Before revision: The increased synthesis of brain serotonin occurs because of a higher level of tryptophan, the serotonin precursor, in the blood and which results in larger amounts of tryptophan crossing the blood–brain barrier.
After revision: The increased synthesis of brain serotonin occurs because of a higher proportion of tryptophan, the serotonin precursor, in the blood and which results in larger amounts of tryptophan crossing the blood–brain barrier.