Background
Proposals that vitamin C might be beneficial in the treatment of asthma date back to the 1940s [
1,
2]. Nevertheless, the role of vitamin C is still undefined. A study of Nigerian asthmatics reported a 78% lower incidence of asthma attacks in those administered vitamin C [
3], whereas a study of British asthmatics found no effect of vitamin C on the symptoms or on the FEV
1 levels [
4]. Three trials found that vitamin C reduces bronchoconstriction caused by exercise in subjects who suffer from exercise-induced bronchoconstriction (EIB) [
5‐
7]. Although these three EIB studies imply that vitamin C may have an effect on lung function, the findings cannot be generalized to patients with other variants of asthma.
There is no well-defined mechanism whereby vitamin C may have an effect on asthma. Nevertheless, vitamin C influences the production of various prostanoids in lung tissues [
8‐
11]. Indomethacin reverses the effect of vitamin C on bronchoconstriction in guinea pigs [
10‐
13] and humans [
14,
15]. Thus, the effect of vitamin C on airways might be, at least partly, mediated by influences on the prostanoid metabolism. Furthermore, in asthmatic patients, the level of vitamin C is low in plasma [
1,
16‐
18] and bronchoalveolar fluid [
19]. Although such a correlation does not imply a causal relationship, it encourages research on vitamin C and asthma.
We have previously carried out a placebo-controlled cross-over trial in which we examined the effect of vitamin C, zinc, omega-3 fatty acids and their combination in Egyptian asthmatic children [
20]. Vitamin C significantly decreased asthma symptoms and increased the FEV
1 levels [
20]. We reasoned that the effect of vitamin C might be greater in children who had low baseline FEV
1 levels and found that the effect was modified by baseline FEV
1 (unpublished). Therefore we decided to carry out a formally planned subgroup analysis of the data.
In this subgroup analysis, we planned to use two clinically relevant outcomes: asthma symptoms as measured by the childhood asthma control test (C-ACT) [
21,
22] and pulmonary function as measured by FEV
1. We planned to examine the effect of six baseline variables: the C-ACT, the FEV
1/FVC ratio, gender, paternal smoking, exposure to dampness or mold in the bedroom, and residential neighborhood. In this subgroup analysis, we decided to use the baseline FEV
1/FVC ratio instead of the baseline FEV
1 since the former adjusts for the variation in the size of lungs.
Results
The essential characteristics of the 60 children are described in Table
1. On average, vitamin C supplementation increased the asthma symptom score, C-ACT, by 3.0 points (Table
2). This effect was modified by the baseline C-ACT so that vitamin C was more effective in those children who had less severe asthma symptoms. The evidence of effect modification was stronger when the baseline C-ACT was included in the statistical model as a continuous variable (P = 0.0002) than as a dichotomous variable (P = 0.004), which indicates that the effect modification was better captured by the continuous baseline C-ACT.
The baseline FEV
1/FVC ratio did not modify the effect of vitamin C on the C-ACT (Table
2). This was inconsistent with the modification caused by the baseline FEV
1, which gave us the motivation for this subgroup analysis. Because of this discrepancy, we considered that the modification by the baseline FEV
1 might be explained by the close correlation between age and FEV
1. Since age significantly modified the vitamin C effect, whereas baseline FEV
1/FVC ratio did not, we concluded that the modification by the baseline FEV
1 was simply reflecting the effect of age on FEV
1 (Table
2). There was no substantial difference between including age as a dichotomous or a continuous variable in the statistical model. Gender and residential area did not modify the effect of vitamin C. There was also no significant difference between the children who were currently or had never been exposed to dampness in the bedroom, or between the children whose fathers were current smokers or had never smoked (Table
2).
