As your "second brain," the state of your gut has been shown to play an important role in your neurological health. Importantly, studies have shown probiotics (beneficial bacteria) can help decrease pathological hallmarks of Alzheimer's disease (AD), including amyloid plaques and tangles.1
One of the most impressive of these studies2 was published in Frontiers in Aging Neuroscience in 2016. Sixty elderly patients diagnosed with Alzheimer's received either a placebo or a probiotic milk product containing Lactobacillus acidophilus, Lactobacillus casei, Bifidobacterium bifidum and Lactobacillus fermentum for 12 weeks.
At the beginning and end of the study, participants underwent a standardized cognitive assessment and a highly sensitive c-reactive protein test, which is a powerful marker of inflammation. As reported by neurologist Dr. David Perlmutter:3
"The results of the study were stunning. The placebo group showed an increase in hs-CRP, the inflammation marker, by an impressive 45 percent. In the group taking the probiotic, on the other hand, hs-CRP … declined by 18 percent, indicating a dramatic reduction in inflammation.
But here's the truly exciting news. Over the 12 weeks, the patients in the placebo continued to decline mentally, as you might expect … But the group on the inflammation reducing probiotics actually demonstrated … improvement, with their MMSE [mini mental state examination] scores going from 8.67 up to 10.57, and that's a huge improvement.
Again, not only was their mental decline stopped in its tracks, these individuals regained brain function! The message here is that inflammation is directly determined by the health and diversity of our gut bacteria, and this has major implications in terms of brain health, function, and disease resistance."
Your Gut and Brain Are Linked
Since then, several other studies have been published, showing probiotics can help improve cognitive function and ward off dementia, including Alzheimer's. While this may seem all too simple to be true, it makes perfect sense when you consider the deep connections that exist between your gut and your brain. As explained by Harvard Health:4
"Research shows that the gut and brain are connected, a partnership called the gut-brain axis. The two are linked through biochemical signaling between the nervous system in the digestive tract, called the enteric nervous system, and the central nervous system, which includes the brain.
The primary information connection between the brain and gut is the vagus nerve, the longest nerve in the body. The gut has been called a 'second brain' because it produces many of the same neurotransmitters as the brain does … What affects the gut often affects the brain and vice versa.
When your brain senses trouble—the fight-or-flight response—it sends warning signals to the gut, which is why stressful events can cause digestive problems like a nervous or upset stomach. On the flip side, flares of gastrointestinal issues like irritable bowel syndrome (IBS), Crohn's disease, or chronic constipation may trigger anxiety or depression."
Probiotic Prevents Amyloid-Beta-Induced Dysfunction
According to a 2017 study5 in Scientific Reports, the Bifidobacterium breve strain A1 may be of particular use in Alzheimer's treatment. Using Alzheimer's disease model mice, the researchers were able to confirm that daily oral administration of B. breve A1 reduced the cognitive dysfunction normally induced by amyloid beta. According to the authors:
"We also demonstrated that non-viable components of the bacterium or its metabolite acetate partially ameliorated the cognitive decline observed in AD mice.
Gene profiling analysis revealed that the consumption of B. breve A1 suppressed the hippocampal expressions of inflammation and immune-reactive genes that are induced by amyloid-β. Together, these findings suggest that B. breve A1 has therapeutic potential for preventing cognitive impairment in AD."
One of the mechanisms behind these protective effects was found to be suppression of amyloid-beta-induced changes in gene expression in the hippocampus. In short, the bacterium had an ameliorating effect on amyloid-beta toxicity.
Interestingly, B. breve A1 did not actually alter the composition of the animals' gut microbiota to any significant degree; rather, many of the benefits appear to be related to significantly raised blood levels of acetate, a byproduct of fermentation by gut bacteria. As explained by the authors:6
"One of the main functions of gut microbiota is the fermentation of dietary fibers in the gut and produce short-chain fatty acids (SCFAs), mainly acetate, propionate, and butyrate, which are the major end products of carbohydrate metabolism in Bifidobacteria."
When looking at SCFA levels in the animals' blood, the mice treated with B. breve A1 had significantly higher levels of acetate, but not propionate or butyrate, compared to controls.
Other microbiota-derived SCFAs may also play a role, however. A 2019 study7 found that rats given both probiotics and prebiotics performed significantly better on spatial memory tests, and this improvement was attributed to increased levels of brain-derived neurotrophic factor (BDNF).
Butyrate — a SCFA produced when gut bacteria ferment fiber — activates the secretion of BDNF. According to the authors, the improvement also correlated with "decreased levels of pro-inflammatory cytokines and better electrophysiological outcomes in the hippocampi."
This led them to conclude that "the results indicated that the progression of cognitive impairment is indeed affected by changes in microbiota induced by probiotics and prebiotics."