, and 10 to 15 years before Alzheimer's or Parkinson's).

The clinical data included more than 502,000 individuals. They also used linear models of UK Biobank data to generate polygenic risk scores and evaluate 1,463 known proteomic biomarkers. The genetic data included more than 487,000 individuals, and the proteomic data included more than 52,000 individuals.

Overall, the researchers found that any coexisting bowel diagnosis contributes to the risk of Alzheimer's or Parkinson's, supporting existing work. There was no statistically significant correlation between the years since diagnosis and the onset of neurodegeneration.

Among these intestinal pathologies, people with noninfectious colitis, gastritis, and esophagitis had a higher rate of developing Alzheimer's or Parkinson's. This relationship also appeared with functional intestinal disorders.

"These data corroborate existing evidence about the so-called gut-brain axis, so it is considered important to pay attention to these intestinal pathologies in order to closely monitor patients and attempt an early diagnosis of neurodegenerative brain diseases," says José Luis Lanciego, senior researcher of the Gene Therapy Program in Neurodegenerative Diseases at the Center for Applied Medical Research (CIMA) of the University of Navarra. Speaking to SMC Spain, the expert explains that the digestive system is often considered "the second brain" because it has numerous neurons in its submucosal layer.

"It is connected to the brain bidirectionally through the vagus nerve. There is considerable evidence demonstrating a key role of the intestinal microbiota and its alterations (known as intestinal dysbiosis) in triggering neurodegenerative brain diseases. In fact, a few days ago it was shown that when a certain bacteria present in people with poor oral health is administered to mice, they develop deposits of the protein called beta-amyloid in the form of plaques very similar to those found in the brains of patients with Alzheimer's disease," Lanciego concludes.