This is my first post, so hope the text comes out in the right place!
I recently read the book "Gut" by Guilia Enders; it has a good contents list but no index, so when I finished reading it, I started re-reading and adding post-it note markers to index the many points of interest.
Of particular relevance to this thread is the subject of fructose intolerance (Giulia is a German and states that a third of Germans are fructose intolerant). If someone does not have sufficient enzyme to digest fructose, it passes straight through the body. Even a fructose intolerant person should be able to cope with an apple a day, however the fructose present in tomato sauce, sweetened yoghurt etc, takes them over the threshold. Fructose intolerance can affect our mood. The amino acid Tryptophan loves to bond with fructose and so in a fructose intolerant person, this important amino acid is lost to the body along with the fructose. We need Tryptophan to make serotonin - the hormone that makes us feel happy.
From reading this book, I have concluded that, food manufacturers like to con the public into thinking their 'products' are healthy as they contain natural fruit sugars. It is most important to read food labels and may be well worth an experiment to see if cutting down/out fructose improves one's mood.