Your Genes, Your Skin, Your Vitamin D: Why One Size Doesn't Fit All

Vitamin D Deficiency Persists Despite Summer Sunshine

Vitamin D plays a critical role in bone health, immune function, and metabolic regulation. While many people assume that summer sunlight naturally restores vitamin D levels, new research suggests that this may not be true for everyone.

Researchers investigating vitamin D status in northern Britain found alarmingly high rates of vitamin D insufficiency and deficiency among older adults and individuals with darker skin pigmentation. The findings challenge the common belief that seasonal sun exposure is enough to correct low vitamin D levels.  

The study analyzed 299 participants screened between December 2024 and August 2025 as part of an ongoing vitamin D supplementation trial. Blood samples were used to measure serum 25-hydroxyvitamin D, the primary biomarker used to assess vitamin D status.  

More Than Half of Older Adults Had Insufficient Vitamin D

Among adults aged 65 years and older, 54.8% had vitamin D levels below the recommended threshold of 50 nmol/L. The situation was even more concerning among adults with darker skin tones (Fitzpatrick skin types IV-VI), where 72.1% showed vitamin D insufficiency or deficiency.

Perhaps the most striking finding was that these rates remained high throughout the year. Researchers observed little evidence that vitamin D status improved substantially during the summer months, despite increased opportunities for sunlight exposure.

Why Sunlight May Not Be Enough

Vitamin D is synthesized in the skin following exposure to ultraviolet B (UVB) radiation. However, several factors can reduce vitamin D production, including age, skin pigmentation, geographic latitude, lifestyle habits, clothing practices, and limited outdoor activity.

Older adults naturally produce less vitamin D through the skin, while higher melanin levels in darker skin reduce the efficiency of UVB-driven vitamin D synthesis. These biological factors may explain why certain populations remain vulnerable even during periods of greater sunlight exposure.  

The researchers conclude that relying solely on sunlight as a public health strategy may be insufficient for maintaining optimal vitamin D levels in high-risk groups living in northern climates.  

Implications for Public Health

Vitamin D deficiency has been linked to impaired bone health, increased fracture risk, reduced muscle function, and a range of chronic health conditions. The findings support growing calls for more targeted supplementation strategies and improved monitoring of vitamin D status in vulnerable populations.

The authors suggest that year-round vitamin D supplementation and proactive screening may be necessary, particularly for older adults and individuals with darker skin living at higher latitudes.  

Why This Matters for GeneFit Readers

At GeneFit, vitamin D represents one of the clearest examples of how genetics, environment, lifestyle, and ethnicity interact to influence health outcomes.

While population-wide recommendations are valuable, individual responses to vitamin D intake and sun exposure can vary substantially. Genetic variants in genes involved in vitamin D metabolism, transport, and receptor function may affect an individual's vitamin D status and biological response to supplementation.

Combining genetic insights with laboratory testing allows a more personalized approach to vitamin D optimization. Rather than relying on generalized recommendations, precision health strategies can help identify individuals at higher risk of deficiency and guide evidence-based supplementation plans tailored to their unique biology.

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Reference

Goddard, A., Watson, A., Tilbury, R., Corfe, B. M., & Fairley, A. (2026). Circannual prevalence of vitamin D insufficiency in older and minoritized ethnic adults in Northern Britain: Screening outcomes from a clinical trial (ISRCTN13778806). European Journal of Clinical Nutrition. https://doi.org/10.1038/s41430-026-01760-z

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