Depression affects 350 million people worldwide. In the light of the global disease burden statistics, the efficacy of current treatments for depression appears insufficient. Thus, research on novel treatment interventions and predictors for good treatment response are warranted. Earlier prospective follow-up studies and intervention studies suggest that several bio-psychosocial factors, including high serum concentrations of vitamin D, are related to better treatment outcomes. In this follow-up study with randomized clinical vitamin D supplementation trial on patients with depression, the investigators aim to

1. clarify how a six-month intervention with vitamin D supplementation affects treatment response, recovery, and the biological pathways related to depression. This aims to finding potential sub-groups getting benefits from vitamin D supplementation. In addition, the investigators want to
2. investigate and characterize factors related to recovery from depression and working ability in depression patients in the long-term. The investigators are especially interested in the bio-psychosocial factors and the aims include examining both the individual's positive resources.

The trial will start with a six-month double-blinded randomized controlled trial with vitamin D supplementation. The aim is to recruit altogether 478 patients with non-psychotic, unipolar depression, aged 18-65 years, who are referred to the recruitment sites for treatment for depression. The participants will be randomized to low (10 µg/day) or high (100 µg/day) vitamin D supplementation group. Clinically necessary antidepressant treatments will continue during the intervention as needed. After six months of intervention, the participants will be followed up at 18 months and at 6 years.

Several measurements will be conducted during the intervention and follow-up period. Participants will fill a variety of clinical questionnaires and questionnaires with background information. All participants give blood samples for biomarker analyses at time points 3, 6, 18 months and 6 years. Clinical interviews of mental disorders (e.g. SCID) and anthropometric measurements (e.g. weight, height, blood pressure) will be carried out.

Depression affects 350 million people worldwide. In the light of the global disease burden statistics, the efficacy of current treatments for depression appears insufficient. Thus, research on novel treatment interventions and predictors for good treatment response are warranted. Earlier prospective follow-up studies and intervention studies suggest that several bio-psychosocial factors, including high serum concentrations of vitamin D, are related to better treatment outcomes.

In this follow-up study with randomized clinical vitamin D supplementation trial on patients with depression, the investigators aim to

1. clarify how a six-month intervention with vitamin D supplementation affects treatment response, recovery, and the biological pathways related to depression. This aims to finding potential sub-groups getting benefits from vitamin D supplementation. In addition, the investigators want to
2. investigate and characterize factors related to recovery from depression and working ability in depression patients in the long-term. The investigators are especially interested in the bio-psychosocial factors and the aims include examining both the individual's positive resources.

The trial will start with a six-month double-blinded randomized controlled trial with vitamin D supplementation. The aim is to recruit altogether 478 patients with non-psychotic, unipolar depression, aged 18-65 years, who are referred to the recruitment sites for treatment for depression. The participants will be randomized to low (10 µg/day) or high (100 µg/day) vitamin D supplementation group. Clinically necessary antidepressant treatments will continue during the intervention as needed. After six months of intervention, the participants will be followed up at 18 months and at 6years.

Several measurements will be conducted during the intervention and follow-up period. Participants will fill a variety of clinical questionnaires and questionnaires with background information. All participants give blood samples for biomarker analyses at time points 3, 6, 18 months and 6 years. Clinical interviews of mental disorders (e.g. SCID) and anthropometric measurements (e.g. weight, height, blood pressure) will be carried out.

Inclusion Criteria:

• patients referred to the recruitment sites for treatment for depression
• mild, moderate or severe depression,
• mild, moderate or severe episode of recurrent depression

Exclusion Criteria:

• bipolar or psychotic depression
• psychotic disorder
• severe substance abuse
• disabilities in senses that affect functioning and severely threat completing the trial
• diseases that affect vitamin D metabolism (such as sarcoidosis, hypercalcemia, hypofunction of kidney)
• pregnancy or lactation
• current use of high dose vitamin D supplementation
• current use of high dose calcium supplementation