Vitamin C and asthma in children: modification of the effect by age, exposure to dampness and the severity of asthma

Table 3 Vitamin C and asthma symptoms: effect modification by age and baseline asthma severity

Age (yr)	C-ACT at baseline	No of. Children	Symptoms (C-ACT) Mean		Arithmetic increase in C-ACT^a)		Percentage increment in C-ACT^b)
			Placebo	Vitamin C	Mean	95% CI	Mean	95% CI
7.0-8.2	18-19	12	17.6	21.8	4.2	3.3-5.2	24%	18-31%
	16-17	10	16.3	20.0	3.7	2.4-5.0	23%	15-32%
	13-15	8	15.5	18.5	3.0	2.4-3.6	19%	16-23%
8.3-10	18-19	8	17.2	21.0	3.7	1.6-5.8	23%	9-36%
	16-17	10	16.8	19.2	2.4	1.5-3.3	14%	9-20%
	14-15	12	15.8	17.2	1.3	0.1-2.5	9%	1-17%

a) Arithmetic increase in the C-ACT. When age and baseline C-ACT as continuous variables were included in the linear model explaining the effect of vitamin C, the model was improved by χ²(2 df) = 19.1, P = 0.00007 (model parameters are shown in Table 4). There was no second order interaction between the baseline C-ACT and age in their modification of the vitamin C effect (χ²(1 df) = 2.1, P = 0.2).
b) Percentage increment in the C-ACT. When age and baseline C-ACT as continuous variables were included in the linear model explaining the effect of vitamin C, the model was improved by χ²(2 df) = 14.0, P = 0.0009. There was no second order interaction between the baseline C-ACT and age in their modification of the vitamin C effect (χ²(1 df) = 2.6, P = 0.1).

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