In 1919 it was found that sunlight cured rickets (Minisola et al, 2019). Following trials in 1917, which involved babies being fed cod liver oil daily, the discovery of fat-soluble vitamin D in 1922 was then linked to rickets, despite researchers not understanding the reason for its benefit (Dobson et al, 2018). Vitamin D is derived from food such as vitamin D3 (cholecalciferol) and from sunlight, specifically ultraviolet B (UVB), which is the easiest way to generate adequate serum concentrations of 25-hydroxyvitamin D (25(OH)D), the crucial metabolite to determine the critical vitamin D marker. UVB acting on the skin converts 7-dehydrocholesterol (7DHC) – also a cholesterol precursor – to vitamin D (Grave et al, 2016).
We get 10-20% of our vitamin D from food, and 80-90% of vitamin D from sunlight (Dobson et al, 2018); and our main natural food sources of vitamin D are fish, eggs, meat and dairy (Minisola et al, 2019). However, because of the risk of uncontrolled exposure to sunlight causing skin cancer, the question has been posed as to whether Vitamin D supplementation should be considered as an alternative.
One question is how to best establish a concentration range of 25(OH)D within which vitamin D status can be determined as deficient, insuffidiecnt or adequate. The First International Conference on Controversies in vitamin D in 2017 announced that ‘a central controversy in vitamin D research is how to define hypovitaminosis D’. In the absence of consensus, the conference recommended that 25(OH)D values below 30 nmol/L should be associated with an increased risk of rickets/osteomalacia, and 25(OH) D values between 50 nmol/L and 125 nmol/L deemed adequate (Sempos et al, 2018).
If we assume that a consensus does exist on what constitutes sufficiency and deficiency, one might infer that vitamin D-deficient individuals would benefit most from supplementation, and that appropriate studies would have included vitamin D-deficient participants. However, when Bolland et al (2018a) evaluated 137 randomised controlled trials (RCTs) of vitamin D supplementation, they found that many were wasteful because ‘up to 70% of RCTs of vitamin D with clinical endpoints, 71% of large completed RCTs, and 100% of ongoing large RCTs could be considered research waste as they studied cohorts that were not vitamin D deficient’.
It is worth bearing this conclusion in mind in the light of another paper from Bolland et al (2018b). This paper investigates vitamin D supplementation in musculoskeletal health. From their analysis of 81 RCTs (53 537 participants). The authors concluded that vitamin D supplementation is neither protective against fractures or falls, nor does it have clinically meaningful effects on bone mineral density: ‘there is little justification to use vitamin D supplements to maintain or improve musculoskeletal health. This conclusion should be reflected in clinical guidelines.’
‘Healthy individuals, through modest exposure to sunlight and by consuming unprocessed non-junk food need not rely on vitamin D supplements’
This conclusion is not reflected in clinical guidelines. For example, Public Health England (PHE) not only states that to protect bone and muscle health everyone needs an average daily vitamin D intake of 10 micrograms, but also that ‘since it is difficult for people to meet the 10 microgram recommendation from consuming foods naturally containing or fortified with vitamin D, people should consider taking a daily supplement containing 10 micrograms of vitamin D in autumn and winter’ (PHE, 2016). Dobson et al (2018) note that vitamin D deficiency and insufficiency are widespread in the UK, citing the UK Scientific Advisory Committee on Nutrition's advice recommending supplementation in the general population of 10 micrograms daily, with no need for serum concentration testing.
Professor of Genetic Epidemiology Tim Spector, of Kings College London is not persuaded, as one might infer from his piece entitled ‘Vitamin D: a pseudo-vitamin for a pseudo-disease’ (Spector, 2018), in which he states that ‘we have created another pseudo-disease that is encouraged by vitamin companies, patient groups, food manufacturers public health departments and charities. Everyone likes to believe in a miracle vitamin pill’.
With government agencies promoting the virtues of low-fat foods and warning against too much sunlight, Spector's approach seems refreshing. At a time when proper foods – meat, fish, eggs, dairy – are under challenge from what some have called the ‘fake-food’ industry and its promotion of vitamin-fortified foodstuffs, I find it appealing to think that healthy individuals, through modest exposure to sunlight and by consuming unprocessed non-junk food need not rely on vitamin D supplements.