Understanding Vitamin D
Vitamin D comes from two main sources:
- Food sources – fatty fish, egg yolks, cheese, and mushrooms (particularly those exposed to UV light)
Vitamin D exists in multiple forms: D2 (ergocalciferol) found in mushrooms and yeast, and D3 (cholecalciferol), which is the primary form in humans. In countries where vitamin D-rich foods aren’t commonly consumed, fortified products like milk, butter, and cereals help address potential deficiencies (Magagnoli et al., 2025).
- Skin production – synthesis of vitamin D when exposed to sunlight
Research indicates that 80-90% of our vitamin D requirements are naturally met through skin synthesis. A 20-minute whole-body exposure to summer sunlight can produce approximately 250 μg of vitamin D3, achieving recommended serum levels of 25-hydroxyvitamin D (>30 ng/mL). The effectiveness of this synthesis varies based on exposure duration, season, and geographical latitude.
While vitamin D is best known for its role in calcium metabolism and bone health, its receptor (VDR) has been identified throughout the human body, including the skin, brain, immune cells, and pancreas. This widespread presence explains vitamin D’s beneficial effects on cardiovascular health, diabetes management, cancer prevention, mental health, cognitive function, multiple sclerosis, and fall prevention in elderly populations.
Today’s limited sun exposure due to clothing, indoor living, and varied climate conditions has made vitamin D deficiency a common issue, often requiring supplementation (Janoušek et al., 2022).
Testing and Supplementation
Vitamin D metabolism involves multiple steps before becoming biologically active. When vitamin D3 enters the bloodstream (whether from skin production or diet), it’s first processed in the liver to form calcidiol (25(OH)D3), then converted in the kidneys to the active form calcitriol (1,25(OH)2D3). Maintaining optimal liver stores of 25D3 is essential for proper vitamin D utilization (Kallioğlu, 2024).
Vitamin D deficiency represents a significant global health concern. Approximately 10% of Europeans have severe deficiency (less than 12 ng/mL), while deficiency rates (less than 20 ng/mL) reach around 20% in Northern Europe, 30-60% in other European regions, and as high as 80% in the Middle East.
Supplementation recommendations vary by organization and individual factors. The National Institutes of Health recommends 400-800 IU/day depending on age, with upper limits ranging from 1000-4000 IU/day. The Endocrine Society suggests higher upper limits (10,000 IU/day) than the Institute of Medicine (4000 IU/day). For individuals with normal levels, a standard dose of 1000 IU daily typically raises blood levels by 10 ng/ml over 3-4 months, though individual responses vary based on age, weight, skin color, and health conditions.
Natural vitamin D synthesis through sunlight exposure follows predictable patterns:
- UVB exposure occurs primarily between 10:00-16:00, peaking at 12:30 pm
- Active D3 synthesis happens from early March through late October in temperate regions
- UVB radiation is minimal or absent during the winter months (November-February)
- Skin type significantly affects production time – lighter skin (type I) requires about 5 minutes for 1000 IU, while darker skin (type VI) needs approximately 25 minutes
- Latitude matters – for every degree away from the equator, D3 production decreases by 105 IU in summer and 237 IU in spring
In addition to general advice, it’s important to:
- Get blood tests to check your actual vitamin D status
- Adjust supplementation based on your specific levels
- Remember that a vitamin D overdose, while rare, can be toxic
Sun Exposure and Alternatives
In my opinion, it’s better to protect skin from cancer and premature ageing by avoiding unprotected sun exposure (without SPF, sunglasses, hat, appropriate clothing) and instead relying on vitamin D supplementation.
Balancing sun exposure is important:
- Some sunlight is essential for our circadian rhythm and sleep quality
- From a longevity perspective, protecting our skin with sunscreen and sunglasses is recommended
Does SPF block vitamin D production? This remains controversial!
According to recent research, “The existing evidence supports that sunscreen can impair vitamin D3 synthesis, and as a result decrease serum 25(OH)D levels” (Gatta & Capelli, 2025). However, many studies note that sunscreen would need to be applied as a thick layer and regularly reapplied to fully block vitamin D production.
My recommendation: prioritise skin protection with SPF and other measures, then supplement vitamin D and incorporate more vitamin D3-rich foods to maintain healthy skin for years to come.
Effective Supplementation
Research shows that consistent, smaller daily doses of vitamin D are more effective than large weekly doses.
A clinical trial comparing equivalent doses of vitamin D3 (600 IU/day, 4200 IU/week, and 18,000 IU/month) in nursing home residents found daily administration most effective, while monthly dosing was least effective. After four months of treatment, 35% of those receiving monthly doses still had insufficient levels. Interestingly, calcium supplementation provided no additional benefit (Chel et al., 2007).
