How Diet Affects Your Mental Health

There's a line you'll hear in wellness circles a lot: "90% of your serotonin is made in your gut." And it's true. But it's also one of the most misunderstood facts on the internet, and if we're going to talk about diet and the mind, we should probably start by getting that right.

Yes, roughly 90 to 95% of your body's serotonin is produced in the gut, by specialised cells called enterochromaffin cells. That's been well established in the research for decades. But here's the part most people skip over: that gut serotonin doesn't cross the blood-brain barrier. It can't just float up to your brain and make you feel better. The brain makes its own serotonin from tryptophan, an amino acid you get through food.

So does that mean your gut doesn't affect your mood? Not even close. The connection is real. It's just more complex than the headline suggests.

Your gut and your brain are in constant conversation

The gut-brain axis is a two-way communication network linking your digestive system and your central nervous system. It works through the vagus nerve, through hormones, through immune signalling, and through the metabolites produced by your gut bacteria.

A 2025 review published in Cureusdescribed how dietary patterns directly influence the composition of the gut microbiome, which in turn affects emotional regulation, cognitive function, and neurological health. The researchers noted that imbalances in gut bacteria (known as dysbiosis) have been linked to inflammation and neurotransmitter disturbances associated with anxiety, depression, and cognitive decline.

What makes this worth paying attention to is the growing body of evidence suggesting that what you eat can shift the composition of your microbiome in ways that either support or undermine this communication. It's not magic. It's biology.

The tryptophan question

Your brain needs tryptophan to make serotonin. You can't produce tryptophan on your own. You have to eat it. Foods like eggs, turkey, salmon, nuts, seeds, tofu, and cheese are good dietary sources.

But here's where it gets interesting. Tryptophan can go down one of two metabolic pathways: toward serotonin production, or toward a substance called kynurenine, which plays a role in niacin production. In people dealing with chronic inflammation or stress, more tryptophan gets shunted down the kynurenine pathway, which means less raw material available for serotonin in the brain. The state of your gut microbiome appears to influence which pathway gets prioritised.

So it's not as simple as "eat more tryptophan, feel happier." But ensuring a consistent dietary supply, combined with supporting your gut health, creates better conditions for your brain to do its job.

What the research says about specific dietary patterns

The Mediterranean diet keeps showing up in this research. High in fibre, vegetables, legumes, whole grains, olive oil, and oily fish, it's consistently associated with better microbial diversity and lower rates of depression and anxiety across multiple studies and populations.

On the other end, Western dietary patterns, heavy in ultra-processed foods, saturated fats, refined sugars, and low in fibre, are associated with reduced microbial diversity and increased systemic inflammation. A 2025 review in Food Science & Nutritioncompared the two approaches and found that Mediterranean-style eating improved immune regulation through positive changes in gut bacteria, while Western diets actively promoted dysbiosis and impaired immune function.

None of this means you need to eat a perfect diet to function. But the pattern matters more than any single meal or supplement.

Why this matters if you have ADHD

If you're neurodivergent, the diet-brain connection adds another layer. A 2025 study from the University of Roehampton, published in Frontiers in Nutrition, found that both children and adults with ADHD showed deficiencies in several nutrients directly tied to neurotransmitter function. Omega-3 fatty acids, zinc, magnesium, vitamin D, and B vitamins were all below recommended levels. The study also found significant relationships between nutrient levels and ADHD symptom severity. For example, lower magnesium was correlated with more severe behavioural symptoms.

Then there's the ultra-processed food connection. A 2026 study published in Pediatric Research found that children with ADHD consumed significantly more ultra-processed food than their neurotypical peers, even after adjusting for other factors. The researchers proposed several potential pathways: displacement of nutrient-dense foods, dopaminergic dysregulation from high sugar loads, the impact of artificial additives on neuroinflammation, and disruption of the gut microbiome.

This isn't about blame. If you have ADHD, you already know that impulsive eating, sensory food preferences, and the sheer executive function cost of meal planning make "just eat better" one of the least helpful pieces of advice going. The research into food relationships in ADHD and autism specifically highlights the role of sensory issues, avoidant/restrictive eating patterns (ARFID), and food avoidance as real barriers.

