There are many explanations for the cause-and-effect relationship between food and mood. The following are some examples:
Fluctuations in blood sugar levels are associated with changes in mood and energy, and are affected by what we eat.
Brain chemicals (neurotransmitters, such as serotonin, dopamine and acetylcholine) influence the way we think, feel and behave. They can be affected by what we've eaten.
There can be abnormal reactions to artificial chemicals in foods, such as artificial colourings and flavourings.
There are reactions that can be due to the deficiency of an enzyme needed to digest a food. Lactase, for instance, is needed to digest lactose (milk sugar). Without it, a milk intolerance can build up.
People can become hypersensitive to foods. This can cause what are known as delayed or hidden food allergies or sensitivities.
Low levels of vitamins, minerals and essential fatty acids can affect mental health, with some symptoms associated with particular nutritional deficiencies. For example, links have been demonstrated between low levels of certain B-vitamins and symptoms of schizophrenia, low levels of the mineral zinc and eating disorders, and low levels of omega-3 oils and depression.
What's the psychological relationship?
It's generally accepted that how we feel can influence what we choose to eat or drink (mood to food). What is less well known is how what we eat can affect our mental functioning (food to mood). The use of caffeine is one example of a complex relationship. Caffeine, found in tea, coffee, cola drinks and chocolate, is probably the most widely used behaviour-modifying drug in the world.
We often choose to drink it if we are feeling tired and irritable, because it can give us a boost and help us to concentrate. Having a cup of coffee or tea also has a lot of positive psychological associations. We meet a friend for 'coffee and a chat' or give ourselves a break by sitting down with a cup of tea, and these things are very important. But too much caffeine (which is a different amount for each of us) can cause symptoms, such as anxiety, nervousness and depression.
Any exploration into food and mood needs to take into account this two-way relationship and include the psychological aspect behind what we are choosing to eat.