Vitamin D: What's new a year on from the COVID‐19 outbreak?

When COVID19 hit the UK in early 2020, few would have envisaged that this catastrophic public health crisis would still be posing an unprecedented threat to human health, healthcare systems, public life and the economy a year on. Saving lives and slowing the worldwide pandemic remains an urgent priority. A great deal has been learnt but COVID19 is still a relatively new disease and research into preventative measures and treatments continues to evolve. During the pandemic, there has been an increased clinical and public interest in the role of diet and health, particularly in supporting immunity, sparking media stories suggesting that certain food components and supplements are capable of ‘boosting’ the immune system (BBC, 2020; Mirror, 2020). No specific food or supplement can simply ‘boost’ immunity. To support normal immune function, the body requires adequate energy and nutrition (Calder, 2020), and specific roles have been demonstrated for a number of vitamins, trace elements, amino acids and fatty acids, which together contribute to the body's immune defences, including ability to reduce risk of infections (Calder, 2020). For example, the European Commission has approved health claims for vitamin A, vitamin B6, folate, vitamin B12, vitamin C, vitamin D, zinc, selenium, iron and copper recognising their contribution to the normal function of the immune system (EC, 2021) (see Lockyer, 2020 for more details on these nutrients and their role in immunity). However, early on in the pandemic, the spotlight landed on vitamin D presumably because vitamin D status is typically lowest in late winter when respiratory tract infections peak (Berry et al., 2011; LanhamNew et al., 2020). Whilst this is purely an association rather than indicative of causation, interest continued to grow once it was realised that severe COVID19 outcomes were particularly common in those population groups known to be at greater risk of low vitamin D status, including people living with obesity and those from ethnic minorities (discussed in more detail later on) (PHE, 2020). Therefore, a potential beneficial effect of vitamin D in protecting against COVID19 infection and disease severity was considered plausible and has stimulated much research to investigate whether this is actually the case. Vitamin D is a generic name for two different secosteroid compounds, ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3) (Buttriss et al., 2021). Vitamin D3 is found in a limited number of foods such as oilrich fish, eggs, red meat and vitamin Dfortified products, but the main source results from synthesis in the skin following exposure to sunlight (UVB radiation at 290– 315 nm), which converts 7dehydrocholesterol to vitamin D3. In contrast, vitamin D2 production is not triggered by sunlight in humans but can be obtained from fungi (e.g. wild mushrooms or UVB treated cultivated mushrooms) and yeasts (LanhamNew et al., 2020). In 1940, a mandatory requirement of vitamin D (and vitamin A) fortification of margarines was introduced, which was removed in 2013, although many fat spreads still contain vitamin D, added on a voluntary basis (Defra, 2014). Vitamin D status is usually measured by the level of the circulating metabolite 25hydroxyvitaminD [25(OH) D], produced by hepatic hydroxylation of vitamin D sourced from either skin or the gut via oral intake (SACN, 2016). 25(OH)D is the most commonly used marker of vitamin D status because it has a relatively long halflife (several weeks) and its concentration in serum/plasma reflects dietary intakes, UVB exposure and biological reserves of vitamin D (Buttriss et al., 2021). In the UK, Received: 13 April 2021 | Revised: 19 April 2021 | Accepted: 19 April 2021