Given that the baseline C-ACT and age modified the effect of vitamin C, we analyzed the combined effect of these two variables (Tables
3 and
4). When both of these variables were simultaneously included in the linear model, it was substantially improved (P = 0.0001), so that the proportion of variance in the vitamin C effect explained by these two variables was 27% (R
2 = 0.27). There was no second order interaction between these two variables in their influence on the vitamin C effect (Table
3). The greatest effect of vitamin C on the C-ACT was seen in the younger children who had mild asthma symptoms (4.2 point increase), whereas the smallest effect was seen in the older children who had severe asthma symptoms (1.3 point increase). The estimated influence of the baseline C-ACT and age on the vitamin C effect is shown in Table
4.
Table 4
Effect of vitamin C on the C-ACT level: parameters estimating the effect modification by age and baseline C-ACT
Age 7.0 yr, baseline C-ACT 13 points | +2.12 (SE 0.65) |
Age (per year over 7.0 yr) | -0.52 (SE 0.22) |
Baseline C-ACT (per point over 13) | +0.46 (SE 0.13) |
In our analysis of the C-ACT change, we used the absolute difference as the primary outcome. However, as we found a greater effect in those children who had a high initial C-ACT score, we also analyzed Table
3 heterogeneity by using the percentage increment in the C-ACT score. With this secondary outcome, we also found strong evidence of heterogeneity in vitamin C effect between the children (P = 0.001).
On average, vitamin C increased the FEV
1 level by 29% (Table
5). This effect was modified by age, and continuous age was better than dichotomous age in capturing the interaction (Table
5). The effect of vitamin C on FEV
1 was also modified by dampness in the bedroom. Our test of interaction was restricted to the children who were currently or had never been exposed to dampness in the bedroom. However, the effect of vitamin C was smallest in the children who were exposed to dampness in their earlier childhood. Other tested baseline variables did not modify the effect of vitamin C (Table
5).
Table 5
Effect of vitamin C on the FEV1 levels of asthmatic children
All | 60 | 1.125 | 1.446 | 28.9% | 27.3-30.4% | |
C-ACT at baseline | | | | | | |
13-15 | 30 | 1.16 | 1.48 | 27.5% | 25.4-29.6% | 0.07a) |
16-19 | 30 | 1.09 | 1.41 | 30.3% | 28.0-32.6% | |
FEV1/FVC (%) | | | | | | |
< 59 | 28 | 1.11 | 1.43 | 29.4% | 27.0-31.9% | 0.5a) |
≥59 | 32 | 1.14 | 1.46 | 28.4% | 26.3-30.5% | |
FEV1 at baseline (L/s) | | | | | | |
< 1.1 | 29 | 1.00 | 1.31 | 31.1% | 28.6-33.6% | 0.004a) |
≥1.1 | 31 | 1.24 | 1.58 | 26.8% | 25.1-28.5% | |
Age (yr) | | | | | | |
7.0-8.2 | 30 | 1.00 | 1.31 | 31.1% | 28.7-33.5% | 0.003a) |
8.3-10 | 30 | 1.25 | 1.58 | 26.7% | 24.9-28.4% | |
Weight (kg) | | | | | | |
23-28 | 29 | 1.12 | 1.44 | 29.7% | 27.1-32.3% | 0.3a) |
29-37 | 31 | 1.13 | 1.45 | 28.1% | 26.2-30.0% | |
Gender | | | | | | |
Girl | 22 | 1.07 | 1.37 | 28.6% | 25.6-31.6% | 0.8 |
Boy | 38 | 1.16 | 1.49 | 29.0% | 27.2-30.9% | |
Dampness exposure | | | | | | |
Never | 25 | 1.16 | 1.53 | 32.4% | 30.2-34.6% | 0.001b) |
During past 1 yr | 20 | 1.11 | 1.42 | 27.6% | 25.9-29.4% | |
Earlier | 14 | 1.07 | 1.34 | 25.0% | 21.2-28.8% | |
Smoking by the father | | | | | | |
Never | 21 | 1.17 | 1.52 | 30.1% | 28.0-32.2% | 0.4c) |
Current | 22 | 1.08 | 1.38 | 28.5% | 25.3-31.7% | |
Ex-smoker | 17 | 1.12 | 1.43 | 27.9% | 24.9-30.9% | |
Residential area | | | | | | |
Urban | 34 | 1.09 | 1.40 | 29.2% | 27.2-31.3% | 0.6 |
Rural | 26 | 1.17 | 1.50 | 28.4% | 25.8-31.0% | |
When both age and exposure to dampness were included in the same statistical model to explain FEV
1 changes, the model was significantly improved (P = 10
-10) (Tables
6 and
7). The proportion of variance in the vitamin C effect explained by the two variables was 58% (R
2 = 0.58). There was no second order interaction between age and exposure to dampness in their influence on the vitamin C effect. The greatest effect of vitamin C on FEV
1 was seen in the younger children who had never been exposed to dampness or mold in their bedroom (37% increase in FEV
1), whereas the smallest effect was seen in the older children who had been exposed to dampness more than one year prior to the study (21% increase in FEV
1)(Table
6). The estimated influence of age and exposure to dampness on the vitamin C effect is shown in Table
7.