But understanding the mechanisms can help you make small, realistic shifts. Not an overhaul. Not a restrictive diet. Just slightly better conditions for your brain, when you can manage it.

What actually helps (practically speaking)

This isn't a meal plan. It's a set of principles based on what the current evidence actually supports.

Feed your gut bacteria. Fibre is the single biggest lever you have. Vegetables, legumes, whole grains, and fruit feed the beneficial bacteria that produce the short-chain fatty acids your gut lining needs to stay intact. A diverse plant intake supports a diverse microbiome, and microbial diversity is consistently associated with better mental health outcomes.

Get your omega-3s. Oily fish (salmon, mackerel, sardines) twice a week is a reasonable target. If fish isn't your thing, algae-based supplements provide DHA directly. The evidence for omega-3 supplementation in ADHD is modest but consistent enough to be worth considering, particularly if your dietary intake is low.

Don't fear protein at breakfast.Tryptophan comes from protein-rich foods. Starting the day with eggs, yoghurt, or nuts gives your brain raw material to work with and can help stabilise blood sugar, which matters for focus and mood regulation.

Reduce ultra-processed food where you can. This isn't about perfection or food guilt. It's about recognising that heavily processed foods tend to displace the nutrient-dense ones your brain needs, while also potentially disrupting your gut bacteria. Small swaps over time make more difference than dramatic dietary overhauls that last three days.

Consider fermented foods. Yoghurt, kefir, sauerkraut, kimchi, and miso introduce beneficial bacteria directly. The evidence for specific probiotic strains in mental health is still developing, but regularly eating fermented foods is a low-risk, potentially beneficial habit.

Eat regularly. For ADHD brains especially, blood sugar crashes can amplify inattention, irritability, and emotional dysregulation. An evenly timed eating pattern helps maintain a steadier supply of nutrients to the brain throughout the day.

A note on what this isn't

This is not a claim that diet cures depression, anxiety, or ADHD. The research is promising, and the mechanisms are increasingly well understood, but most reviews are careful to note that individual microbiome variability is huge, and we still need more long-term clinical data before making strong prescriptive claims.

What the evidence does support is that nutrition is one meaningful input among many. It works alongside sleep, movement, stress management, breathwork, social connection, and, where appropriate, medication and professional support.

Think of it less as a solution and more as a foundation. You can't meditate or breathe your way out of a nutrient deficiency. And you can't supplement your way out of chronic stress. But when you stack the basics, each one works a little better.

Your gut is talking to your brain whether you're paying attention or not. You might as well give it something worth saying.

Cian ODriscoll is the founder of Low Tide Calm, a Wicklow-based wellness practice offering structured breathwork and mindfulness programmes for people navigating ADHD, burnout, and nervous system dysregulation. He is a certified mindfulness teacher, Buteyko breathing instructor, and complementary therapist.

Patil et al. (2025). "The Gut-Brain Axis and Mental Health." Cureus, 17(7), e88420. PMC

Ahmed et al. (2025). "Gut-Brain Axis in Obesity: How Dietary Patterns Influence Psychological Well-Being." Food Science & Nutrition, 13(7), e70689. Wiley

Hunter, Smith, Davies, Dyall & Gow (2025). "A Closer Look at the Role of Nutrition in Children and Adults with ADHD and Neurodivergence." Frontiers in Nutrition, 12, 1586925. PMC

Mottis et al. (2026). "ADHD and Ultra-Processed Food Consumption Among Children." Pediatric Research. Nature

Hsiao Lab, Caltech (2015). "Microbes Help Produce Serotonin in Gut." Cell. Caltech

PMC (2020). "Does Serotonin in the Intestines Make You Happy?" PMC

Psychology Today (2023). "Depression, Serotonin, and the Gut." Psychology Today

Pelsser et al. (2021). "Correlation Between Brain Function and ADHD Symptom Changes Following a Few-Foods Diet." Scientific Reports. Nature