Your gut contains roughly 100 million neurons. It produces about 95% of your body's serotonin. It communicates directly with your brain through the vagus nerve. Calling the gut your "second brain" isn't a metaphor. It's closer to a literal description.
The Gut-Brain Highway
John Cryan and Ted Dinan at University College Cork have spent years mapping the gut-brain axis. Their research shows that the trillions of bacteria in your digestive system (the microbiome) directly influence mood, anxiety levels, and stress responses.
In one striking experiment, germ-free mice (raised without any gut bacteria) showed exaggerated stress responses and anxiety-like behaviour. Introducing a single bacterial strain, Lactobacillus rhamnosus, reduced anxiety and altered GABA receptor expression in the brain.
The food you eat shapes your microbiome, which shapes your mood. This isn't a loose correlation. It's a causal chain with identifiable mechanisms.
What the Diet Studies Show
Felice Jacka at Deakin University in Australia ran the first randomised controlled trial testing whether dietary improvement could treat depression. The SMILES trial (2017) took people with moderate to severe depression and put half of them on a modified Mediterranean diet for 12 weeks. The dietary group showed significantly greater improvement than the social support control group. A third of the diet group achieved full remission.
The diet wasn't exotic. More vegetables, fruits, whole grains, legumes, nuts, olive oil, and fish. Less processed food, refined sugar, and fried items. Nothing that would surprise your grandmother.
Similar findings have emerged from the PREDIMED trial in Spain, the SUN Project cohort studies, and Almudena Sanchez-Villegas's research linking Mediterranean dietary patterns to lower depression risk across thousands of participants.
Sugar, Inflammation, and Mood
The mechanism connecting processed food to low mood runs partly through inflammation. Diets high in refined sugar and ultra-processed foods increase systemic inflammation. Michael Berk's research at Deakin has demonstrated that inflammatory markers are consistently elevated in people with depression.
This doesn't mean sugar causes depression in a simple, linear way. But chronic, low-grade inflammation from poor diet creates a biological environment where mood disorders thrive more easily.
The practical takeaway: you can't out-supplement a bad diet. No amount of omega-3 capsules will offset a daily routine built on crisps and fizzy drinks.
Cooking, Sharing, and the Social Side
Food's relationship to happiness isn't purely biochemical. Cooking is a creative act. Sharing a meal is a social one. Research by Robin Dunbar (the same primatologist behind "Dunbar's number") found that people who eat socially more often feel happier, are more trusting of others, and are more engaged with their communities.
The meal itself matters less than the act of sharing it. A bowl of pasta eaten with friends at a crowded kitchen table delivers something a perfectly optimised meal eaten alone at your desk never will.
Masamichi Souzou's approach to happiness treats these connections seriously. What you eat, how you eat, and who you eat with form an interconnected system. Designing your food life with intention (not obsession, not guilt, just attention) is one of the most tangible ways to shift your daily wellbeing.