The dietary issue is infection (usually two or more, basic shit like candida in the gut, mold exposure, EBV recurrence, Lyme, parasites, more) causing systemic malnourishment, and lack of homeostasis, causing nervous and endocrine issues/misfires,

(at which point you'll often notice immunodeficiencies, minor "unexplainable" abnormalities in bloodwork),

and indeed cognitive issues, depression, anxiety, and other more severe brain malfunctions.