Genes & Vitamin D: Why Are Some People Deficient Despite the Sun?

Key Takeaways

• Your blood vitamin D level depends not only on sun and diet, but also significantly on your genes.
• Genes like VDR, GC, CYP2R1 affect how vitamin D is made, transported, and used.
• Some people stay low even with enough sun exposure, because of genetics.
• Knowing your genetic tendency helps you plan vitamin D supplementation more appropriately.

Living in a sunny tropical country like Thailand, yet your blood test shows low vitamin D? This is more common than you'd think, and part of it is each person's genetics. This article explains the role genes play in vitamin D levels.

Why Does Vitamin D Matter?

Vitamin D isn't just about bones — it's also involved in the immune system, muscles, and mood. The body makes vitamin D when skin is exposed to sunlight, and gets some from certain foods, but converting it into its usable form takes several gene-controlled steps.

The Genes Involved: VDR, GC, CYP2R1

Several genes play a role. CYP2R1 helps convert vitamin D into the form measured in blood, GC makes the protein that transports vitamin D in the bloodstream, and VDR (the vitamin D receptor) determines how well cells respond to vitamin D. Genome-wide studies found that variants near the GC, CYP2R1, DHCR7, and CYP24A1 genes are clearly associated with blood vitamin D levels (Wang et al., The Lancet, 2010). Variation in these genes explains why two people with the same sun exposure can have different vitamin D levels.

Why Are You Deficient Despite the Sun?

Many are surprised to be deficient while living in a hot country. There are many factors: sunscreen use, staying indoors/in air-con, skin tone, and genetics — with estimates suggesting genetic variation explains around 7.5% of the differences in blood vitamin D levels (Jiang et al., Nature Communications, 2018). People with certain gene variants may make or maintain vitamin D less easily, even with enough sun. Combining a blood test with genetic data gives a fuller picture.

What Can You Do Once You Know Your Genetic Tendency?

If you know you have a genetic tendency toward low vitamin D, you can plan precisely: regular blood monitoring, safe sun exposure, and supplementation per a doctor's advice. This concept aligns with nutrigenomics, which tailors nutrients to each person's genes.

Author's Final Note

Vitamin D is a great example of "same action, different result" because everyone's genes differ. Knowing your own tendency helps you manage health more precisely, not by guesswork. Check your blood level and consult a doctor before taking high doses. Start understanding your genes with a DNA test.

References

1. Wang TJ, Zhang F, Richards JB, et al. Common genetic determinants of vitamin D insufficiency: a genome-wide association study. The Lancet. 2010;376(9736):180-188. PubMed
2. Jiang X, O'Reilly PF, Aschard H, et al. Genome-wide association study in 79,366 European-ancestry individuals informs the genetic architecture of 25-hydroxyvitamin D levels. Nature Communications. 2018;9:260. Nature Communications