Plain language summary
Neurotransmitters (NTs) are chemical messengers, found mainly within the nervous system. Common NTs are acetylcholine (ACh), glutamate, γ-aminobutyric acid (GABA), dopamine, serotonin (5-HT), and histamine. Many foods are natural sources of NTs that may influence the nervous system, and therefore mood and mental health. This review paper looked at books and studies and discusses the NT content of foods and the possible implications for human health. Acetylcholine is a NT involved in muscle movement, learning and memory. Its presence is documented in more than 50 plant species, including squash, aubergine spinach and nettles. Glutamate is the most abundant excitatory NT in the brain. Glutamic acid naturally occurs in foods with high protein content. Seaweeds, cheeses, fish sauces, soy sauces, fermented beans, and tomato showed high levels of free glutamic acid. Dried cod, salami, caviar, and instant coffee powder are other sources of this amino acid. Salts of glutamic acid, such as monosodium glutamate, are added to certain foods as flavour enhancers. GABA is a calming NT. Studies have found the highest contents of GABA in raw spinach, potato, sweet potato and cruciferous vegetables such as kale and broccoli. Shiitake mushrooms and chestnuts also contained a significant amount of GABA. Dopamine plays an essential role in the coordination of body movements, motivation, and reward. Information on the content of dopamine foods is very limited. Bananas, plantains and avocado were reported to contain high levels of dopamine. 5-HT pathways modulate behaviours, eating, and sleep, and in the gut are involved in the regulation of gastrointestinal motility. In recent years, the number of studies on the content of 5-HT in plants has increased. 5-HT appeared to be prevalent in the green bananas, with higher concentrations found in the peel compared with the flesh. 5-HT was also found in peppers, paprika, hazelnut, tomatoes, pineapple, plum, passion fruit, papaya and kiwi fruit. Histamine is involved in arousal, attention, and reactivity, as well as in local immune responses. The presence of histamine in processed foods, such as aged cheeses, contributes to characteristic flavours and textures. Wine and beer may contain a significant amount of histamine. Fermented foods contain histamine. The food industry generally aims to maintain the levels of amines in foods as low as possible; consumption of fish, cured meat products, sauerkraut, and cheese varieties such as Cheddar, Swiss, Gruyère, and Gouda have been associated with amine poisoning. The significance of dietary NTs intake needs to be further investigated, as there is little data about their bioavailability or clinical implications. New studies should consider if dietary NTs can be transported across the blood-brain barriers or act on the central nervous system via other organs. The authors suggest that in future, including or excluding particular foods containing NTs could be beneficial for patients suffering from Alzheimer’s disease or dementia (an ACh diet), epilepsy or migraines (a glutamate-free diet), anxiety or insomnia (a GABA diet), Parkinson’s disease (a dopamine diet), depressive disorders (a serotonin diet), and vascular headaches (a histamine-free diet).