The idea that the stomach and the brain are somehow connected has long been reflected in common sayings like “gut feeling” or “butterflies in the stomach.” For centuries, these phrases were considered mere figures of speech, but modern neuro-science has revealed that they are rooted in biological truth.

The gut and the brain are intimately linked through a complex communication network known as the gut–brain axis, and this connection means that what we eat can profoundly affect how we feel and think, giving a whole new level of meaning to the old adage; you are what you eat.

Nerve and Gut Linkage

The gut–brain axis functions as a two-way communication system between the digestive tract and the central nervous system. Messages travel along multiple channels—through nerves, hormones, and immune signals. One of the most important of these channels is the vagus nerve, which runs from the brainstem to the abdomen.

It acts like a high-speed cable, constantly relaying information about hunger, satiety, and stress between the two organs. When we experience anxiety, for example, signals travel from the brain to the gut, causing cramps or nausea. On the flip side, when the gut becomes inflamed or imbalanced, distress signals can travel upward and influence our emotions and mental clarity.

The Second Brain Down Under

Lining the digestive tract is a vast network of about 100 million neurones known as the enteric nervous system, sometimes referred to as the “second brain.” This system can function independently, coordinating digestion, motility, and enzyme release even without direction from the brain.

“beneficial nutrients, polyphenols—plant compounds abundant in berries, green tea, olive oil, and dark chocolate—act as prebiotics, feeding good microbes while reducing oxidative stress in the brain“

Yet it also communicates continuously with the central nervous system, forming a feedback loop that influences not only digestion but also mood and cognition. This interplay helps explain why gastrointestinal disorders are so often accompanied by anxiety or depression, and why psychological stress can worsen gut symptoms. Gut symptoms in turn, constant pain or even a continuous but mild churning of the stomach, may adversely affect one’s day.

Friendly Gut Microbiome

At the heart of this connection lies the gut microbiome, the trillions of bacteria, fungi, and viruses that live within our intestines. These microorganisms outnumber our human cells by roughly ten to one, forming an ecosystem as intricate as a rainforest.

They help digest food, produce vitamins, and regulate the immune system—but their influence extends far beyond the gut. Gut microbes produce many of the same chemicals that the brain uses to regulate mood, such as serotonin, dopamine, and gamma-aminobutyric acid (GABA).

In fact, around ninety percent of the body’s serotonin, a key neurotransmitter for mood balance, is produced in the gut. When this microbial community is balanced and diverse, it supports emotional stability and cognitive sharpness.

When it becomes disturbed, a condition known as dysbiosis, the resulting inflammation and chemical imbalances can contribute to low mood, anxiety, and mental fog. Alcohol, for example, releases strong levels of GABA which, due to their strong inhibitory nature could greatly lead to grogginess  and diminished energy levels the following day. 

Breeding Friendly Microbiome 

Diet is the single most powerful factor shaping the gut microbiome, yet often ignored. Every meal we eat nourishes certain groups of bacteria while discouraging others. Diets rich in plant fibre, for instance, feed beneficial microbes that ferment the fibre into short-chain fatty acids such as butyrate, acetate, and propionate.

“The iconic boba tea has solidified its role as a coping mechanism for all age groups at times of stress, with consumers being oblivious to its high levels of sugar which perpetuates an entire vicious cycle of stress and fatigue due to spikes of glucose rise and fall in the gut“

These compounds have anti-inflammatory properties and help maintain the integrity of the gut lining, preventing harmful substances from leaking into the bloodstream. Butyrate has even been shown in laboratory studies to enhance memory and learning, suggesting a direct link between dietary fibre and cognitive performance.

When fibre intake is low, however, beneficial bacteria starve, and harmful species gain dominance, increasing inflammation and reducing the brain’s resilience to stress. Some reach for a course of antibiotics when the bowel feels unwell, hoping for a quick fix but unwary of the detrimental consequences to pre-existing healthy microflora which takes weeks to months to re-cultivate. 

Fermented foods such as yogurt, kefir, kimchi, sauerkraut, and miso provide another route to a healthy gut. These foods contain live bacteria—probiotics—that can replenish and diversify the microbiome.

People who eat fermented foods regularly tend to have lower levels of inflammation and improved emotional well-being. In one study, participants who consumed fermented foods daily for ten weeks showed measurable improvements in mood and reductions in stress-related biomarkers.

High Sugar Diet and the Vicious Cycle

In contrast, diets high in sugar and processed foods disrupt the microbial balance and promote inflammation. Excess sugar feeds bacteria that produce toxic byproducts, weakening the gut barrier and triggering immune responses that can reach the brain.

These same foods cause rapid swings in blood sugar, leading to irritability, fatigue, and the familiar “sugar crash.” In these modern era, access to sugar rich diet is even easier than before with the availability of canned drinks and fast food.

The iconic boba tea has solidified its role as a coping mechanism for all age groups at times of stress, with consumers being oblivious to its high levels of sugar which perpetuates an entire vicious cycle of stress and fatigue due to spikes of glucose rise and fall in the gut. Chronic inflammation, driven by poor diet, has been increasingly recognised as a major contributor to depression and cognitive decline. 

