The Importance of Vitamin D in the Winter Months

The Importance of Vitamin D in the Winter Months

Vitamin D, also known as the ‘sunshine’ vitamin, is vital for health and vitality all year round. We are all aware of its role in bone and tooth health via calcium and phosphorus modulation, but this wonder nutrient also supports brain health, immune response, muscle function and more. Produced in the skin when exposed to the ultraviolet (UV) rays from direct sunlight, vitamin D is present in almost every cell of the body and involved in hundreds of metabolic processes – so what happens when direct sunlight is scarce and how do we get our vitamin D in the winter months?

The longer nights and darker days of the northern hemisphere mean we do not get enough UV light to produce sufficient levels of vitamin D to support optimal functioning and health. As such, UK guidelines recommend adults and children over 4 years old supplement with vitamin D in the winter months; from September to around March, we should all be supplementing with 10mcg (400IU) of vitamin D daily. Whether you choose a capsule or spray is down to personal preference, however it’s vital to take note of the form of vitamin D you are supplementing with. While both vitamin D2 and D3 have been shown to absorb well in the gut, evidence suggests vitamin D3 is more effective at raising serum blood level and maintaining that level for longer (1).

Let’s dive into the diverse roles that Vitamin D play in human health….

Immune function

Lower temperatures during the winter months increase the likelihood of becoming ill, as the immune system does not function as effectively in the colder weather, making us more susceptible to circulating viruses. That, alongside more time indoors for most of us in close proximity to others, explains why the winter months are notorious for bringing along their fair share of viruses like the flu, COVID-19 and the common cold – hence why supporting the immune system to function as optimally as possible is of upmost importance during this time.

The immune system’s primary role is to protect the body against pathogenic organisms such as viruses, bacteria and parasites. Immune response is variable from person-to-person and dependent on a number of factors, including age, sleep quality, stress, genetics and of course, diet. Many nutrients are involved in immune response, such as zinc, selenium, vitamin C and vitamin D, therefore sufficient nutrient intake is vital for optimal immune function. In recent years, the essential role of vitamin D in immunity has been recognised. It is now understood that vitamin D receptors are present in B cells, T cells and antigen presenting-cells – all vital immunological cells that help to form the immune system. Vitamin D has been shown to modulate both the innate and adaptive immune responses, and deficiency has been linked to increased susceptibility to illness and even autoimmune diseases (2). Finally, vitamin D has potent anti-inflammatory properties; it has been demonstrated that vitamin D supplementation may reduce pro-inflammatory mediators and increase anti-inflammatory cytokines (3). Inflammation is an essential mechanism in the immune response, but chronic, irregulated inflammation can negatively alter its effectiveness, therefore vitamin D’s mediating effects on inflammation are crucial in promoting healthy immunity.

Mood regulation

Rates of mood disorders such as anxiety and depression have sadly been rising annually, with further increases noted since the pandemic. The darker months of winter bring the additional risk of Seasonal Affective Disorder (SAD); often referred to as ‘winter depression’ due to its prevalence during this time, SAD is a mood disorder thought to be caused by a dysregulation of hormone production due to a lack of sunlight. While clinical trials on vitamin D supplementation for SAD are sparse, the body of evidence for vitamin D’s protective role in mood disorders is vast and growing.

It is thought that vitamin D supports mood regulation through several mechanisms of action. Firstly, vitamin D is required in the production and release of serotonin (4). Serotonin is a neurotransmitter known as the ‘happy hormone’ because of its role in mood regulation, with lower levels found in those with low mood and depression. It also promotes good sleep quality – another important risk factor in the development of mood disorders. Furthermore, the immune system is heavily involved in the development and severity of mood disorders, with increased levels of chronic inflammation associated with higher risk of anxiety and depression (5). Due to vitamin D’s role in mediating inflammation, it may be supportive in reducing mental health disorders; this evidence may help to explain the inverse relationship between circulating vitamin D levels and depression found in many research papers.

