Why does our memory decline with age?

We all know that as we age, our memory and cognitive function decline. This is often accepted as a natural part of aging, but have we stopped to ask why this happens? And more importantly, whether we can slow it down without just doing more sudokus? Increasingly, research suggests that the answer may lie not only in the brain, but also in the gut.

So, what exactly is the gut microbiome? This term is frequently mentioned in the media, often without much explanation. In simple terms, the gut microbiome refers to the enormous ecosystem of bacteria and other microbes living in our intestines. These microbes are not just passengers, they play essential roles in digestion, immune function, metabolism, and even brain health.

The idea that the gut and brain are connected is relatively new in modern science, but evidence for this “gut–brain axis” has grown rapidly. The intestine has its own network of nerves, which communicates with the brain through pathways such as the vagus nerve. In addition, gut microbes produce many biologically active molecules that can influence mood, immunity, and brain function. Given this close connection, it is reasonable to suspect that changes in the gut could also affect memory.

As we age, the brain gradually becomes less efficient, with declines in memory, learning, and the ability to process information. A recent study published in Nature, one of the most prestigious scientific journals, has provided new insight into how the gut microbiome may contribute to this process. Using mouse models, researchers were able to manipulate the gut microbiomes of young and old animals. When the microbiome from an aged mouse was transferred into a young mouse, the young mouse developed memory impairments. Remarkably, when the microbiome was reduced using antibiotics, these cognitive problems could be reversed.

The researchers also identified a specific bacterial species, Parabacteroides goldsteinii, which increased in abundance with age. When this bacterium was introduced into mice that otherwise had no gut microbes, it was sufficient to cause memory impairment. Importantly, the scientists showed that the effect was not due to structural damage in the brain. Instead, the problem appeared to lie in communication between the gut and the brain, particularly through the vagus nerve.

Further experiments revealed that this bacterium produces molecules called medium-chain fatty acids (MCFAs). These are small fat-derived molecules that the body can use as a rapid source of energy, but in this context, they had an unexpected effect. Increased levels of these molecules reduced activity in the vagus nerve and weakened the signals being sent from the gut to the brain. Even more interestingly, the researchers found that MCFAs act by activating a receptor on immune cells known as GPR84. When this pathway was blocked, memory function could be restored in the mice.

These findings suggest that age-related changes in the microbiome may contribute to cognitive decline by altering immune activity and disrupting gut-to-brain signalling, rather than by directly damaging the brain itself. In the future, this could open the possibility of slowing memory decline by targeting the gut microbiome, the immune system, or the vagus nerve.

It is still too early to know whether these results will translate to humans, but the study highlights how important the gut may be for brain health. Far from being just a digestive organ, the gut appears to be part of a complex system that helps regulate how the brain functions throughout life.

You can read the original paper here:
https://www.nature.com/articles/s41586-026-10191-6