Brain scans show changes in neurotransmitters and grey matter volume in people who have a poor diet, versus those who adhere to a Mediterranean style diet, which is considered very healthy. The researchers also found that these changes are associated with rumination, a part of the diagnostic criteria for conditions affecting mental health, such as depression and anxiety.

This research was carried out by the University of Reading, Roehampton University, FrieslandCampina (Netherlands), and Kings College London, and is published in Nutritional Neuroscience.

When someone eats a poor quality diet, there is reduced gamma aminobutyric acid (GABA) and elevated glutamate — both neurotransmitters, along with reduced grey matter volume — in the frontal area of their brain. This could explain the association between what we eat, and how we feel.

Dr Piril Hepsomali, University of Reading, said: “We can eat ourselves well! Ultimately, we see that people who have an unhealthy diet — high in sugar and saturated fat — have imbalanced excitatory and inhibitory neurotransmission, as well as reduced volume of grey matter in the frontal part of the brain. This part of the brain is involved in mental health issues such as depression and anxiety.”

The exact reason that diet affects the brain in this way is still under investigation. It’s possible that obesity and dietary patterns that are high in saturated fats cause changes in glutamate and GABA metabolism and neurotransmission, as has been shown in animal studies.

Distinct alterations of the gut microbiome, due to dietary patterns that are high in saturated fats, is thought to have an impact on cell machinery that drives both GABA and glutamate production.

A high saturated fat, high sugar, diet has also been shown to reduce the number of parvalbumin interneurons, which perform the role of delivering GABA to where it is needed.

Unhealthy diets also have an impact on glucose, making blood glucose and insulin higher. This increases glutamate in the brain and plasma, thus reducing GABA production and release. Having a diet high in fat and cholesterol can cause changes in cell membranes that alter the release of neurotransmitters, too.

These changes in brain chemistry might lead to changes in the brain grey matter volume, as observed in this study.

Dr Hepsomali continued: “I would like to note that GABA and glutamate are intimately involved in appetite and food intake, too. Reduced GABA and/or increased glutamate might also be a driving factor in making unhealthy food choices. So, there may be a circular relationship between eating well, having a healthier brain and better mental wellbeing, and making better food choices to eat well.”