Vitamin D, aptly referred to as the “sunshine vitamin”, is a crucial fat-soluble vitamin that supports nearly every system in the body in maintaining overall health and well-being. Even though its best known for its role in supporting bone health, vitamin D also has many other functions such as in muscle strength, mood, immunity, and cellular function (Mayo Clinic, 2025).
Key Roles of Vitamin D
1. Bone Health: Vitamin D is best known for its role in maintaining healthy bones, as it promotes calcium absorption in the gut and bone mineralization, which is positively associated with bone mineral density. The higher the bone mineral density, the less likely bones are to break as they contain more minerals therefore are denser and stronger. Without enough vitamin D, bones can become thin and brittle which can lead to rickets in children, and osteomalacia in adults (NIAMS, 2025; Laird et al., 2010).
2. Immune Support: Vitamin D can strengthen both the innate and adaptive immune system. In the innate immune system, vitamin D can modulate macrophage activity and inhibit inflammatory cytokine production to reduce excessive inflammation. In the adaptive immune system, vitamin D can influence the differentiation of T cells, which can also inhibit inflammatory cytokine production. Vitamin D has also been found to support immune tolerance and reduce the risk and severity of autoimmune diseases, especially after transplantation (Aranow, 2012; Prietl et al., 2013).
3. Muscle Function: Vitamin D plays an important role in skeletal muscle function, structural integrity, and regeneration. Vitamin D has been shown to regulate myoblast growth, size, and gene expression, all of which are essential for muscle growth. Vitamin D deficiency has also been shown to decrease muscle grip strength and gait speed among elderly adults (Agoncillo et al., 2023).
4. Nervous System Function: Vitamin D plays a protective role in brain health by protecting neurons through soluble factors released from microglia that inhibit neuroinflammation, and promoting communication between brain cells. Adequate vitamin D levels have also been shown to protect against neurodegenerative diseases such as Parkinson’s, Alzheimer’s, and Multiple Sclerosis (Shi et al., 2024).
5. Cell Growth & Inflammation Control: Vitamin D helps to regulate cell growth and inflammation by influencing cell proliferation and differentiation. Vitamin D can also have anti-cancer effects by modulating key mediators of apoptosis (programmed cell death) in cancer cells (Umar et al., 2018).
6. Mood Regulation: Vitamin D plays a role in regulating serotonin production in the brain, also known as the “happy hormone”. Low levels of vitamin D are associated with depression, anxiety, fatigue, and seasonal affective disorder (SAD) (Patrick et al., 2015; Akpinar et al., 2022).
Vitamin D Deficiency
1 billion people worldwide suffer from vitamin D deficiencies worldwide. This is especially prevalent in populations with limited sun exposure and those living in higher latitudes, those with a darker skin pigmentation, cultural practices that may limit sun exposure due to clothing preferences, those with malabsorption disorders, and older age adults. Symptoms can include (Cleveland Clinic, 2022):
· Rickets – bone deformities in children
· Osteomalacia – soft, weak bones in adults
· Increased risk of fractures and osteoporosis
· Muscle weakness
· Fatigue, weakness, and low mood
· Weakened immune response and frequent illness
Vitamin D Toxicity
While rare, vitamin D toxicity can occur due to excessive supplementation, making it essential to follow dosage guidelines and consult with healthcare providers prior to beginning supplements. Symptoms of vitamin D toxicity include (Mayo Clinic, 2025):
· Hypercalcemia – calcium buildup in the blood which can lead to bone pain, kidney stones, and heart damage
· Nausea and vomiting
· Weakness and frequent urination
Sources of Vitamin D
1. Sunlight: Vitamin D is free. When UVB rays from sunlight interact with a specific type of cholesterol in the skin, it synthesis vitamin D3, which is the form of vitamin D that the body uses. Around midday, during the summer, is the best time to get sunlight as UVB rays are the most intense at that time, requiring less time in the sun to make sufficient vitamin D (Raman, 2024).
