While the specific mechanisms involved are largely still to be defined, there is plenty of data to sink our teeth into. A tool called the Dietary Inflammatory Index (DII), which assesses the potential of an individual's diet to promote inflammation, has been used in numerous clinical trials. The emerging consensus is that the higher the inflammatory potential of a person's diet, the worse their brain function will tend to be. A large prospective study that followed thousands of people for an average of five years found that those with a more pro-inflammatory diet had a greater risk of developing depression.
According to the DII, the foods and nutrients with the most anti-inflammatory potential include fibre, beta carotene (found in brightly coloured vegetables), garlic, ginger, turmeric, vitamins A, C, D and E (fruit, vegetables, oily fish, nuts), green and black tea, flavones and flavonoids (berries), and Omega-3 fats (oily fish and seafood). (We'll discuss how to incorporate more of these into your diet below.) There are a number of other pathways through which food could affect mood. The gut is colonised by trillions of microbes, collectively known as the gut microbiome. In return for being supplied with the right substrate, these microbes help to maintain the integrity of the gut lining and produce a range of byproducts that can be beneficial for mood regulation. And food might play an even simpler role in mood regulation through the resolution of nutritional deficiencies. For example, Vitamin B6 is essential for the synthesis of serotonin, dopamine, GABA and noradrenaline, meaning that deficiency in this nutrient leads to the downregulation of these mood-regulatory neurotransmitters.