 Recent studies like this suggest that dietary patterns characterized by high intakes of vegetables, fruit, mushrooms, and soy products are associated with fewer depressive symptoms. The year before it was this study of the association between dietary patterns in mental health and early adolescence, showing improved behavioral scores significantly associated with higher intakes of leafy green vegetables and fresh fruit. Would any of this be because of the psychoactive substances found in plant foods? The neurotransmitter serotonin, often referred to as the happiness hormone, is found in plant foods. But serotonin doesn't cross the blood-brain barrier, so it shouldn't affect our mood no matter how much we eat. The precursor to serotonin, however, what your body makes serotonin out of, is an amino acid called tryptophan, and there's a transport protein in the brain that plucks tryptophan out of the bloodstream, and so what you eat can end up affecting our mood. Back in the 1970s they did tryptophan depletion experiments, where you give people specially concocted tryptophan deficient diets, and indeed, their mood suffers. They get irritable, annoyed, angry, depressed. Their body just can't make enough serotonin. Likewise, you can give people tryptophan pills to improve their mood, and indeed, it became a popular dietary supplement until people started dying from something called eosinophilium myelgia syndrome, an incurable, debilitating, and sometimes fatal flu-like neurological condition caused by the ingestion of tryptophan supplements. It may have been due to some unknown impurity, but better safe than sorry. Instead of supplements, there are dietary strategies one can use to improve mood, which we'll talk about next.