My advice: develop a consistent habit of taking vitamin D with breakfast daily. Separate calcium supplements aren’t necessary for vitamin D absorption.
Vitamin D and Longevity
Recent research demonstrates vitamin D’s potential impact on longevity and healthy ageing. The DO-HEALTH trial, examining 777 participants, found that vitamin D (2,000 IU daily), omega-3 (1g daily), and regular exercise showed additive benefits in slowing biological ageing markers. Over three years, these interventions demonstrated measurable protective effects on DNA methylation age markers (Bischoff-Ferrari et al., 2025).
Vitamin D has also gained attention as a neuroprotective agent. Low serum levels (<20 ng/mL) are associated with a 2.3-fold increased Alzheimer’s disease risk, while supplementation appears to slow cognitive decline in those with mild cognitive impairment (Li Y et al., 2025).
Further research indicates distinct roles for vitamin D in biological ageing processes, highlighting the importance of maintaining adequate levels for cognitive and physical health in older adults. Even among those with normal vitamin D levels, preserving cognitive function significantly slows biological ageing (Li M et al., 2025).
Vitamin D deficiency has been identified as a risk factor for decreased mobility in older individuals. Monitoring levels should be prioritised across clinical settings to minimise complications associated with deficiency, particularly regarding mobility (Luiz et al., 2025).
Studies examining exceptional longevity have found associations between vitamin D levels and cardiovascular health. Lower 25(OH)D levels are linked to increased risk of cardiovascular disease, hypertension, atherosclerosis, atrial fibrillation, and heart failure. Conversely, higher concentrations predict lower long-term mortality and cardiovascular disease incidence (Pareja-Galeano et al., 2015).
Recent research has also revealed vitamin D’s role in mitochondrial function. The vitamin D receptor (VDR) interacts with mitochondrial DNA, suggesting vitamin D’s involvement in cellular energy production and ageing processes (Gezen-Ak et al., 2023).
Take-home message
Vitamin D is essential for bone health and numerous body functions. While our bodies produce it naturally through sun exposure, modern lifestyles often lead to a deficiency. For optimal health, consider these key points:
- Get your vitamin D levels tested to determine your personal needs
- Daily supplementation (typically 1000-2000 IU) is more effective than weekly or monthly doses
- Protect your skin with SPF and rely on supplements rather than unprotected sun exposure
- Adequate vitamin D levels contribute to longevity, cognitive health, and reduced cardiovascular disease risk
References:
Bischoff-Ferrari, H., et al. (2025). Individual and additive effects of vitamin D, omega-3 and exercise on DNA methylation clocks of biological aging in older adults from the DO-HEALTH trial. Front. Aging, 12:1581612.
Chel, V., et al. (2007). Efficacy of different doses and time intervals of oral vitamin D supplementation with or without calcium in elderly nursing home residents. J Clin Endocrinol Metab, 92(7):2604-2609.
Gatta, L., & Capelli, G. (2025). Sunscreen and 25-Hydroxyvitamin D Levels: Friends or Foes? A Systematic Review and Meta-Analysis. Journal of Dermatology, 44(2):109-121. doi: 10.1016/j.eprac.2025.03.014
Gezen-Ak, D., et al. (2023). Vitamin D receptor regulates transcription of mitochondrial DNA and directly interacts with mitochondrial DNA and TFAM. J Nutr Biochem, 116:109322. doi: 10.1016/j.jnutbio.2023.109322
Kallioğlu, T. (2024). UV index‑based model for predicting synthesis of (pre‑)vitamin D3 in the mediterranean basin. Int J Biometeorol, 68(5):765-778.
Li, M., et al. (2025). Association of serum 25(OH)D3 and cognitive levels with biological aging in the elderly: a cross-sectional study. Front Nutr, 12:1581610. doi: 10.3389/fnut.2025.1581610
Li, Y., et al. (2025). The relationship between vitamin D levels and Alzheimer’s disease risk: insights from a centenarian study of Chinese women. Front Nutr, 12:1628732. doi: 10.3389/fnut.2025.1628732
Luiz, L.C., et al. (2025). Is serum 25-hydroxyvitamin D deficiency a risk factor for the incidence of slow gait speed in older individuals? Evidence from the English longitudinal study of ageing. Age and Ageing, 54(2):239-246.
Pareja-Galeano, H., et al. (2015). Vitamin D, precocious acute myocardial infarction, and exceptional longevity. Int J Cardiol, 199:405-6. doi: 10.1016/j.ijcard.2015.07.082
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