Vitamin D

Editor – In their recent article, Griffin et al suggest that all adults should receive 800–1,000 IU vitamin D daily supplementation, perhaps to reduce the impact of the COVID-19 pandemic.¹ They point to an observed association between low serum 25-hydroxyvitamin D (25(OH)D) concentration and COVID-19 seropositivity in a recent study involving two of the current article's authors.² In addition, they highlight a small, open label, pilot study from Spain (n=76), which suggested a lower rate of intensive care unit admission with vitamin D supplementation compared with placebo (baseline and on-treatment 25(OH)D concentrations not reported).³

There is a familiar tale of vitamin D that has been repeated over the last few decades. Observational studies associate low serum 25(OH)D with numerous adverse health outcomes.^4,5 Yet despite tens of thousands of people being randomised into studies, the evidence for any health benefit from vitamin D supplementation still evades us.^6–7 The reason for the association of low 25(OH)D and adverse outcomes is multifactorial; in part related to people with poorer health status getting less sunlight exposure and having reduced dietary intake; in part due to the negative acute phase response of serum 25(OH)D (ie it is lowered in times of bodily inflammation).⁸

Through the majority of their article, Griffin et al consider the serum concentration at which 25(OH)D would justify supplementation. The argument for a higher threshold (50 nmol/L) being mainly supported by the observation that elevating serum 25(OH)D concentration suppresses parathyroid hormone release. But does this lead to any tangible health benefits?

There is some evidence that only 25(OH)D concentrations of 10 nmol/L or below create significant biochemical disorders, such as hypocalcaemia.⁹ Improved bone health has been examined as a potential major benefit of vitamin D supplementation. A study that randomised 2,578 people aged over 70 years (mean age 80) compared vitamin D supplementation with placebo over a median follow-up of 3.5 years.¹⁰ Despite higher serum 25(OH)D concentrations achieved with supplementation (60 nmol/L vs 23 nmol/L), no reduction in fracture rate was detected. These data do not support accepting <50 nmol/L as the threshold for 25(OH)D deficiency.

In any case, Griffin et al are not advocating a treat-to-target approach, instead a blanket supplementation for all adults. Although the risk of harm may be small, adverse effects would include some gastrointestinal symptoms and occasional cases of hypercalcaemia.⁷ Additional tablets would contribute to the growing burden of polypharmacy for many people. But perhaps most importantly, what would be the opportunity cost? Instead of using resources putting unwanted and unopened boxes of pills in every home in Britain and Ireland, we could be investing in shared decision-making processes to encourage health improvement for individuals under our care. Promoting non-pharmacological approaches, including smoking cessation and exercising outdoors, would have a far greater impact on our nations’ well-being.