2. Food Sources: Good sources of vitamin D include (British Heart Foundation, 2022):
· Fatty fish e.g. salmon, mackerel, sardines
· Cod liver oil
· Egg yolks
· Mushrooms
· Beef liver
· Fortified milk and cereals
· Fortified orange juice
3. Supplements: Vitamin D supplements are available in 2 main forms: D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D3 is often recommended since its more bioavailable and effective in raising and maintaining vitamin D levels in the body (Office of Dietary Supplements, 2022). However, healthcare professionals should always be consulted for accurate, safe, and tailored advice.
Conclusion
All in all, vitamin D plays a vital role in the human body, from maintaining health bones, and muscles, to regulating the immune and nervous system. It is easily accessible (and free!) through daily sunlight, and can also be obtained through a balanced diet and supplements if required. Obtaining an optimal level of vitamin D can easily be achieved, and should be prioritized, as it is a key driver in supporting our long-term wellness and health outcomes.
References
Agoncillo, M., Yu, J., & Gunton, J. E. (2023). The Role of Vitamin D in Skeletal Muscle Repair and Regeneration in Animal Models and Humans: A Systematic Review. Nutrients, 15(20), 4377. https://doi.org/10.3390/nu15204377
Akpınar, Ş., & Karadağ, M. G. (2022). Is Vitamin D Important in Anxiety or Depression? What Is the Truth?. Current nutrition reports, 11(4), 675–681. https://doi.org/10.1007/s13668-022-00441-0
Aranow, C. (2011). Vitamin D and the immune system. Journal of investigative medicine: the official publication of the American Federation for Clinical Research, 59(6), 881–886. https://doi.org/10.2310/JIM.0b013e31821b8755
British Heart Foundation. (2022, June 1). Foods high in vitamin D. Retrieved July 9, 2025, from https://www.bhf.org.uk/informationsupport/heart-matters-magazine/nutrition/ask-the-expert/foods-high-in-vitamin-d#:~:text=The%20best%20food%20sources%20of,plant%20milks%20and%20fat%20spreads
Cleveland Clinic. (2022, August 2). Vitamin D (vitamin D deficiency). Retrieved July 8, 2025, from https://my.clevelandclinic.org/health/diseases/15050-vitamin-d-vitamin-d-deficiency
Laird, E., Ward, M., McSorley, E., Strain, J. J., & Wallace, J. (2010). Vitamin D and bone health: potential mechanisms. Nutrients, 2(7), 693–724. https://doi.org/10.3390/nu2070693
Mayo Clinic. (2025, March 21). Vitamin D. Retrieved July 8, 2025, from https://www.mayoclinic.org/drugs-supplements-vitamin-d/art-20363792
Mayo Clinic. (2025, February 20). Vitamin D toxicity: What if you get too much? Retrieved July 8, 2025, from https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/vitamin-d-toxicity/faq-20058108
National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). (2025, February). Bone mineral density tests: What the numbers mean. National Institutes of Health. Retrieved July 8, 2025, from https://www.niams.nih.gov/health-topics/bone-mineral-density-tests-what-numbers-mean
Office of Dietary Supplements. (2022, November 8). Vitamin D. National Institutes of Health. Retrieved July 8, 2025, from https://ods.od.nih.gov/factsheets/VitaminD-Consumer/
Patrick, R. P., & 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. FASEB journal: official publication of the Federation of American Societies for Experimental Biology, 29(6), 2207–2222. https://doi.org/10.1096/fj.14-268342
Prietl, B., Treiber, G., Pieber, T. R., & Amrein, K. (2013). Vitamin D and immune function. Nutrients, 5(7), 2502–2521. https://doi.org/10.3390/nu5072502
Raman, R. (2024, November 26). How to safely get vitamin D from the sun. Healthline. https://www.healthline.com/nutrition/vitamin-d-from-sun
Shi, Y., Shi, Y., Jie, R., He, J., Luo, Z., & Li, J. (2024). Vitamin D: The crucial neuroprotective factor for nerve cells. Neuroscience, 560, 272–285. https://doi.org/10.1016/j.neuroscience.2024.09.042
Umar, M., Sastry, K. S., & Chouchane, A. I. (2018). Role of Vitamin D Beyond the Skeletal Function: A Review of the Molecular and Clinical Studies. International journal of molecular sciences, 19(6), 1618. https://doi.org/10.3390/ijms19061618
