MOST Malaysians have a complicated relationship with their gut.
It grumbles when breakfast gets skipped, protests loudly after a particularly ambitious plate of nasi lemak and stages a full revolt after that questionable roadside food at midnight.
But the idea that the trillions of bacteria living inside the digestive system could actually be pulling strings on your emotions, anxiety levels and even depression is something else entirely.
Is it really true that gut bacteria can influence your mood?
Verdict:
TRUE, BUT
It sounds like something out of a science fiction film, but the evidence is real, peer-reviewed and growing fast enough that even the most sceptical researchers have stopped raising their eyebrows.
The gut is home to roughly 100 trillion microorganisms, and they do considerably more than help digest nasi kandar.
Think of them as a vast, invisible workforce living in the digestive system, producing chemicals, sending signals to the brain and quietly influencing how a person feels from one hour to the next.
The connection runs through what scientists call the gut-brain axis, a two-way communication highway linking the digestive system directly to the brain through nerve signals, hormones and the immune system.
Here is the fact that tends to stop people mid-conversation: approximately 90% of the body’s serotonin, the neurotransmitter most closely associated with happiness and emotional wellbeing, is not produced in the brain.
It is produced in the gut, and the bacteria living there play a direct role in making it.
Research published in the journal Cell, conducted at the California Institute of Technology, finds that specific gut bacteria are essential for triggering the gut cells that produce serotonin, and that mice raised without any gut bacteria produce about 60% less serotonin than those with normal bacterial colonies.
When normal gut bacteria are reintroduced, serotonin levels recover, demonstrating that the damage is reversible.
A comprehensive review published in the United States National Institutes of Health’s research database, examining the relationship between the gut microbiome and mental health, confirms that imbalances in gut bacteria are consistently associated with anxiety, depression and stress-related conditions.
The mechanism is not just about serotonin.
Gut bacteria communicate with the brain through the vagus nerve, a long nerve running from the brainstem all the way down to the abdomen, essentially acting as a private telephone line between the stomach and the brain.
Peer-reviewed research in 2025 confirms that in animal studies, cutting this nerve completely eliminates the mood benefits associated with probiotic use, suggesting the vagus nerve is a critical part of how the gut communicates with the brain.
Gut bacteria also regulate inflammation throughout the body, including in the brain, and influence the body’s stress response system, adding further layers to an already surprisingly complicated picture.
Human clinical trials back the theory.
A systematic review and meta-analysis of randomised controlled trials, the gold standard of medical research, published in BMC Psychiatry in 2025, finds that probiotic supplementation produces measurable improvements in anxiety, depression and sleep quality compared to placebo groups across multiple studies.
A separate 2023 randomised controlled trial finds that six weeks of multi-strain probiotic use in healthy adults reduces depression and anger, improves fatigue and sleep quality, and produces favourable changes in mood markers compared to those taking a placebo.
The important caveat is that the field is still developing.
Many studies are conducted on relatively small groups, the specific bacterial strains that produce the most consistent mood benefits are still being identified, and researchers are careful to stress that probiotics are not a replacement for established mental health treatments.
What the science does clearly support is a simpler and more actionable message: diet shapes gut bacteria, and gut bacteria shape mood.
Foods that nourish a healthy and diverse gut microbiome, including fermented foods like tempeh and yoghurt, fibre-rich vegetables, legumes and whole grains, appear to support better mental wellbeing.
Diets high in processed food, sugar and refined carbohydrates are consistently linked to reduced microbial diversity and poorer mood outcomes.
For a country where mental health remains significantly underdiagnosed and where the national diet has been shifting steadily towards more ultra-processed foods, the idea that what ends up on the plate today might influence how a person feels tomorrow is turning out to be considerably more literal than most Malaysians had ever imagined.
Sources:
1. https://pmc.ncbi.nlm.nih.gov/articles/PMC12007925/
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC12038870/
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC12028401/
4. https://www.frontiersin.org/journals/microbiomes/articles/10.3389/frmbi.2025.1701608/full
5. https://www.caltech.edu/about/news/microbes-help-produce-serotonin-gut-46495
6. https://www.nature.com/articles/s41598-022-05756-0
7. https://link.springer.com/article/10.1186/s12888-025-07644-z
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC10501394/
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC12196188/
