SIR—There is increasing interest in the influence of vitamin D on tissues other than bone [ 1 ]. Active vitamin D (1,25-dihydroxyvitamin D 3 ) is an important regulator of the immune system and has been most studied with regard to autoimmunity, where it acts as an immunosuppressant [ 2 ]. The influence of 1,25-dihydroxyvitamin D 3 on infection in vitro and in animal models has been little studied [ 3 ]. Mouse studies have suggested that 1,25-dihydroxyvitamin D 3 enhances Mycobacterium bovis killing [ 4 ], but increases the susceptibility to Toxoplasma gondii [ 5 ]. 1,25-dihyroxyvitamin D 3 directly regulates antimicrobial peptide gene expression, which could enhance host defence against infection [ 6–8 ]. Case control studies have found associations between vitamin D deficiency and tuberculosis in Asians in the UK [ 9 ] and severe acute lower respiratory and gastrointestinal infections in children in Ethiopia, Turkey, India and Jordan [ 10–13 ]. Peripheral blood T lymphocytes were significantly decreased in Turkish children with rickets [ 14 ], and children with rickets were found to have reduced neutrophil motility [ 15 ] and phagocytosis [ 16 ]. We have been unable to find any randomised controlled trials of vitamin D supplementation, which have examined the incidence of infection in at-risk groups. Rehman [ 17 ] reported reductions in respiratory infections and febrile illnesses in a group of Indian children with high alkaline phosphatase levels, but no clinical evidence of rickets as a result of vitamin D and calcium supplementation. A randomised trial of co-administration of intramuscular 1,25-dihydroxyvitamin D 3 with influenza vaccine was not found to enhance humoral immunity in healthy young volunteers, whose vitamin D status was not described [ 18 ]. At a conservative estimate, at least 8% of UK community-dwelling older people, and 32% of older people living in institutions have vitamin D deficiency (serum 25-hydroxyvitamin D 3 levels below 25 nmol/l) [ 19 , 20 ]. However, there is some evidence that the optimal serum 25-hydroxyvitamin D 3 level for health could be as high as 90–100 nmol/l [ 21 ]. As an adjunct to the RECORD trial [ 22 ], a blinded, randomised, placebo-controlled trial of oral vitamin D 3 and/or calcium supplementation for the secondary prevention of osteoporotic fractures, we examined whether vitamin D was associated with a reduction in self-reported infections and antibiotic use. Five thousand two hundred and ninety-two participants were randomised within a factorial design to 800 IU (20 µg) daily vitamin D 3 , 1,000 mg calcium (calcium carbonate), both, or placebo, and followed up for 24–62 months. The trial was based in 21 centres in England and Scotland. Ethical approval was obtained from the Multicentre Research Ethics Committee for Scotland and each centre's Local Research Ethics Committee. Participants gave written informed consent. Full details and main results of the trial are reported elsewhere [ 22 ]. In March 2002, when 25-hydroxyvitamin D 3 levels are lowest in older people in the northern hemisphere [ 19 ], all trial participants who were alive, or had not withdrawn from filling in questionnaires were sent a reply-paid postal questionnaire. This asked if they had had an infection or received antibiotics in the previous week. We compared all participants who had been randomised to take vitamin D 3 with all participants who had not been randomised to D 3 (i.e. intention-to-treat analysis), using multiple logistic regression (adjusted for the trial minimisation factors of gender, age, type of enrolling fracture and time since fracture). Three thousand four hundred and forty-four participants responded to the questionnaire at a median (interquartile range) time of 18 (11–25) months since randomisation. Based on questionnaire responses, at least 55% of trial participants were still taking their tablets. Of the respondents randomised to vitamin D 3 , 17.2% (300/1,740) reported an infection, compared with 18.8% (321/1,704) on placebo (adjusted odds ratio 0.90, 95% confidence interval 0.76 to 1.07, P = 0.23) (Table 1 ). A percentage of 6.4 (111/1,737) randomised to vitamin D 3 reported antibiotic use compared with 7.5% (128/1,703) on placebo (adjusted odds ratio 0.84, 95% CI 0.64 to 1.09, P = 0.18). Baseline characteristics and reported infections and antibiotic use during window week Baseline characteristics and reported infections and antibiotic use during window week ‘Per protocol’ analyses, based on tablet taking when completing the questionnaire, showed smaller odds ratios for infection (0.80, 95% CI 