Table 6
Vitamin C and FEV1: effect modification by age and exposure to dampness
7.0-8.2 | Never | 10 | 0.98 | 1.34 | 37.0% | 33.9-40.2% |
| During past 1 yr | 11 | 1.04 | 1.34 | 29.0% | 27.4-30.6% |
| Earlier | 9 | 0.99 | 1.25 | 27.1% | 21.5-32.6% |
8.3-10 | Never | 15 | 1.28 | 1.66 | 29.3% | 27.6-31.1% |
| During past 1 yr | 9 | 1.21 | 1.52 | 25.9% | 22.5-29.4% |
| Earlier | 5 | 1.23 | 1.49 | 21.3% | 18.0-24.5% |
Table 7
Effect of vitamin C on the FEV1 level: parameters estimating the effect modification by age and exposure to dampness
Age 7.0 yr, no exposure to dampness | +37.9% (SE 1.2%) |
Age (per year over 7.0 yr) | -3.37% (SE 0.52%) |
Exposure to dampness during past 1 yr | -5.8% (SE 1.2%) |
Exposure to dampness earlier | -9.6% (SE 1.4%) |
Since exposure to dampness was composed of four indicator items, we explored whether there might be differences between the indicators; mold odor was reported only by 3 children, and it was excluded from this comparison. Within the accuracy of the confidence intervals, there were no differences between the three other indicators in the modification of the vitamin C effect on the FEV1 level (data not shown).
Discussion
We found that age modified the effect of vitamin C on asthma symptoms (C-ACT) and on the FEV1 level in this group of Egyptian children. In addition, the vitamin C effect on asthma symptoms was modified by baseline C-ACT, and the vitamin C effect on the FEV1 level was modified by exposure to dampness in the bedroom.
Previously, an age-dependent variation in the vitamin C effect on common cold duration was noted, but it was not evident whether the greater effect on children than on adults was caused by age
per se or by a higher dose per weight unit since children weigh less [
26,
27]. In the current study, we found a greater vitamin C effect on younger children, and this was not explained by weight differences (Tables
2 and
5). Still, it is possible that the heterogeneity over age might be caused by some factors closely correlated with age; however, this possibility does not challenge the strong evidence indicating that substantial heterogeneity exists across this group of children.
When planning this subgroup analysis, we reasoned that the effect of vitamin C might be greater in children who had the lowest baseline C-ACT level and FEV
1/FVC ratio, and in children who had been exposed to dampness (Additional file
1). However, we found the opposite direction for the modification by C-ACT, namely the effect of vitamin C was greater in those who had a high baseline C-ACT level. We also found that the relation between the baseline FEV
1 and the vitamin C effect, which gave us the motivation for this study, was explained by age and not by the baseline FEV
1/FVC ratio. In addition, contrary to our expectation, exposure to dampness in the bedroom was associated with a decreased effect of vitamin C. Thus, although the baseline C-ACT and dampness modified the vitamin C effect, the modification was in a direction opposite to our expectation.
