Clinically, symptoms such as apathy, anhedonia, excessive sleepiness and reduced motivation tend to worsen during autumn and winter, which directly correlates with a significant reduction in the skin’s production of cholecalciferol (vitamin D3). Although traditionally associated with calcium and phosphate balance, vitamin D also acts as a potent neurosteroid. Its receptors (VDR) are present in key areas of the brain responsible for emotional regulation, and deficiency can disrupt neurotransmitter balance, including serotonin production.
For individuals living in the United Kingdom, this issue is particularly relevant. The temperate maritime climate, characterised by frequent cloud cover and low UV levels, limits effective vitamin D synthesis for much of the year. Observations from London and surrounding areas indicate that between October and March, the angle of sunlight is insufficient to support meaningful vitamin D production in the skin. As a result, people living in the UK, including the Polish community, are at increased risk of chronic deficiency, which may contribute to the development or worsening of seasonal depression.
This article explains how vitamin D affects the central nervous system and outlines recommendations for supplementation in conditions of limited sunlight exposure.
How vitamin D works in the brain: VDR receptors and neurotransmitters
To understand the role of vitamin D in mental health, it is important to recognise its function as a biologically active neurosteroid. Unlike many other vitamins, cholecalciferol can cross the blood–brain barrier and bind to vitamin D receptors (VDR), which are found in neurons and glial cells in areas such as the prefrontal cortex, hippocampus and hypothalamus. These regions play a crucial role in mood regulation, highlighting vitamin D’s
involvement in emotional processing.
Vitamin D is also involved in the production of key neurotransmitters. It activates tyrosine hydroxylase, an enzyme necessary for the synthesis of dopamine, noradrenaline and adrenaline. In addition, it influences the expression of tryptophan hydroxylase (TPH2), which is responsible for converting tryptophan into serotonin in the brain. Serotonin plays a central role in regulating mood, sleep and appetite. Reduced levels, often linked to vitamin D deficiency, are strongly associated with symptoms of depression, including seasonal depression.
Beyond neurotransmission, vitamin D has neuroprotective effects. It helps regulate calcium levels in nerve cells, protecting them from overstimulation and damage. It also supports the production of neurotrophic factors such as brain-derived neurotrophic factor (BDNF), which promotes neuronal survival and neuroplasticity. Additionally, vitamin D helps reduce inflammation in the brain by inhibiting pro-inflammatory cytokines, which is important given the growing evidence linking chronic low-grade inflammation with mood disorders.
Vitamin D deficiency and symptoms of seasonal depression
There is increasing clinical evidence linking low blood levels of 25-hydroxyvitamin D [25(OH)D] with mood disorders. Many symptoms of seasonal depression overlap with those seen in vitamin D deficiency, including persistent fatigue, muscle weakness, excessive sleepiness and difficulty concentrating. In London and other parts of the UK, where sunlight exposure is limited for much of the year, these conditions often coexist, intensifying both physical and psychological symptoms.
Studies show that individuals with low vitamin D levels are at greater risk of experiencing depressive episodes. In SAD, the seasonal drop in UVB exposure leads to reduced vitamin D production, which coincides with the onset of low mood. This relationship is not coincidental - vitamin D deficiency may impair the brain’s ability to adapt to environmental stressors. This can present as anhedonia (reduced ability to experience pleasure) and increased social withdrawal.
Seasonal depression is also associated with metabolic changes, including increased cravings for foods high in simple carbohydrates. This may represent a compensatory mechanism, as the body attempts to boost serotonin levels when natural production is reduced. Recognising the biochemical basis of low mood during winter months shifts the focus of treatment towards correcting underlying deficiencies rather than simply managing symptoms.
Testing and safe supplementation of vitamin D
Effective management of seasonal mood changes should be based on objective medical assessment rather than self-prescribing supplements. The first step is a blood test measuring 25-hydroxyvitamin D [25(OH)D], which reflects the body’s vitamin D status. In the UK, where deficiency is common, optimal levels for overall health, including mental wellbeing, are typically considered to be between 30–50 ng/mL (75–125 nmol/L).
The appropriate supplementation dose should be tailored to the individual, taking into account baseline levels, body mass index (BMI) and overall health. In the UK, vitamin D doses are often expressed in micrograms (µg), where 1 µg equals 40 IU. As vitamin D is fat - soluble, it should be taken with food containing fat to improve absorption.
High-dose supplementation without medical supervision is not recommended, as excessive intake can lead to hypervitaminosis D, hypercalcaemia and potential kidney damage. Regular monitoring of vitamin D levels is therefore essential to ensure safe and effective treatment.
At our private medical clinic in London, we take a comprehensive approach to patients experiencing symptoms of seasonal depression. Alongside appropriate blood tests, our clinicians assess results in the context of individual symptoms. This allows us to provide targeted supplementation and support that not only corrects deficiencies but also helps improve cognitive function and stabilise mood. Book a consultation to access professional diagnostics and personalised care, helping you manage the effects of limited sunlight exposure in the UK safely and effectively.
