Important note: this article does not suggest any medical advice, but rather provides information on the nationwide vitamin D insufficiencies facing Ireland and why the Health Services Executive (HSE) has certain recommendations in place. Always consult with a doctor before taking any supplements.
Cold, dreary, and shorter days are creeping up on us slowly in Dublin. Bundling up for cold, windy days and cloud cover makes sun exposure difficult as we head towards the winter months. It’s no surprise Ireland is not the sunniest country, with 184 rainy days in Dublin during 2025 alone, as reported by Met Éireann. With all this rainfall, could the gloomy sky be hijacking our mental health and weakening our immunity to the lingering freshers’ flu?
Ireland’s Climate
As an island, Ireland has a temperate maritime climate heavily influenced by the Atlantic Ocean: the dreary sky and rainfall in Ireland are known to come from oceanic currents and pressure systems. Warm currents carried by the North Atlantic Drift keep the surrounding air warmer, and a warmer atmosphere is more conducive to moisture absorption. The high moisture content, in turn, cultivates a cloudy and drizzly atmosphere that lasts most of the year. Meanwhile, low-pressure depressions in the Atlantic (which are described as rotating storm systems where warm and cold air collide, creating low atmospheric pressure responsible for stormy conditions) bring in rain and grey skies. This is especially prominent from September to April: during these months, the general cloudiness means the average ultraviolet radiation (UV) falls to 0 to 1 on the index.
UV as low as 2 does not require any additional sun protection, according to Met Éireann. However, despite a lower risk of skin cancer from lower UV, the Cancer Council Australia does not note a UV of two and lower to be optimal for vitamin D generation from direct skin exposure.
Molecular Nutrition and Why You Need Vitamin D
Vitamin D is a fat-soluble vitamin that acts as a hormone and is critical in maintaining optimal serum levels of calcium and phosphate, which reinforces the skeletal and immune systems as well as muscular function. Our bodies mainly receive vitamin D from the sun through endogenous synthesis, which means that the body internally manufactures chemical compounds, proteins, and active nutrients after UV exposure to the skin. In a literature review, Professor Mladen Mavar and his colleagues explain that humans synthesize vitamin D through photolysis – a process that turns sunlight (specifically ultraviolet light) into pre-vitamin D3. This pre-vitamin is then converted into vitamin D3 via a thermal reaction, which is classified as a sigmatropic hydrogen shift. Next, the vitamin is metabolized into the bloodstream and reaches the liver, where it creates 25-hydroxyvitamin D (or calcidiol) through enzymatic processes involving the enzyme CYP2R1. After this step, the vitamin is transported back through the bloodstream to the kidneys, where another enzymatic process involving the CYP27B1 enzyme brings a rise to 1,25-dihydroxyvitamin D, or calcitriol.
Calcitriol is the true active form of vitamin D and has the greatest affinity to vitamin D receptors in tissues. In essence, this active hormone is one of the main regulators of calcium homeostasis on a day-to-day basis. Calcitriol also promotes phosphate metabolism, which on a cellular level can promote energy production by facilitating adenosine triphosphate (ATP) production and boosting mitochondrial activity, as described by Dr Ravi Kumar, Assistant Professor of Neurosurgery at the University of North Carolina. This is important because cells rely on ATP as an energy currency used to carry out necessary cellular functions. Kumar also notes that it improves the structural integrity of the skeletal system concerning calcium and phosphate and their ability to form hydroxyapatite, the mineral that strengthens and hardens bones. In addition, Prof. Mavar makes some observations on the vitamin’s impact on immunity:
“VDRs [vitamin D receptors] are expressed on immune cells, which makes them vitamin D targets. By preventing the invasion of harmful bacteria, inhibiting inflammation, and preserving barrier function, the activation of VDRs is crucial in controlling intestinal homeostasis. In addition, VDR signaling has a critical role in controlling other anti-inflammatory and proliferative pathways.” This evidence suggests the vitamin’s vital role in immune health.
A review titled “Athletic Performance and Vitamin D”, authored by J.J. Cannell, an American physician and founder of the Vitamin D Council, states that vitamin D increases the abundance of Type II muscle fibers, fast-twitch muscles that are utilized during rapid bursts of movement. Cannell also states that vitamin D is directly linked to musculoskeletal performance in individuals, which is because sufficient levels are associated with better physical performance in elderly populations.
Conditions of a Deficiency
Due to current climate conditions in Ireland, especially during winter months, sun exposure above UV 1 is rare. Without a diet rich in vitamin D, it is difficult to maintain sufficient levels from UV synthesis alone. This can lead to insufficient or deficient levels that have a transcending effect on overall health and wellbeing. Some common symptoms of a vitamin D deficiency include soft bones in adults and rickets in children, general bone pain, muscle weakness, fatigue, and mood changes, as mentioned by the Cleveland Clinic. Additionally, vitamin D deficiency is associated with an increased vulnerability to immune infections and autoimmune disorders. In terms of mental health, severe vitamin D deficiency is associated with some mental health disorders such as depression, bipolar disorder, schizophrenia, and ADHD. However, it is important to note that despite associated links, the deficiency in vitamin D is not necessarily causal to these disorders, and a deficiency does not imply that one automatically has the disorder.
Supplements
The HSE suggests that everyone in Ireland needs to take a vitamin D supplement. They note that everyone is at risk of a deficiency, but some groups are more at risk than others, such as people aged above 65, individuals with darker skin tones, pregnant women, those who stay mostly indoors, and those who wear clothes that cover most skin. These populations are at risk all year round of a vitamin D deficiency.
Vitamin D can be taken in several forms: tablets, oral drops, sprays, and powders, and testing levels of vitamin D can be performed by a simple blood test. Supplements can serve as preventive care as well as deficiency treatment, and they are both available in most pharmacies and affordable. Consider reaching out to your GP and scheduling a blood test: are your vitamin D levels sufficient?
