This happens on two levels. If the food intolerance is not treated and more fructose is ingested, then the person will experience the symptoms described elsewhere (see “Symptoms of fructose malabsorption”). In the long run these symptoms will lead to the deterioration of the psychological and social situation.
But there is also a biochemical cause that can make fructose malabsorption responsible for depression.
Serotonin (frequently called the "happy hormone") has a diverse array of tasks. One of them is to act as a neurotransmitter which is responsible for us feeling "happy". Serotonin is produced from tryptophan, an essential amino acid. Essential means that the body cannot synthesise this amino acid by itself but needs to supply it from ingested food. Tryptophan will form a stable complex with the fructose in the gut and thus cannot be absorbed by the body when a high concentration of fructose is present. Because of this many people with fructose malabsorption have insufficient tryptophan and thus also too little serotonin. The result will be, amongst other problems, a state of depression.
The disastrous consequences: The body knows that it needs the tryptophan, and the person tends to eat more foods that will contain this substance. However, nearly all these foods tend to have a very high fructose content. In addition, we are often told that we should eat more fruit because it is healthy and it helps. But it doesn't help at all. Instead, if a person is suffering from fructose intolerance, it just makes the whole situation much worse.
Other food intolerances, too, can aggravate this depressive state. It is always recommended that you have all possible causes investigated by a medical professional!
A deep depressive state should always be looked into by a doctor!
If the depression is only down to fructose malabsorption, then it will disappear completely if the patient sticks to an appropriate low-sugar and low-fructose diet, avoiding fructose. This will reinstate the tryptophan balance and thus the serotonin levels will return to where they should be!
Ledochowski M, Widner B, Bair H, Probst T, Fuchs D.; Fructose- and sorbitol-reduced diet improves mood and gastrointestinal disturbances in fructose malabsorbers.; Scand J Gastroenterol. 2000 Oct;35(10):1048-52.