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Can vitamin D help prevent respiratory tract infection and asthma exacerbation?

National surveys show that about one in five people in the UK population have low vitamin D levels in the blood.1

Scientists recognise that adequate vitamin D status has important skeletal and muscular health benefit. Low blood vitamin D levels, currently defined in the UK as less than 25nmol/l, increase the risk of conditions like rickets in children, and osteomalacia in adults, causing bone pain and muscle weakness.2 While it is also acknowledged that vitamin D contributes to the normal function of the immune system (an approved European health claim has been published to this effect), there are still differing views within the scientific community on the strength of evidence for an effect of vitamin D on prevention of acute respiratory tract infections, including colds and flu, and for reducing asthma attacks.3

Vitamin D in the prevention of respiratory infections

To explore this area, researchers led by Professor Adrian Martineau from Queen Mary University of London conducted two systematic reviews in 2017.4,5 Looking at respiratory infections, the researchers pooled together individual data from double blind, randomised controlled trials where vitamin D was compared with a placebo, which together investigated around 11,000 people from 14 countries on four continents, including the US and the UK.4 Their analysis suggested daily or weekly vitamin D supplementation (daily dose equivalents ranging from <20 to >50µg) was useful in prevention, with a reported 12% reduction in the proportion of participants experiencing at least one acute respiratory tract infection. Importantly, supplementation was particularly protective for people who had low blood levels of vitamin D.

However, other scientists have argued that because of the mixture of diverse conditions looked at in these studies, ranging from self-reported colds and flu, ear infections, laboratory-confirmed influenza or radiograph-confirmed pneumonia, it is difficult to know whether a reduction in this mixture of conditions is applicable to the general population and how it should be interpreted clinically. They argue that larger well-designed randomised controlled trials in specific groups with low blood concentrations of vitamin D are needed before any conclusions can be made.

Vitamin D and asthma attacks

Asthma exacerbations are commonly triggered by viral upper respiratory infections, and some studies have suggested that vitamin D may be protective. To gain a better understanding of this association, a review was undertaken looking at the effect of vitamin D supplementation (compared to placebo) on the incidence of asthma exacerbations requiring treatment with systemic corticosteroids.5 When pooling 955 adult and child participants in seven studies, vitamin D supplementation was associated with a 26% reduced risk of asthma exacerbation. Sub analysis revealed, however, that it was only in people with low vitamin D status (less than 25nmol/l n=92) that a statistically protective effect was noted.

It should be mentioned that the small number of trials, participants and number of exacerbations in this analysis can give less precise risk estimates. In addition, the dosing and duration of treatment varied from study to study. Furthermore these results are largely based on data from adults with mild to moderate asthma – children and adults with severe asthma were relatively under-represented in the dataset. Therefore larger trials are needed, particularly with patients who experience frequent severe exacerbations, before definitive clinical recommendations are made or guidelines changed.

In the meantime, the general population is advised to consider taking a 10µg vitamin D supplement, particularly in the autumn and winter months when there are low levels of sunlight, and this would also apply to patients with asthma.6

Vitamin D supplement recommendations in the UK are detailed further below:

Current UK Recommendations on Vitamin D Supplementation7
General population aged five years and over, including pregnant women
  • Between late March/early April and September, the majority will probably obtain sufficient vitamin D from sunlight, alongside foods that naturally contain or are fortified with vitamin D. As such, they might choose not to take a vitamin D supplement during these months
  • From October to March everyone should consider taking a daily supplement containing 10µg of vitamin D as vitamin D is found only in a small number of foods and it might be difficult to get enough from foods that naturally contain vitamin D and/or fortified foods alone
Infants and young children
  • All children aged one to four years of age should be given a daily supplement containing 10µg of vitamin D
  • As a precaution it is recommended that infants from birth to one year of age, whether exclusively or partially breastfed, should be given a daily supplement containing 8.5-10µg of vitamin D
  • Infants fed infant formula should not be given a vitamin D supplement unless they are receiving less than 500 ml (about a pint) of formula a day
People with very little or no sun exposure
  • People with very little or no sunshine exposure should take a daily supplement containing 10µg vitamin D throughout the year. For example:
  1. People who are seldom outdoors (such as frail or housebound individuals) and those who are confined indoors (e.g. in institutions such as care homes)
  2. People who habitually wear clothes that cover most of their skin while outdoors

Intakes of vitamin D can be increased by consuming oily fish, a rich source of vitamin D. Dietary recommendations are to include at least two portions of fish a week, of which one should be oily. Other sources of vitamin D in the diet include eggs, fortified cereals and fat spreads.

Dietary Sources of Vitamin D8
Food Portion size Vitamin D
Salmon red, canned 140g 15.3µg
Sardines, canned 140g 4.6µg
Mackerel, grilled 140g 11.9µg
Tuna, canned 140g 1.5µg
Boiled 50g 1.6µg
2 egg omelette/scrambled 120g 3.4µg
Fortified cereals
Fortified breakfast cereals 30g 1.4µg
Fat spreads 10g 0.8µg
Butter 10g 0.1µg

Vitamin D continues to be a hot topic and the debate of the strength of the evidence base in non-musculoskeletal disease is likely to continue. What is clear though is that health professionals can help prevent low vitamin D status through advice on adequate sun exposure (less than the amount of time needed for skin to become red), regular consumption of oily fish and appropriate supplementation, particularly in at risk groups.

Ayela Spiro is a nutrition science manager with the British Nutrition Foundation


  1. Public Health England. National Diet and Nutrition Survey: Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015 to 2015/2016). London;PHE:2018
  2. Scientific Advisory Committee on Nutrition. Vitamin D and Health. London;SACN:2016
  3. EFSA Panel on Dietetic Products, Nutrition and Allergies. Vitamin D and contribution to the normal function of the immune system: evaluation of a health claim pursuant to Article 14 of Regulation (EC) No 1924/2006. EFSA Journal 2015;13:4182
  4. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ 2017;356:i6583
  5. Jolliffe DA, Greenberg L, Hooper RL, et al. Vitamin D supplementation to prevent asthma exacerbations: a systematic review and meta-analysis of individual participant data. Lancet Respir Med 2017;5:881-90
  6. NHS Vitamin D may prevent asthma worsening for some NHS: 2017
  7. NHS. Vitamin D. London;NHS:2017
  8. Food Standards Agency, Institute of Food Research, Public Health England. McCance and Widdowson’s The Composition of Foods. 7th ed. Cambridge;Royal Society of Chemistry:2014