Gender differences have been found in the vitamin C effects on the common cold [
28‐
30], but in this study we did not find any differences between boys and girls. Urban and rural neighborhoods differ in the type of outdoor air pollution, and passive smoking causes irritation of the airways, but we found no modification of the vitamin C effect by residential neighborhood or paternal smoking.
We found substantial heterogeneity in the effect of vitamin C, over two-fold variation in the effect between the extremes of the subgroups in Tables
3 and
6. Thus, the effect of vitamin C on asthma seems to be context dependent. This heterogeneity in the vitamin C effect seems important since it indicates that no universal effect should be sought. Instead, the characteristics and living conditions of asthma patients who would get the greatest benefit from vitamin C should be targeted.
The heterogeneity we found within these children also has implications for the interpretation of previous studies. Two randomized, double-blind, placebo-controlled trials found divergent effects of vitamin C in asthmatic patients. In Nigeria, Anah et al. found a 78% reduction in the incidence of asthma attacks in 15 to 46 year-old patients administered 1 g/day of vitamin C [
3]. In the UK, Fogarty et al. did not find any effect of 1 g/day of vitamin C on asthma symptoms or FEV
1 levels in 18 to 64 year-old asthmatics [
4]. Since differences in nutrition or lifestyle, or other differences between the participants of the two trials may explain the divergent findings, the newer trial [
4] should not be considered a refutation of the older trial [
3].
Asthma is a collection of different phenotypes, rather than a single disease [
31,
32]. These phenotypes are categorized under the broad umbrella of "asthma" because they meet the criteria for the clinical diagnosis of the disease. Allergic sensitization that triggers asthma may be the largest phenotype. There is also evidence that molds are an important environmental trigger for asthma exacerbations, and the effects of molds are possibly caused, at least partly, by their mycotoxins [
24,
33‐
35]. Furthermore, the mechanisms behind EIB seem to be different from those of ordinary asthma [
36]. Given the variety of mechanisms causing asthma-type symptoms, it seems plausible that vitamin C has different effects on different types of asthma. Thus, although three trials consistently found a benefit of vitamin C against EIB [
5‐
7], those studies cannot be extrapolated to other types of asthma. In this study, we found that the effect of vitamin C on the FEV
1 level of asthmatic children was significantly modified by exposure to dampness or mold in the bedroom.
A number of subgroup comparisons were carried out in our study, and therefore the multiple comparison problem might be of concern. However, the particularly low P-values seen in Tables
2 and
5 are not easily explained by the 18 subgroup comparisons in these two tables. Furthermore, the proportion of variance in the vitamin C effect explained by the statistical models (R
2) in Tables
3 and
6 is high. Therefore, we do not consider that the differences identified might be easily explained by multiple testing.
Our study was randomized, double blind and placebo controlled. Nevertheless, our study has various limitations. Our study subjects were Egyptian children, and it is not clear whether the same modifying factors might apply to children in industrialized countries or in other developing countries or to adults. There may have been inaccuracy in the measurement of mold exposure; however, nondifferential misclassification would move the estimate of interaction effect towards the null value (of no interaction) and cannot generate an artificial difference between the exposed and unexposed [
37]. The duration of vitamin C administration was only 6 weeks, and it is not evident whether the observed effect lasts substantially longer. In a cross-over study, the carry-over effect from the intervention phase to the placebo phase could reduce the difference between the two phases, but cannot bias in the direction of greater effect. The dose of vitamin C was rather low, 0.2 g/day, and our study does not give any information about dose dependency: whether higher doses might cause a greater effect or whether similar effects might be caused by even lower doses. Such issues should be considered in future studies on vitamin C and childhood asthma.
Competing interests
This study was not funded by external sources. We have no conflicts of interest.
Authors' contributions
HH and MB wrote the protocol for the subgroup analysis. MB and AB carried out of the trial which is analyzed in this subgroup analysis. HH wrote the first version of the manuscript and MB and AB participated in the critical revision of the manuscript. All authors read and approved the final manuscript.