India is projected to have 20% of its population over the age of 60, by the year 2050, accounting for 15.4% of worldwide population in the same age category [1]. This demographic trend reflects the rising longevity; 42.9 years in 1970 to 70.4 years as in 2020 [2]. Age is the strongest and best known risk factor for dementia. A 2025 study from Chennai found that nearly half of older adults screened reported subjective cognitive decline and alarmingly, this was prevalent even among individuals without major comorbid conditions such as diabetes or hypertension [3].

There have been studies to identify potential causes to cognitive decline outside of the brain. In the recent past, the focus is towards gut health and its impact on other organs and organ systems.

Gut microbiota changes with age, characterized by reduced diversity, expansion of pathobionts (opportunistic microbes that are normally harmless but can cause harm when the gut environment is disrupted), and alteration of small molecule metabolites (crucial building blocks, energy sources, and signalling agents).

Communication between the gastrointestinal (GI) tract and the brain is critical in maintaining food intake control and overall energy regulation. The key conduit of communication between the gut and the brain is the vagus nerve, whose afferent/sensory neurons (vagal afferent neurons, VAN) transmit a wide array of metabolic information.

The hippocampus, a brain region crucial for memory formation, storage, and recall, is known to decline with age, the exact mechanisms behind this memory loss are not yet fully understood.

The article “Intestinal interoceptive dysfunction drives age-associated cognitive decline [4]”, presents a detailed work on gut health and its impact on cognitive decline. In the article, the experiments are done on lab bred mice, it is a significant step to understand a similar impact of human diet and its implication on gut health and its corresponding impact on vagal nerve functioning.

The article establishes a core three-step pathway to Memory Loss.

Microbiome shift -> Gut immune inflammation -> Vagal silencing of hippocampus -> Memory Loss.

In the experiments conducted on mice, below are the key findings

1. Aging changes the gut bacteria. A specific bacterium, Parabacteroides goldsteinii, increased in abundance with age and was transmissible between animals through co-housing and microbiota transplantation.

2. The paper’s key mechanistic link, P.goldsteinii, produces Medium Chain Fatty Acids (MCFA), which activate GPR84 receptors on myeloid immune cells, triggering the inflammatory response.

3. The vagus neuron activity is suppressed.

4. Brain responses in the Hippocampus (HPC) is weakened due to low signals received. This leads to cognitive decline, leading to memory loss. Several additional brain areas, including the nucleus tractus solitarii (NTS), and somatosensory and entorhinal cortices, showed reduced neuronal activation. Given the role that these regions of the brain play in the sensory processing, the researches postulated that ageing leads to reduced transduction of interoceptive information through gut-innervating sensory neurons

The researchers tested interventions that improved memory in the older mice successfully

1. Bacteriophages (viruses that kill a specific type of bacteria) to reduce the inflammatory bacteria, Parabacteroides distasonis. The paper found it reduced intestinal MCFA levels, through transcriptional effects on P. goldsteinii.

2. GPR84 inhibitors – Drugs that block the inflammatory signal

3. Vagus nerve stimulation – To restore gut-brain communication

This research suggests that age-related memory decline isn't just about the brain itself—it's also about the communication breakdown between your gut and brain. The study opens up new possibilities for treating age-related cognitive decline by targeting the gut microbiome or restoring gut-brain signalling, rather than only focusing on the brain.

Citations:

[1]: International Institute for Population Sciences (IIPS), NPHCE, MoHFW, Harvard T.H. Chan School of Public Health (HSPH), and the University of Southern California (USC). Longitudinal Ageing Study in India (LASI) Wave 1, 2017-18, India Report. International Institute for Population Sciences; 2020.

[2]: United Nations, Department of Economic and Social Affairs, Population Division. World Population Prospects 2019 United Nations; 2020. Accessed September 3, 2020.

[3]: Menon J, Kantipudi SJ, Vinoth S, Kuchipudi JD. Prevalence of subjective cognitive decline and its association with physical health problems among urban community dwelling elderly population in South India. Alzheimers Dement. 2025 Feb;21(2):e14505. doi: 10.1002/alz.14505. Epub 2025 Feb 12. PMID: 39935341; PMCID: PMC11848155.

[4]: Cox, T.O., Devason, A.S., de Araujo, A. et al. Intestinal interoceptive dysfunction drives age-associated cognitive decline. Nature (2026). https://doi.org/10.1038/s41586-026-10191-6