0.64 to 1.01, P = 0.06) and antibiotic use (0.74, 95% CI 0.52 to 1.06, P = 0.10). Adverse events such as hypercalcaemia, renal stones and renal impairment were rare and did not differ between those people receiving vitamin D 3 or not. Although the observed differences were consistent with vitamin D reducing the risk of infection, the results were not statistically significant. ‘Per protocol’ analyses showed smaller odds ratios for infection and antibiotic use, but the concern about such ‘per protocol’ analyses is that they are prone to bias, however, rates of tablet taking were similar for vitamin D 3 and placebo groups. Non-compliance may therefore have reduced the protective effect. Our questions may also have been too crude, and it might have been preferable to ask participants to keep diaries for longer and to record the length, type and severity of infections. Only 6% of the RECORD trial participants could not walk out-doors unaccompanied: hence, our participants were likely to have had higher sunlight exposure for vitamin D manufacture in the skin than less mobile older people and be less likely to benefit. The 25-hydroxyvitamin D 3 level from a small sample of 60 trial participants in Southampton and Newcastle, measured from February to July before supplementation averaged 38 (SD 16) nmol/l by high-performance liquid chromatography [ 22 ], similar to the mean level for older people in institutions in recent UK surveys [ 19 , 20 ]. After 1 year of supplementation in these participants 25-hydroxyvitamin D 3 was 62 (SD 16) nmol/l. Based on data from studies of bone mineral density, lower-extremity function, falls, fractures, colorectal cancer and dental health, Bischoff-Ferrari and colleagues have argued that serum concentrations of 25-hydroxyvitamin D 3 should be 75 nmol/l or more, with optimal levels at least 90–100 nmol/l [ 21 ]. In older people, an intake of ⩾1000 IU (25 µg) vitamin D 3 could bring 25-hydroxyvitamin D 3 to 75 nmol/l in more than 50% of the population [ 21 ]. However, the studies contributing these data used different methods for determining serum 25-hydroxyvitamin D 3 and rarely reported compliance. Currently available assays for 25-hydroxyvitamin D 3 do not agree well, so that assay-specific decision limits have been suggested for assessing compliance and 25-hydroxyvitamin D 3 status [ 23 ]. Further research is needed to assess the effect of a higher dose of vitamin D on infections in populations at high risk of insufficiency. 1,25-dihyroxyvitaminD 3 is an important regulator of the immune system. Vitamin D deficiency is common in older people in the UK, particularly in people living in institutions. Daily supplementation with 800 IU vitamin D 3 (20 μg) was well tolerated by older people living in the community, with no detectable adverse effects. Supplementation was associated with fewer self-reported infections and antibiotics, but the estimated 10–15% reduction was not statistically significant. Further randomised trials of vitamin D and infections are warranted in at-risk populations. None declared for the authors. Conflict of interest statements for other members of the RECORD Trial Group are given in reference number 22 . Health Services Research Unit, University of Aberdeen, Aberdeen, UK (AM Grant, A Avenell, MK Campbell, AM McDonald, GS MacLennan, GC McPherson); University of Southampton, UK (FH Anderson); MRC Epidemiology Resource Centre, University of Southampton, UK (C Cooper); Freeman Hospital, Newcastle-Upon-Tyne (RM Francis); Centre for Health Services Research and Business School—Economics, University of Newcastle-Upon-Tyne, UK (C Donaldson); The Hull York Medical School, Hull, UK (WJ Gillespie); Royal Infirmary of Edinburgh, UK (CM Robinson); Department of Health Sciences, York, UK (DJ Torgerson); Queens Medical Centre, Nottingham, UK (WA Wallace). WD Fraser, Royal Liverpool University Hospital, Liverpool. Further members of the RECORD Trial Group are listed in reference22 We thank the patients who took part in the RECORD study, without whose help this study would not have been possible. The MRC funded the central organisation of RECORD, and Shire Pharmaceuticals funded the drugs, which were co-funded and manufactured by Nycomed. Shire Pharmaceuticals and Nycomed were given the opportunity to comment on the penultimate version of this trial report. However, the trial was conducted, analysed and reported independently of all the funding parties.
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Avenell et al. (2007) studied this question.
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