Tryptophan, an amino acid found in foods like turkey and carbohydrates, is a precursor to serotonin.

“Dopamine also called 5HT, essentially derives from a precursor called tryptophan. Tryptophan arrives into our system through our diet, okay, tryptophan is an amino acid. Tryptophan is found in Turkey, it's found in carbohydrates. And that should therefore raise the idea, hmm, I wonder if one of the reasons why people who are depressed have such an appetite for carbohydrate late in foods is because they're trying to get more tryptophan, and therefore more serotonin. And indeed that's the case.”