Public health measures such as hand washing, social distancing, and when appropriate, self-isolation, are the way to reduce transmission of any virus. However, following contact and infection with a virus – including Covid-19 – diet, nutrition and nutritional status are important in supporting immune function to help combat the illness. Much evidence exists to show that several nutrients are essential for good immune function.
Vitamins, including vitamins A, B6, B12, C, D, E and folate, as well as trace elements, including zinc, iron, selenium, magnesium and copper, play established roles in the immune system. All of these micronutrients, apart from vitamin E and magnesium, are allowed by the European Commission (EC) to make a health claim that they contribute to the normal function of the immune system.
Micronutrients support both innate and adaptive immunity.
Amongst other things, micronutrients support growth and activity of innate cells, including neutrophils and macrophages, and recovery from inflammation through cytokine production and antioxidant activity.
Micronutrients support adaptive immunity through antibody production, phagocytosis and cytokine production.
The omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) also play important and complementary roles in supporting the immune system,1 mainly through balancing the inflammatory response.
Deficiencies or suboptimal status of micronutrients adversely affect immune function and can increase risk of infection. While the extent to which the immune system can be modified by supplementation is unclear, research is promising. Evidence exists for vitamins C and D in terms of preventing respiratory tract infections and pneumonia, and for zinc in improving immunity.
A 2013 Cochrane review2 evaluating evidence from three studies found an 80 per cent or greater reduction in pneumonia with vitamin C supplementation, particularly in those with poor dietary intakes. In older people, severity of disease and risk were reduced, particularly in those with low vitamin C plasma levels.
A 2017 review3 concluded that three controlled trials found that vitamin C prevented pneumonia and that two trials observed a treatment benefit of vitamin C in patients with pneumonia. Doses of ≥200mg saturate vitamin C levels in the blood have evidence supporting a reduction in risk, severity and duration of both upper and lower respiratory tract infections. Requirements for vitamin C increase during infection and doses of 1-2g daily have been given in studies involving people who are sick.
Deficiencies or suboptimal status of micronutrients adversely affect immune function and can increase risk of infection
Vitamin D has well established effects in regulating immune function. Vitamin D receptors are found in immune cells such as T and B cells, and it has been suggested that immune cells can convert vitamin D to its active form.4
Recent meta-analyses have indicated that vitamin D supplementation reduces the risk of upper respiratory tract infections and asthma exacerbations,5 particularly in those with low (<25nmol/litre) 25(OH)D plasma levels.6 However, a recent UK Biobank paper did not support a potential link between vitamin D concentrations and risk of Covid-19 infection, nor that vitamin D concentration may explain ethnic differences in Covid-19 infection.7
• Several nutrients are essential for immune function
• The extent to which the immune system can be modified by supplementation is unclear
• Further research is needed on the doses of vitamins that might reduce risk of Covid-19.
Studies in people of South Asian origin find low intakes of vitamin D and also 25(OH)D levels below 25nmol/litre throughout the year, even in summer. Low levels of vitamin D and Covid-19 seem to disproportionately affect black and minority ethnic individuals.
Other recent research – in this case a working paper from Indonesia8 – did find a significant association between vitamin D deficiency and Covid-19 deaths. A study in 20 European countries also found an inverse link between mean levels of 25(OH)D and both Covid-19 cases and mortality (per million people).9
Clinical studies show a link between vitamin E and respiratory tract infections. In a study in 617 nursing home residents, vitamin E 200iu daily for a year reduced the risk of upper respiratory tract infections but not those of the lower respiratory tract.10 Vitamin E enhances T-cell mediated immune function and several other immune function factors in elderly men and women.11
Zinc deficiency also impairs immune function as zinc is needed for cellular responses involved in both innate and adaptive immune function. Zinc deficiency has been associated with respiratory tract infection in children in developing countries.
Of note is that almost one in 10 adults (8 per cent) in the UK have below lower reference nutrient intake levels of zinc. A recent narrative review12 highlighted the need for further study with regards to Covid-19, suggesting that zinc may provide a protective effect through reducing inflammation, improving mucus clearance, preventing ventilator-induced lung injury, and influencing antiviral and antibacterial immunity.
Pharmacists should include nutritional strategies in their recommendations to improve public health and maintain immune function. Many people across the UK have below recommended intakes and low status of several vitamins and minerals, many of which are involved in immune function.
Further research is needed on the doses of vitamins that might reduce risk of Covid-19. But in the meantime, a healthy diet and a multivitamin/mineral supplement containing recommended amounts of micronutrients will help to bridge the dietary gap and support immune function.