by Nina Fajs
Suicide is a complex public health problem, influenced by a wide range of psychological, social, environmental, and biological factors (WHO, 2025). While much of the focus in suicide prevention has traditionally been focused on psychosocial risk factors, there is a growing interest in biological markers. Indeed, biological markers might help us better understand what changes in the body before and during periods of mental vulnerability. Researchers are particularly interested in biological markers that are easy to measure and can potentially be modified (Johnston et al., 2022; Goossens et al., 2021).
One such candidate is vitamin D.
While historically vitamin D is best known in relation to bone health, vitamin D is increasingly being recognised for its broader role in the body, including immune regulation, cardiovascular health, and brain function (Delrue & Speeckaert, 2023). Vitamin D is often called ‘The Sunshine Vitamin’ as most of it is produced through skin exposure to sunlight. It is often considered as a link between geography, seasonality, and various mental health outcomes. Suicide rates, for instance, have long been observed to be higher in countries further from the equator, where sunlight exposure is lower throughout the year (An et al., 2023).
What did our study reveal?
To explore the relationship between vitamin D levels and suicide outcomes, our study utilised data from the UK Biobank, a large population cohort of more than 500,000 participants from England, Scotland and Wales.
We analysed data from over 200,000 people who had completed a mental health questionnaire that asked about lifetime suicidal thoughts and behaviours, and who had valid blood measurements of serum 25-hydroxyvitamin D (25[OH]D), the standard marker of vitamin D status.
Across both time points, we observed a consistent pattern: people with suboptimal vitamin D levels (less than 50nmol/L) were more likely to report suicidal thoughts and behaviours compared to those with sufficient levels.
Notably, this association was seen across the full spectrum of suicide outcomes, including thoughts of suicide and self-harm, self-harm, and suicide attempts and deaths. People with vitamin D deficiency (<25nmol/L) had approximately 25-30% higher relative risk of reporting a lifetime suicide attempt, compared to those with optimal levels.
These patterns remained even after adjusting for a wide range of potential confounding factors, including demographic variables, socioeconomic conditions, lifestyle factors, and measures of mental and physical health. Importantly, the associations also persisted when we excluded individuals with severe psychiatric conditions, suggesting that the relationship was not solely driven by pre-existing mental illness.
We also found that associations were generally stronger in men compared to women, although they were present in both groups.
Potential mechanisms
Importantly, these findings do not suggest that low vitamin D levels cause suicidal thoughts or behaviours. Rather, they reveal a potential association that may reflect shared underlying biological or environmental pathways to suicide risk.
One possibility is that vitamin D may be involved in neurobiological processes linked to inflammation and stress regulation. Chronic inflammation has been increasingly implicated in suicidal thoughts and behaviour, and vitamin D is known to play a role in modulating immune responses (Calati et al., 2020). Another possibility is that vitamin D acts as a proxy for broader environmental or lifestyle factors, such as reduced sunlight exposure, poorer physical health, or reduced time spent outdoors (Tanaka & Matsubayashi, 2025).
It’s also important to consider whether vitamin D levels predict suicidal thoughts and behaviours or whether suicidal thoughts and behaviours predict vitamin D levels. For instance, people with suicidal thoughts may be more likely to spend less time outdoors, leading to lower vitamin D levels. This makes it hard to untangle cause and effect.
Why this matters for suicide prevention?
Even with these caveats, the findings raise interesting questions for suicide prevention research. Vitamin D deficiency and insufficiency are a common finding, relatively easy to measure, and potentially modifiable through sunlight exposure, diet modification, or supplementation. In the UK, it is recommended to take a daily vitamin D supplement (400 IU/10 μg) during the autumn and winter months (October-March), although decisions about supplementation should ideally be made in consultation with a clinician, especially for people with existing health conditions (NICE, 2022).
However, it is important to be cautious. Vitamin D should not be viewed as a standalone protective factor or treatment for suicidal thoughts or behaviours. The complexity of suicide risk means that no single biological marker is likely to be sufficient on its own.
Instead, these findings add to a growing picture in which biological, psychological, and social factors interact in shaping suicide risk.
Final thoughts
This study adds to a growing body of evidence suggesting that biological factors, including those influenced by environment and geography, may play a role in shaping vulnerability to suicidal thoughts and behaviours. While vitamin D is unlikely to be a standalone explanation, it may represent one piece of a much larger biological puzzle. Understanding this puzzle, alongside its social and psychological context, is essential if we are to meaningfully inform public health and suicide prevention strategies.
The study is currently available as a preprint.
Suicidal Behaviour Research Laboratory 