How vitamin D regulate mood and reduce depression?

With respect to mood,

• the serotonergic pathway would predict that,
• in people who have insufficient levels of vitamin D,
• mood may improve when serum 25(OH)D increases as this may increase the availability of serotonin in the brain via TPH2 expression.

study indicates direct link between vitamin D levels and mental health is so strong that adding just 1,000 IU of vitamin D per day can reduce: Fatigue. 

Feelings of Depression or Anxiety.