Almost all of the biological, psychological, and social factors implicated in mental illness can be linked to metabolism. Stress, trauma, sleep, diet, exercise, inflammation, hormones, substance use, medications, social adversity, and genetics all influence metabolic function.
One reason I believe metabolism—and more specifically mitochondria—deserves consideration as a central factor in mental disorders is that metabolism is fundamental to life itself.
Metabolism doesn't explain everything, but it is one of the few frameworks that can plausibly connect a vast range of risk factors and biological findings across psychiatric disorders.
"Saying most mental illnesses are related to metabolism is reductionistic."
Only if you think this chart is "reductionistic."