The Gut–Brain Axis. How Nutrition Shapes Your Mood, Focus, and Mental Well-being

When most people think about mental health, they think about the brain. But some of the most important conversations affecting your mood, memory, and stress response are happening nearly two feet away, in your gut.

I’m Jessica, a functional medicine nutritionist, and the gut–brain axis is one of the most exciting areas of nutrition research today. It’s a complex, two-way communication network linking your digestive system and your brain, and it influences far more than digestion. It plays a measurable role in your mood, stress response, mental clarity, sleep, and overall mental well-being.

Here’s what the science says and what it means for how you eat.

What Is the Gut–Brain Axis?

The gut and brain are constantly in conversation through interconnected systems. Rather than functioning independently, they work together to regulate everything from digestion to emotional health. Three main pathways carry the traffic.

The Nervous System

The vagus nerve is one of the body’s main communication highways, carrying signals between the gastrointestinal tract and the brain in both directions.

This is why you might feel butterflies before a presentation or lose your appetite during stressful times. It also works the other way: problems in the gut can send signals back to the brain that influence mood, stress resilience, and cognitive function. Your gut isn’t simply responding to your emotions; it’s actively communicating with your brain every day.

The Immune System

Around 70% of the body’s immune cells live in the gut. When the intestinal lining is compromised or the gut microbiome loses diversity, inflammatory signals can increase throughout the body.

Chronic low-grade inflammation has been linked to depression, fatigue, anxiety, and cognitive impairment. Inflammation is rarely the sole cause of these conditions, but it may contribute to symptom severity, which is why supporting gut health matters for the brain as well as the digestive system.

Hormones and Neurotransmitters

Your gut bacteria produce and influence compounds that communicate with the brain and affect neurotransmitter activity, including serotonin, dopamine, and GABA.

One often-cited figure is that around 90–95% of the body’s serotonin is made in the gut. This serotonin doesn’t cross the blood–brain barrier, but it communicates with the brain through immune, hormonal, and nervous system pathways. Meanwhile, the brain produces its own serotonin from the amino acid tryptophan, which we get from dietary protein. Nutrition and gut health both support brain function through distinct yet interconnected pathways.

Your Gut Microbiome: More Than Just Digestion

Trillions of bacteria live inside your digestive tract. These microbes help digest food, produce vitamins, strengthen the immune system, and manufacture compounds called short-chain fatty acids (SCFAs) that help maintain the intestinal barrier, calm inflammation, and support gut–brain communication.

Emerging research suggests the gut microbiome may influence mood, stress resilience, memory, motivation, cognitive function, and sleep quality. Scientists are also investigating its potential role in conditions such as depression, Parkinson’s disease, and the cognitive symptoms experienced by some people with long COVID.

The research is still evolving, but the message is increasingly clear: a healthy gut supports a healthy brain.

How Diet Shapes the Gut–Brain Axis

The foods we eat have a profound impact on both our gut microbiome and our brain.

Diets high in ultra-processed foods and added sugars can reduce microbial diversity, promote inflammation, and contribute to unstable blood glucose. Those blood sugar swings often show up as low energy, irritability, cravings, and difficulty concentrating.

Diets rich in fibre, vegetables, fruit, legumes, whole grains, healthy fats, and high-quality protein do the opposite: they feed beneficial gut bacteria, support stable blood sugar, reduce inflammation, and provide the raw materials for healthy brain function.

You don’t need to eat perfectly. You just need to eat in a way that consistently tips the balance in that direction.

Six Habits That Support a Healthy Gut–Brain Axis

Rather than chasing a single “superfood” or miracle supplement, focus on your overall pattern. These six habits are the ones I come back to most often with clients:

1. Eat a wide variety of plant foods each week. Aiming for around 30 different plants a week (herbs, spices, nuts, and seeds all count) is one of the best-supported strategies for microbial diversity.
2. Include protein with every meal to support neurotransmitter production and satiety.
3. Prioritise fibre-rich foods like vegetables, fruit, legumes, and whole grains.
4. Include fermented foods regularly.
5. Limit ultra-processed foods and foods high in added sugars.
6. Keep meal timing reasonably regular to support stable blood sugar and appetite regulation.

Small, consistent changes will always beat short bursts of perfection.

Why Fibre Matters More Than You Think

One of the biggest gaps I see in practice is low fibre intake. Most people eat only about half the recommended daily amount (about 15 g, compared with 25–30 g).

Fibre isn’t just about preventing constipation. It acts as a prebiotic, feeding the beneficial bacteria in your gut. Those bacteria then produce short-chain fatty acids that maintain the gut lining, regulate inflammation, and support gut–brain communication.

Many people reach for a probiotic before asking a more important question: Are you feeding the beneficial bacteria you already have? Without enough fibre, even the best probiotic may struggle to have a lasting effect.

Easy ways to add fibre without overhauling your diet:

• Add a tablespoon of chia or ground flaxseed to breakfast
• Swap white rice or pasta for wholegrain versions half the time
• Add quinoa and beans to write rice (https://ahnestkitchen.com/food/jagokbap)
• Keep tinned beans, lentils, or chickpeas on hand to add to soups, salads, and curries
• Leave the skin on fruit and vegetables where possible
• Choose a  smoothie over  juice 

Should You Eat Fermented Foods?

