Vitamin D and Longevity: Correcting a Deficiency, Not Chasing a Number

Vitamin D is a hormone the skin makes from sunlight, and that we also get from a few foods (oily fish, egg yolk, fortified products) and supplements. It is central to calcium balance and bone health, with receptors found in many tissues throughout the body (Bouillon et al., 2019). The blood level is measured as 25-hydroxyvitamin D (25-OH-D).

Expert panels generally treat levels below about 30 nmol/L (12 ng/mL) as deficient and around 50 nmol/L (20 ng/mL) as sufficient for most people's bone health (Ross et al., 2011). Deficiency is widespread at high latitudes and in winter, when sunlight is too weak for the skin to make enough.

Clear deficiency causes bone disease — rickets in children, osteomalacia and increased fracture risk in adults — and observational studies consistently link low 25-OH-D to higher rates of many conditions and to higher mortality (Holick, 2007; Chowdhury et al., 2014).

The catch with those observational links is that low vitamin D is often a marker of poor health rather than its cause: illness, inactivity and little time outdoors all lower it. That is exactly why randomised trials are needed to separate cause from correlation.

When large randomised trials tested vitamin D supplements in generally replete adults, the results were sobering: in the VITAL trial, supplementation did not reduce cardiovascular disease or cancer overall (Manson et al., 2019), and it did not lower fracture risk in adults who were not deficient. In other words, more is not better once you have enough.

The consistent signal is narrower and real: correcting a genuine deficiency benefits bone health and is sensible, whereas high-dose supplementation on top of sufficiency has not been shown to extend healthspan and can, at very high doses, cause harm.

If you live far from the equator, spend little time in the sun, have darker skin, are older, or carry excess weight, you are more likely to be low — a simple 25-OH-D blood test settles it. If you are deficient, a modest daily dose to reach sufficiency is well justified; if you are already replete, there is little reason to chase a higher number.

Treat vitamin D as a deficiency to correct, not a lever to maximise. Retest occasionally, avoid very high unmonitored doses, and set your target with a qualified professional in the context of your overall health.