Healthy Neuronal Building Blocks

Healthy fats also play a crucial role. Omega-3 fatty acids, found in fish like salmon and sardines as well as in flaxseeds and walnuts, are essential for maintaining the structure of brain cell membranes and reducing inflammation.

They support the growth of beneficial gut bacteria and protect against “leaky gut,” a condition in which gaps in the intestinal wall allow toxins to enter the bloodstream. Another group of beneficial nutrients, polyphenols—plant compounds abundant in berries, green tea, olive oil, and dark chocolate—act as prebiotics, feeding good microbes while reducing oxidative stress in the brain.

Diets rich in these compounds, such as the Mediterranean diet, have repeatedly been linked with lower rates of depression and dementia.

Leaky Gut and Neuroinflammation

The concept of “leaky gut” has led to what researchers call the “leaky brain” hypothesis. Leaky gut usually stems from diet rich in fried food and processed red meat which contain high levels of free radicals and inflammatory agents.

When the gut barrier becomes compromised due to inflammation, stress, or poor diet, bacterial toxins can enter the bloodstream and activate immune cells in the brain. This neuroinflammation can disrupt the delicate chemical signaling required for healthy mood and cognition.

“When fatigue and brain fog sets in, we are quick to search for easy fixes such as caffeine and energy drinks but the cultprit may be indigestion and bowel inflammation from an unhealthy portion of meat served the day before”

Such processes are increasingly being studied in connection with depression, anxiety, autism spectrum disorder, and even neurodegenerative diseases like Alzheimer’s. In this light, protecting gut health through good nutrition is not merely a digestive concern but a form of mental self-care.

Evidence supporting the diet–mood connection has grown rapidly in recent years. The SMILES Trial, conducted in Australia in 2017, was one of the first controlled studies to show that dietary change can directly improve depression.

Participants with moderate to severe depression who adopted a Mediterranean-style diet for twelve weeks experienced significantly greater improvements in mood than those who received social support alone.

Other population studies have shown that traditional diets, rich in vegetables, legumes, whole grains, and fermented foods, are associated with lower rates of depression and cognitive decline compared to Western-style diets high in sugar, red meat, and refined carbohydrates.

However the role of diet is easily overlooked. When fatigue and brain fog sets in, we are quick to search for easy fixes such as caffeine and energy drinks but the cultprit may be indigestion and bowel inflammation from an unhealthy portion of meat served the day before.

The effects of gut health extend to cognition as well as mood. A balanced microbiome helps regulate inflammation, produces neurotransmitter precursors, and generates short-chain fatty acids that serve as fuel for brain cells.

It also influences the stress response system, known as the hypothalamic–pituitary–adrenal axis, reducing the damaging effects of chronic stress hormones like cortisol. When the gut is out of balance, people often report symptoms such as brain fog, poor concentration, and fatigue—all of which can improve when the microbiome is restored through diet and lifestyle changes.

Eat Your Rainbows

Fortunately, supporting the gut–brain axis doesn’t require exotic supplements or extreme diets. The most effective strategies are simple and grounded in everyday habits. Eating a variety of colourful fruits and vegetables ensures a steady supply of fibre and antioxidants that nourish good bacteria.

Following a Mediterranean-style diet—emphasising olive oil, nuts, legumes, fish, and whole grains—provides the nutrients and fats that both the gut and brain need. Including a small serving of fermented food each day can further enhance microbial diversity. It is equally important to limit added sugars, processed foods, and refined carbohydrates, which feed harmful microbes and destabilise mood.

Hydration, adequate sleep, stress management, and regular exercise all support a balanced microbiome as well. Physical activity in particular has been shown to increase microbial diversity and promote the release of brain-derived neurotrophic factor, a molecule that supports memory and learning.

Breeding Healthy Micro Allies from Within

The study of the gut–brain axis has opened the door to a new field of research known as psychobiotics—probiotic strains specifically designed to influence mood and cognition. Early trials suggest that certain species of Lactobacillus and Bifidobacterium can reduce anxiety, stress, and even symptoms of depression.

Advances in microbiome sequencing now allow scientists to identify bacterial patterns associated with particular mental states, raising the possibility of personalised nutritional and microbial therapies in the future.

This emerging understanding challenges the traditional separation between body and mind. It reminds us that mental health is not confined to the brain but is deeply intertwined with the rest of the body, especially the gut.

Every meal we eat sends biochemical messages to the brain—some soothing and stabilising, others inflammatory and disruptive. Our emotions, resilience, and clarity of thought are therefore shaped not only by genes and psychology but also by the quality of our diet.

Conclusion

To care for the mind, then, we must also care for the gut. A diet rich in whole, unprocessed foods and living microbes can nurture a balanced microbiome, reduce inflammation, and promote mental clarity.

The old saying “you are what you eat” is truer than ever—but modern science adds a new layer: how you feel and think may also depend on what your gut bacteria are eating. In the intricate dialogue between brain and body, every bite is a message, and a healthy gut speaks the language of calm, focus, and well-being.

This article is contributed by Dr Chong Siew Koon, Psychiatrist, Hospital Kuala Lumpur, and Dr Ong Wan Xi, Psychiatrist, Hospital Taiping.