Where is serotonin produced?

I was told that 10 percent of serotonin is produced in the brain stem. Is that correct, and if so, where is the other 90 percent produced?

 An estimated 10 percent of serotonin is produced in the central nervous system or CNS, specifically in the brain stem. The remaining 90 percent of serotonin is produced in the enteric nervous system or ENS located in the GI or gastrointestinal system. Serotonin is a special type of neurotransmitter that impacts many complex functions. For example: modulating mood, cognition or thinking, sleep, learning, and memory. It also manages many physiological processes including nausea and vomiting. Eating too few healthy carbs may be one of the reasons that habitual dieters may have low serotonin levels in both their brain and gastrointestinal tract—a situation that can lead to bingeing.

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