Weight management

Although weight is not a comprehensive indicator of health, we cannot ignore the overwhelming evidence linking excess body fat to an increased risk of disease development. A person with overweight or obesity is at much higher risk of developing type 2 diabetes, heart disease, various types of cancers and strokes. While maintaining a healthy weight is important all year round, this may be of particular significance during the winter months which tend to be a time of overindulgence(!) and weight gain for much of the population.

Obesity has been shown to increase pro-inflammatory adipokine secretion, negatively affecting both immune and metabolic systems, which may be in part contributing to increased disease risk in this sub-group of people. Furthermore, vitamin D deficiency is more common in those with excess adiposity, thought to be related to decreased bioavailability of vitamin D due to its fat solubility (6). As such, it has been suggested that increasing vitamin D levels in those with obesity may promote weight loss and subsequent improved health outcomes. For example, one study found that high-dose vitamin D (50,000IU) supplementation over a 6-week period resulted in significant decreases in body weight and body mass index scores, alongside improvements in waist and hip circumference (7); however, it is important to note that these results have not been replicated in all studies, therefore evidence for vitamin D’s effectiveness for weight loss is currently inconclusive. Despite this, it is clear that there is a reverse causation relationship between vitamin D levels and adiposity, whereby higher excess fat results in suboptimal concentrations of vitamin D, which in turn may pose additional disease risk. With overweight and obesity on the rise worldwide, it is even more imperative that adequate vitamin D levels are maintained across these populations for future health outcomes.

Vitamin D in the Winter Months

As you can see, maintaining optimal vitamin D levels is of particular importance during the dark winter months to support physical and emotional health and wellbeing. While research is still ongoing, there is strong evidence to support vitamin D’s role in immune function, mood regulation and weight management, and it is vital to promote supplementation during this time when sunlight is inadequate. UK vitamin D supplementation guidelines are modest, and an upper limit of 25mcg (4000IU) is in fact safe for most people. As demonstrated in many studies, a moderate dose – as delivered in our Vitamin D3 spray – may be more effective in achieving the desired health benefits.


  1. Tripkovic L, Lambert H, Hart K, Smith CP, Bucca G, Penson S, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: A systematic review and meta-analysis. Am J Clin Nutr 2012;95:1357-64.
  2. Aranow C. Vitamin D and the immune system. J Investig Med. 2011 Aug;59(6):881-6. doi: 10.2310/JIM.0b013e31821b8755. PMID: 21527855; PMCID: PMC3166406.
  3. Krajewska, M. et al. (2022) “Vitamin D effects on selected anti-inflammatory and pro-inflammatory markers of obesity-related chronic inflammation. Frontiers in Endocrinology, 13.
  4. Patrick, R.P. and Ames, B.N. (2015) “Vitamin D and the omega‐3 fatty acids control serotonin synthesis and action, part 2: Relevance for ADHD, bipolar disorder, schizophrenia, and impulsive behavior,” The FASEB Journal, 29(6), pp. 2207–2222.
  5. Menon, V. et al. (2020) “Vitamin D and depression: A critical appraisal of the evidence and future directions,” Indian Journal of Psychological Medicine, 42(1), pp. 11–21.
  6. Mallard, S.R., Howe, A.S. and Houghton, L.A. (2016) “Vitamin D status and Weight Loss: A systematic review and meta-analysis of randomized and nonrandomized controlled weight-loss trials,” The American Journal of Clinical Nutrition, 104(4), pp. 1151–1159.
  7. Khosravi ZS, Kafeshani M, Tavasoli P, Zadeh AH, Entezari MH. Effect of Vitamin D Supplementation on Weight Loss, Glycemic Indices, and Lipid Profile in Obese and Overweight Women: A Clinical Trial Study. Int J Prev Med. 2018 Jul 20;9:63. doi: 10.4103/ijpvm.IJPVM_329_15. PMID: 30123437; PMCID: PMC6071442
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Elisabeth Philipps

Karen Devine

Karen Devine

Shelley Harvey

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depressionImmune HealthImmune Supportmental healthmoodserotoninsunshinevitamin dVitamin D3Weight Managementwinter

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