• It is possible for vitamin D production to be affected by age 
• The focus on vitamin D is quite understandable given its involvement in supporting the immune system
• There are many factors that influence the susceptibility to a virus ranging from health inequalities to geography and genetics

It's inevitable that the science of nutrition involves the activity of individual nutrients and, in these unusual times, the spotlight has naturally fallen on those involved in supporting a robust immune system. In the early days of the COVID-19 pandemic it was vitamin C that garnered the most attention, but the advice was less about eating, say, peppers or kiwi fruits but more about how much to take in supplement form. As the weeks passed it was vitamin D that we heard more about but unlike vitamin C, getting regular amounts of vitamin D can be a little more complicated than just the diet.

Vitamin D is the collective name for a group of chemicals but in short, exposure to UV light allows one variant, D3, to be made in the skin, which is then transported to the liver where it is turned into calcitriol, the active form of the vitamin. Therefore, sunlight is vital when it comes to our vitamin D status, and around 15 minutes of direct exposure, ideally between 10am and 2pm, and without blocking UV protection for enough vitamin D production to take place in the skin. It’s not enough to have just the face in the sun, ideally the arms, legs and even some torso would be exposed too for optimal effect. The NHS suggest that this is not a problem from April until September, but in reality, sunlight is not guaranteed in this part of the world, even in the summer months.

It is possible for vitamin D production to be affected by age in part as the process of making it on exposure to UV light may be less efficient, but also as older people may spend more time inside and thus the potential for exposure to UV light is reduced. Vitamin D levels can be further compromised by excess weight, as fat cells can effectively hold vitamin D making it unavailable. In addition, darker skins may also be less efficient at creating D3 with UV exposure.

The National Institute for Health and Care Excellence (NICE) estimate that 1 in 5 adults and 1 in 6 children in England alone may have low vitamin D status and so the official advice is for everyone over the age of 5 to take a supplement of 10mcg daily. This is in addition to foods that contain the dietary form of D3, such as oily fish, soya, shiitake mushrooms egg yolks and liver. It is also to be found as D2, sourced in soya, mushrooms and some algae and thus suitable for anyone following a vegetarian or vegan diet.

The focus on vitamin D is quite understandable given its involvement in supporting the immune system, notably respiratory conditions, not unlike COVID. Vitamin D plays many roles including potentially enhancing the activity of T cells, which activate other immune cells in part by producing cytokines, messengers that call in various mediators of inflammation. In the short-term inflammation is an important part of repair and healing, but in the longer-term cytokines may continue to trigger an inflammatory response in the longer term, known as hyper-inflammation, in which tissue damage can occur. In some cases, this can be fatal.

It has been suggested that supplementing vitamin D may play a significant role in reducing the incidence and severity of respiratory infections, yet studies have produced inconsistent results. For example, the most recent paper at the time of writing, a follow up of a cohort study by the Germany Cancer Research Study involving almost 10,000 adults, found that reduced levels of vitamin D was linked to increased respiratory mortality. The researchers don’t make any claims as to why this may be, or indeed that increasing intake of the nutrient will change the outcome. To confuse matters a little more, there are some medics who maintain that it is the respiratory infections themselves that diminish vitamin D levels, not the other way around and thus concluding that increasing vitamin D will change the outcome is incorrect.

That said, a systematic review and meta-analysis published in the BMJ in 2017 noted that ‘Observational studies report consistent independent associations between low serum concentrations of 25-hydroxyvitamin D (the major circulating vitamin D metabolite) and susceptibility to acute respiratory tract infection’. Importantly, the review concluded that supplemental vitamin D showed an ‘overall protective effect’ but that was strongest in those people that have low vitamin D status in the first place.

Amid the media reports about vitamin D in the early days of the pandemic NICE updated their advice in June 2020 relating to supplementation and COVID saying that there was no evidence that it could prevent or treat the infection and that advice to take 10mcg (400iu) daily remained unchanged.

There are many factors that influence the susceptibility to a virus ranging from health inequalities to geography and genetics. Whilst the evidence for the effectiveness of vitamin D is not absolutely conclusive, taking a supplement that is almost risk free and a very low-cost intervention at that seems prudent.

It may be that there is no extra benefit in taking larger amounts than health authorities advise but bear in mind that the upper limit is around 100mcg (4000iu), a level at which there are no predicted adverse health effects.