Brain and gut are connected, the health of one depends on the health of the other. When there are alterations in the microbiota, which is the set of bacteria that make up our intestinal flora, this can cause a series of processes that lead to neuroinflammation, damage to neurons and finally, in the long term, even degenerative diseases such as Alzheimer's. What can we do? Definitely take care of our microbiota, as emerges from a very recent review published in the Nutrients journal by a Lithuanian team (Megur et al, Nutrients, Dec 2020).
For a long time, it was believed that the microbiota had a role only with regard to the processes that occur at the level of the gut, therefore digestion and barrier for pathogens that have reached the gastrointestinal tract. However, more and more research confirms that the microbiota actually plays a much larger role, supporting the immune system, protecting against overweight and chronic inflammatory conditions. Not only that, the microbiota also communicates with the brain and its imbalance leads to inflammation that causes activation of the immune response and an alteration in the release of neurotransmitters and therefore in the functioning of the brain. For example, different strains of gut bacteria can synthesize serotonin, the feel-good hormone, which then enters the bloodstream and reaches the brain. If these bacteria can no longer work effectively due to changes in the bacterial flora and inflammation, then the circulating serotonin is also reduced. Not only that, it has been observed that, in people with Alzheimer's, the microbiota is depleted of those bacteria capable of producing butyric acid, with an anti-inflammatory action and capable of protecting the brain. In fact, low butyric acid levels are associated with neuroinflammation and cognitive impairment. And that's not all, very recent studies have observed that Alzheimer's can even start from the gut and then spread to the brain. In particular, what has been found is that some intestinal bacteria, not belonging to the so-called good bacteria of the microbiota, such as Escherichia coli, Salmonella enterica or Staphylococcus aureus, can produce accumulations of amyloid proteins. Today we know that Alzheimer's is the result of the accumulation of amyloid proteins in the brain. These accumulations produced by bacteria are not the same as those that form in the brain but they are very similar. It is believed that the appearance of these accumulations in the intestine increases the immune response and induces the release of beta amyloid aggregates in the brain.
Probiotics, as pointed out in the article, can help normalize the intestinal microbiota and the processes that connect the intestine and brain. It has been observed, for example, that the intake of probiotics, in particular Bifidobacterium brevis, in an animal model protected cognitive function in the case of Alzheimer's. Lactobacillus casei was found to be useful in reducing neuroinflammation. Lactobacillus acidophilus, Bifidobacterium bifidum, and Lactobacillus fermentum bacteria have also been shown to protect the brain and its function. It is believed that, in addition to the anti-inflammatory and regulating action of intestinal and brain exchanges, probiotics are also able to activate the immune response capable of removing the already formed beta amyloid protein plaques. However, much remains to be understood and above all it is necessary to study the right doses of probiotics for a possible treatment against Alzheimer's. Meanwhile, what we can do is follow a healthy and varied diet that also includes probiotics.