MS, Parkinson's, Alzheimer's

Neurological disorders often have an inflammatory aspect, and here too, relationships with an altered microbiome are not uncommon. This may seem unlikely, but the intestines are very rich in nerves and have two-way communication with the brain. Evidence that the microbiome can bring about a direct modulation of the central nervous system, and vice versa, is piling up. Evidence of its relationship with the microbiome has been found in:

In MS patients, abnormalities have been found in the blood concentrations of substances of bacterial origin, which are known to affect neuroinflammation (1). Differences in the composition of the microbiome have also been found in MS patients, with the most notable difference being a lower number of bacteria from species that can dampen autoimmune responses (2-4). With modern techniques, traces of bacteria are now also found in organs that were previously considered sterile. An intriguing study compared bacteria in the brains of deceased MS patients with those in the brains of other deceased individuals and found notable differences there as well, with a strong correlation between the numbers of bacteria found and the disease-related immune response (5). Initial studies on the impact of probiotics in MS patients show shifts in the microbiome toward that of healthy individuals, cautious indications of a decrease in autoimmune activity, and reductions in disability and mental health problems (6-8).

This is the second most common neurodegenerative disease, in which the motor regions in the brain in particular are damaged. An important characteristic of Parkinson's is the reduced production of dopamine. The microbiome regulates the production of enzymes for dopamine production, and bacteria also produce dopamine themselves, accounting for half of all dopamine in the body (9). Many patients appear to suffer from constipation even before the onset of movement problems (9-11). Constipation occurs in about 80% of patients and is accompanied by accumulation of A-synuclein, damage to intestinal nerves, and increased permeability of the intestinal wall (9,11). Abnormalities in the composition of the microbiome have also been found in Parkinson's, with the amount of a certain bacterial genus correlated with the severity of the movement problems (10). Finally, small intestine bacterial overgrowth is also associated with Parkinson's (9). Although there is strong evidence for a role of the microbiome in Parkinson's, no studies have yet been published on a direct modulation of the microbiome (via probiotics or faecal transplantation, for example).

An important hypothesis about the cause of Alzheimer's is that this disease results from accumulation of amyloid plaques. In an animal model, a changed composition of the microbiome was found to contribute to the deposition of amyloid plaques (10). In addition, a different composition of the microbiome has also been found in Alzheimer's patients compared to healthy individuals (12). In the bacterial genera where the abnormalities were greatest, the amount was correlated with the severity of the disease (12). It is remarkable that more and different bacteria have been found in the brains of deceased Alzheimer's patients than in the brains of healthy people (10). In addition, increased levels of endotoxins of bacterial origin (LPS) have been found in the brains of Alzheimer's patients, ranging from 3 times to, in some patients with advanced Alzheimer's, sometimes as much as 26 times as much as in healthy control subjects (13). These substances cause neuroinflammatory responses, similar to the neuroinflammation found in Alzheimer's (14). Finally, a small study on the effect of probiotics in Alzheimer's patients suggests an improvement in cognitive functioning and a reduction in inflammatory factors (7)