Much of part V of this series focues on research which provided strong support for the hypothesis that chronic inflammation creates depression via alterations in tryptophan metabolism that redirect tryptophan away from serotonin and melatonin production and towards the kynurenine pathway and neuroexcitatory and neurodestructive metabolites such as quinolinic acid that are products of this pathway. However, while the body of research supporting this connection is immense, it is not unanimous. What follows next is a review of some fascinating research that suggests the papers arguing against the depression/kynurenine pathway connection are flawed because of a variable that was not controlled for in the populations studied, nutritional status. As you will see, nutritional status, in fact, has a massive impact on both kynurenine metabolism and clinical depression.
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