Winter depression: can vitamin d shed light on mood?

By Ana Espino | Published on January 5, 2026 | 3 min read


Seasonal affective disorder (SAD) is a recurrent form of major depressive disorder or bipolar disorder, characterized by depressive episodes occurring at specific times of the year, predominantly in winter. Despite the partial effectiveness of light therapy, antidepressants, or cognitive therapy, a substantial proportion of patients remain insufficiently relieved. This therapeutic resistance is partly explained by an incomplete understanding of the underlying neurobiological mechanisms.

Among the hypotheses explored, the role of vitamin D has attracted growing interest. Its deficiency during winter, which is common, may promote or exacerbate depressive symptoms. Indeed, brain serotonin—a key neurotransmitter in mood regulation—is synthesized from tryptophan via the enzyme TPH2, whose expression is activated by calcitriol (the active form of vitamin D). Vitamin D deficiency reduces this synthesis, affecting cognitive and emotional functions, particularly during sensitive periods such as the perinatal phase or early childhood, when brain development is at stake.

In this context, this study was initiated to explore the modulatory role of vitamin D in depressive disorders through its interaction with serotonin synthesis, and to assess its clinical relevance in their management.

Vitamin D: a natural antidepressant?

The analyzed data suggest that vitamin D deficiency, common in winter, is associated with altered serotonin metabolism, a central factor in the pathophysiology of depressive disorders. In patients with depression, particularly those with elevated inflammatory markers, inflammation reduces plasma tryptophan levels and serotonergic activity. This mechanism may explain the reduced effectiveness of SSRIs (selective serotonin reuptake inhibitors) in some patients.

Vitamin D supplementation could not only correct deficiency but also enhance the effectiveness of antidepressant treatments. It may be particularly relevant in individuals with inflammatory or autoimmune conditions, which are often comorbid with depression. In addition, UVB-spectrum light (280–320 nm) enables effective cutaneous production of vitamin D, reinforcing the relevance of light therapy in winter management strategies.

Shedding light on winter serotonin

Seasonal affective disorder is a recurrent mood disorder strongly influenced by seasonal variations in light exposure. One of the main challenges remains resistance to standard treatments in certain patients, as well as the lack of precise biomarkers to guide personalized approaches.

This study aimed to explore the pathophysiological role of vitamin D in serotonin regulation and to consider its clinical relevance in the management of depressive disorders. The analyzed data suggest that vitamin D may favorably modulate serotonergic neurotransmission, particularly in patients exposed to chronic inflammatory stress, and thereby improve response to antidepressants.

However, limitations remain and justify further research. Future work should include randomized controlled trials, longitudinal studies with systematic measurement of vitamin D levels, assessment of inflammatory biomarkers, and analysis of interactions with existing pharmacological treatments. Improved identification of responder profiles will allow vitamin D to be more effectively integrated into personalized therapeutic strategies.

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