The Food-Mood Connection - How What You Eat Affects How You Feel

Nutritional Psychiatry, a New Field

Nutritional psychiatry is a relatively new academic field that studies the relationship between diet and mental health scientifically. Since the 2010s, large epidemiological studies and randomized controlled trials have shown one after another that dietary patterns have a significant effect on depression, anxiety disorders and cognitive function.

The SMILES trial (2017), conducted at Deakin University in Australia, guided patients with moderate to severe depression toward a Mediterranean-style diet for 12 weeks and found that depressive symptoms improved significantly compared with the control group. With dietary change alone, about a third of the participants met the criteria for remission of depression.

The Nutrients the Brain Needs

The brain accounts for only about 2% of body weight, yet it is an organ that consumes roughly 20% of the body's total energy. Specific nutrients are indispensable if that high energy demand is to be met and neurotransmitters are to be synthesized properly.

Synthesizing serotonin, the neurotransmitter tied to feelings of well-being, requires tryptophan (an amino acid), vitamin B6, iron and magnesium. Synthesizing dopamine, which is involved in motivation and reward, requires tyrosine (an amino acid), iron and vitamin B9 (folate). When these nutrients run short, neurotransmitter synthesis stalls, which brings unstable mood and a loss of drive.

Omega-3 fatty acids (EPA and DHA) are structural components of brain cell membranes and keep the flow of information between nerve cells smooth. Several sets of epidemiological data show that populations with a higher intake of omega-3 have a lower prevalence of depression.

The Gut-Brain Axis That Links the Two

The gut is called the "second brain", and more than 100 million nerve cells are spread through the intestinal wall (some reports estimate 200 to 600 million). More than 90% of the body's serotonin is made in the gut, but what makes it is not the gut bacteria themselves: it is the enterochromaffin cells (EC cells) in the intestinal lining. The gut microbiome plays a regulating role, prompting serotonin synthesis in EC cells by way of its metabolites. The serotonin released by EC cells acts on the endings of the vagus nerve distributed through the intestinal wall, and that signal is carried to the brain.

Serotonin made in the gut does not, however, cross the blood-brain barrier. It is not a simple matter of more serotonin in the gut meaning more serotonin in the brain: serotonin in the brain is synthesized by the brain itself from tryptophan. The connection between gut and brain is thought to rest on indirect routes - the exchange of signals via the vagus nerve, inflammation, and the supply of the nutrients that serve as raw material.

When the intestinal environment is disturbed, the production of inflammatory cytokines increases, and these are thought to act on the brain through the bloodstream and the vagus nerve, leading to neural inflammation. Several studies have reported that the gut microbiome of people with depression differs significantly from that of healthy people, and the link between the intestinal environment and mental health is becoming established. The gut-brain connection is set out in more detail in a separate article.

Dietary Patterns That Keep Mood Steady

The dietary pattern as a whole matters more than individual nutrients. The Mediterranean diet (built around vegetables, fruit, whole grains, fish and olive oil) and the traditional Japanese diet are both associated with a lower risk of depression.

What they share is that they contain few processed foods, are rich in dietary fiber, include good-quality fats and draw on a wide variety of plant foods. Dietary fiber in particular feeds the gut bacteria, promotes the production of short-chain fatty acids and keeps the gut-brain axis healthy.

Vitamin D, also called the "sunshine vitamin", is synthesized in the skin when you are exposed to sunlight. Vitamin D deficiency caused by a lack of winter sunlight is known as a risk factor for seasonal affective disorder (SAD). Vitamin D takes part in the synthesis of serotonin, and a shortage brings low mood and a loss of drive. Many people in Japan are said to be short of vitamin D, so it is worth being conscious about taking it in from fish (salmon, mackerel and sardines in particular), egg yolk and mushrooms.

In concrete terms: bring colorful vegetables into every meal, eat oily fish 2 to 3 times a week, swap refined grains for whole grains, and take fermented foods (miso, natto, yogurt) every day. These habits contribute to steady mood.

Eating Habits to Avoid

Taking in too much ultra-processed food is associated with a higher risk of depression. An umbrella review of epidemiological meta-analyses published in 2024 reported that the risk of developing depressive symptoms was about 22% higher in the group with high intake (hazard ratio 1.22). The certainty of the evidence was rated as low, however, and diet alone does not determine whether depression develops. Ultra-processed foods are industrially mass-produced, contain many additives and no longer retain the original form of their ingredients. Instant noodles, sweet bread, soft drinks and many frozen foods fall into this category.

Excess refined sugar is also a problem. Sharp rises and falls in blood sugar (blood sugar spikes) bring irritability, anxiety and fatigue. A sweet drink or a snack may lift your mood for a moment, but the sharp fall in blood sugar that follows sends it plunging again. Repeating that cycle produces chronically unstable mood.

Practical Steps for Improving Your Diet

There is no need to change everything at once. Start with just one change, the one with the largest impact. For many people that is "replacing sweet drinks with water or tea". A single 500 ml soft drink contains about 50 g of sugar, and removing it alone improves the stability of your blood sugar considerably.

As the next step, add protein to breakfast: eggs, yogurt, natto. Taking protein in the morning secures the supply of tryptophan and supports serotonin synthesis through the day. Simply changing breakfast from sweet bread and coffee to eggs and whole-grain bread is enough to let you feel the difference in concentration and steadiness of mood through the morning.

Inflammation and Mental Health

Several studies have shown that chronic low-grade inflammation is involved in the onset of depression and anxiety disorders. Taking in too much ultra-processed food, refined sugar and trans fat promotes inflammation in the body, while anti-inflammatory foods such as vegetables, fruit, fish, nuts and olive oil hold it down. One reason the Mediterranean diet is good for mental health is thought to lie in this anti-inflammatory action. Turmeric (curcumin), ginger and green tea (catechins) are also drawing attention as foods with anti-inflammatory properties.

Cases Diet Alone Will Not Solve

Nutritional psychiatry shows how much diet matters, but diet alone does not cure depression or anxiety disorders. Improving what you eat is an aid to treatment, not a replacement for it. If you have already been diagnosed with a mental illness, give priority to the treatment plan set by your doctor.

Care is also needed with superfood-style claims that "eating this particular food will cure depression". What matters is not a single food but raising the overall quality of your dietary pattern. Diet influences mood, but do not forget that it is not a cure-all.