Vitamin D, often referred to as the “sunshine vitamin”, is obtained through sun exposure and dietary sources such as fatty fish and egg yolks [1,2]. Vitamin D can also be taken in through dietary supplements, a topic discussed in more detail in this past article. The importance of vitamin D for bone integrity, muscle health, and the immune system has long been established [3]. But besides that, does vitamin D play any role in our brain? And if so, how does lack thereof affect us? In a world where vitamin D deficiency is more prevalent than ever, such questions need to be addressed. Vitamin D deficiency currently affects a population of 1 billion individuals worldwide [4]. This deficiency can for example be attributed to less sun exposure (more time indoors), increased use of sunscreen, less physical activity, and poor socioeconomic background [5]. This article delves into the role of vitamin D and its impact on the human brain, exploring emerging research on its implications for brain function and development.

Nearly two decades ago, researchers discovered that vitamin D receptors are widely distributed through the human brain, mirroring the patterns seen in rodents [6]. This allowed the investigation of vitamin D’s role as a “neuroactive steroid”, a compound that can quickly exert a change to a neuron’s behavior by interacting directly with its receptors. Specifically, vitamin D has been shown to affect neuronal cell differentiation (helping neurons mature correctly), neurotransmitter synthesis (the messenger in our brain), calcium signaling (helping neurons communicate with one another), as well as creating genes and proteins important for brain structure and function [6]. Considering the importance of vitamin D on various neuronal functions, vitamin D deficiency is expected to exert a number of disruptions. Let’s have a closer look at several scientific studies.

Vitamin D has been found to play a crucial role during brain development, already before birth [7]. Lack of vitamin D in pregnant women has been linked to a higher risk of certain neurodevelopmental disorders, such as schizophrenia, in their children [8]. Curiously, the very season in which a person is born can impact their risk for schizophrenia, with those born in the winter and spring months facing a slightly elevated risk – a phenomenon partly attributed to reduced sunlight exposure [9,10]. This has been shown through both animal and human epidemiological studies [10]. For example, an experimental study looked at the influence of prenatal (before birth) vitamin D deficiency on the brains of mice [11]. They found that lack of vitamin D resulted in larger brains with a thinner neocortex compared to the brains of the control mice. This change was due to increased cell proliferation (growth), as well as a deficient apoptosis process, which is a crucial phenomenon during gestation that eliminates unwanted neurons. Neuronal apoptosis is a key event in mammalian brain development [12]. It is important to keep in mind that this is a rodent study and not a human study. Although the pattern of vitamin D receptors in the human brain has been reported to be strikingly similar to that of rodents, any generalizations from rodents to humans need to be approached with precautions [6]. This is especially the case because rodents are nocturnal animals and their vitamin D needs may vary from that of humans.

As for human epidemiological studies, a systematic review compared maternal vitamin D levels during pregnancy with the psychiatric outcomes of the children [13]. 29 studies were included in this review, including data from various countries in Europe, Australia, China, and the USA. This systematic review finds an association between maternal vitamin D deficiency and autism spectrum disorder, attention deficit hyperactivity disorder, and schizophrenia diagnoses. Furthermore, more positive associations between maternal vitamin D deficiency and psychiatric outcomes were found in northern European countries, countries known to have little sunlight in the winter months. This systematic review however reports several limitations, such as some studies not accounting for the mothers’ psychiatric background or small sample sizes. Furthermore, these are observational associations and therefore no definite conclusions can be drawn regarding the causality of these data.

Even after the brain is fully developed, vitamin D continues to play a crucial role in maintaining brain health. It has been found to induce anti-inflammatory and antioxidant properties, which may help reduce negative effects such as neurodegeneration (death of neurons) and impaired neurotransmission (communication between neurons) [14,15]. Moreover, studies have shown that vitamin D supplementation can offer neuroprotective benefits, potentially slowing cognitive decline associated with dementia [16,17]. Especially older populations are affected by vitamin D deficiency because their body is less able to synthesize vitamin D from the resources available [15,18]. A meta-analysis (a large study collecting information from several studies) conducted in 2024 looked at the relationship between vitamin D deficiency and risk for dementia [19]. They found that vitamin D deficiency was a risk factor for dementia, Alzheimer’s disorder, and cognitive impairment [19]. This suggests that optimal vitamin D levels may promote healthy brain aging [18]. However, studies that looked at vitamin D supplementation in participants who already had Alzheimer’s disease do not find sufficient evidence indicating that this may help improve cognitive functioning in Alzheimer’s [20]. Vitamin D supplementation seems to exert very different effects on dementia prevention as compared to relieving symptoms.

Vitamin D receptors have been found to be particularly present in brain areas essential for cognition and emotional regulation, such as the temporal, cingulate, and orbital cortex [21]. Considering the associations found between vitamin D and disorders such as schizophrenia, autism spectrum disorder, attention deficit hyperactivity disorder, and dementia – all disorders where cognition comes into play – vitamin D does indeed seem to play a crucial role in cognitive brain functions [18].

Lastly, the relationship between vitamin D and mood regulation, particularly in the context of depression, has also been explored [22]. While cross-sectional studies (looking at a population at a specific point in time) have suggested a link between lower vitamin D levels and depression, the causality of this relationship remains unclear. For example, when looking at this association in regard to different factors such as demographics, geographical location, gender, smoking status, or age, the strength of this association appears to fluctuate [22]. Attempts to alleviate depressive symptoms through vitamin D supplementation also yielded mixed results [22]. If and how vitamin D and depression interact therefore remains unclear and needs further investigation.

In conclusion, the above-mentioned findings highlight the multifaceted role of vitamin D on brain health, from its significance during development to its potential neuroprotective role in cognition and mood-regulating effects. As vitamin D deficiency remains a widespread issue, understanding its impact on brain functioning is crucial in order to address it accurately – in regard to psychiatric disorders, as well as to our overall well-being.