One of the easiest ways to support a healthy gut microbiome is to regularly include fermented foods. Research (including a well-known 2021 Stanford study) has shown that eating fermented foods daily can increase gut microbial diversity and reduce markers of inflammation. However, it’s worth noting that once participants reduced their fermented food intake after the intervention, many of these improvements appeared to diminish, highlighting the importance of making fermented foods a regular part of the diet rather than a temporary addition.

Good options include:

• Live yoghurt and kefir
• Sauerkraut and kimchi
• Miso, tempeh, and natto 
• Traditionally fermented pickles (look for “unpasteurised” or refrigerated versions, not shelf-stable vinegar pickles)
• Kombucha, in moderation

If you’re new to fermented foods, start with a spoonful or two and build up. Fermented foods introduce beneficial bacteria; fibre feeds the ones you already have. 

Foods such as tempeh and miso are often cooked or heated before eating, so they may contain few or no live microbes. However, they can still provide beneficial microbial compounds, known as postbiotics (formerly called paraprobiotics), along with other health-promoting fermentation products.

Mental Health Is Biological Too

Mental health is sometimes viewed through a purely psychological lens, but it’s also biological. Sleep, physical activity, inflammation, nutrient status, hormones, blood sugar, and gut health all influence how we think, feel, and function.

This doesn’t mean nutrition can cure anxiety or depression. But it does mean nutrition can support many of the biological systems involved in mental well-being. For some people, improving diet quality can meaningfully complement psychological therapy and medical treatment, creating a stronger biological foundation for long-term mental health. The SMILES trial (Jacka et al., 2017) was the first randomised controlled trial to show that dietary improvement can reduce symptoms of moderate to severe depression, and the evidence has continued to grow since.

Frequently Asked Questions

Can changing my diet really improve my mood?
For many people, yes, particularly if the starting point is a diet high in ultra-processed foods and low in fibre. Nutrition isn’t a replacement for therapy or medication, but improvements in mood, energy, and mental clarity are commonly reported alongside dietary changes.

Do I need a probiotic supplement?
Not usually as a first step. Fibre, plant variety, and fermented foods will do more for most people than a generic probiotic. Specific probiotic strains can be useful for particular conditions, but that’s best guided by a professional.

How long before I notice changes?
Gut microbiome composition can begin to shift within days of a dietary change, but noticeable changes in mood, energy, and digestion usually take two to six weeks of consistent eating.

Is there a “best” diet for the gut–brain axis?

No single diet fits everyone, but the strongest evidence supports dietary patterns based on whole, minimally processed foods. The Mediterranean diet has been studied most extensively, but many traditional Asian diets share the same core principles. Eating plenty of vegetables, fruit, legumes, soy foods, fish, whole grains, fermented foods, and healthy fats—while limiting ultra-processed foods and added sugars—can help support both gut and mental health.

When to See a Professional

If you’re experiencing persistent anxiety, low mood, brain fog, fatigue, digestive problems, or poor concentration, it may be worth considering whether nutrition is part of the picture.

Consider working with a nutritionist or dietitian if you:

• Have ongoing digestive symptoms (bloating, reflux, irregular bowels)
• Are struggling with mood, energy, or focus despite otherwise reasonable habits
• Have been diagnosed with a mental health condition and want nutritional support alongside your existing treatment
• Have tried multiple diets or elimination approaches without lasting results
• Want personalised guidance rather than generic online advice

An integrative approach that combines therapy, medical care when needed, and personalised nutritional support often provides better long-term results than focusing on any one aspect alone.

Final Thoughts

Nutrition isn’t a replacement for therapy or medical treatment, but it is a powerful tool that can support both brain function and emotional well-being. When we look after the gut, we’re also looking after the brain.

If you’d like help working out where to start, I’d love to chat.

Ready to feel calmer, clearer, and more like yourself?
Book a complimentary 10-minute discovery call to talk through your goals.

Jessica Woodruff is the founder of MindNourish Nutrition, a Singapore-based functional nutrition practice that works alongside doctors and psychologists to support women’s mental and physical well-being through evidence-based nutrition.

Disclaimer: This article is for educational purposes only and is not a substitute for medical advice or mental health care. If you’re experiencing significant symptoms of anxiety, depression, or any other mental health condition, please speak to a qualified healthcare professional. Nutrition can support mental well-being but should not replace therapy, medication, or medical treatment where these are needed.

References: Jacka, F. N., et al. (2017), A randomised controlled trial of dietary improvement for adults with major depression (the SMILES trial), BMC Medicine 15:23; Cryan, J. F., et al. (2019), The microbiota–gut–brain axis, Physiological Reviews 99(4); Miller, A. H., & Raison, C. L. (2016), The role of inflammation in depression, Nature Reviews Immunology 16:22–34; Knüppel, A., et al. (2017), Sugar intake from sweet food and beverages, common mental disorder and depression, Scientific Reports 7:6287; Wastyk, H. C., et al. (2021), Gut-microbiota-targeted diets modulate human immune status, Cell 184:4137–4153 (the Stanford fermented foods study); Marx, W., et al. (2021), Nutritional psychiatry: the present state of the evidence, Proceedings of the Nutrition Society.