Biomarkers: Vitamin D and Longevity — Why Deficiency Is So Common and What to Do About It

Vitamin D is not a vitamin in the conventional sense. It's a hormone — and more than a billion people don't have enough of it.

Despite its name, Vitamin D acts like a hormone, not a micronutrient. Every cell in your body has Vitamin D receptors. It regulates calcium absorption and bone mineralisation, modulates immune function, influences cardiovascular health, affects mood and cognition, and plays a role in reducing the risk of certain cancers. Low Vitamin D is one of the most common and most consequential nutritional deficiencies in the modern world.

Why Deficiency Is So Widespread

Your body produces Vitamin D when UVB radiation from sunlight hits your skin. The problem: in most of the world above 35 degrees latitude — which includes all of the UK, Canada, northern Europe, and much of the northern US — UVB radiation from October through March is insufficient to produce meaningful Vitamin D. For six months of the year, the sun doesn't rise high enough for synthesis to occur.

Add to this the modern indoor lifestyle (most of us spend most of our time inside), sunscreen use (which blocks UVB), and darker skin tones (which require more sun exposure for equivalent synthesis), and it becomes clear why deficiency is so prevalent.

What the Research Shows

The clinical sufficiency threshold for Vitamin D is generally set at 50 nmol/L (20 ng/mL) by most national health bodies. This level is designed to prevent rickets and osteomalacia — not to optimise health.

Observational studies consistently associate higher Vitamin D levels — in the 75–150 nmol/L range — with lower rates of cardiovascular disease, autoimmune conditions, depression, and certain cancers. Whether these associations are causal remains debated (the Endocrine Society's 2024 guidelines note that causality is not fully established for many outcomes). What is clear is that deficiency is harmful, and maintaining sufficient levels is beneficial.

For longevity optimisation, we use a target of 100–150 nmol/L (40–60 ng/mL). This reflects the range associated with the most favourable health outcomes in the evidence base while avoiding any risk of toxicity (which becomes a concern above approximately 250 nmol/L).

Testing and Supplementation

Get tested before supplementing — Vitamin D levels are easy and inexpensive to measure, and knowing your starting point makes supplementation more precise.

If your level is below 100 nmol/L:

• A daily dose of 1,000–2,000 IU of Vitamin D3 is a reasonable starting point for most adults and is widely recommended
• Taking it with K2 (100–200 mcg of MK-7) is commonly advised because Vitamin D increases calcium absorption, and K2 helps direct calcium to bones rather than arteries
• Retest after 3 months to confirm your levels have responded

Doses above 4,000 IU/day should be discussed with a doctor, particularly if you have kidney disease or conditions affecting calcium metabolism.

We are not doctors. Discuss supplementation, particularly dosing, with a healthcare professional before starting.

The 100 Great Years perspective

Vitamin D deficiency is one of the most prevalent and most correctable health problems in the modern world. It is also one where the gap between "not deficient" and "optimised" is large and consequential. Bone density, immune resilience, cardiovascular health, and cognitive function all have meaningful Vitamin D dependencies. 100 Great Years tracks Vitamin D because these are not peripheral concerns — they are the biological foundations of the independent, capable later life that the platform is built around. Testing first, then supplementing, is the right order. The data tells you exactly where you stand.

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This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making decisions about your health.