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This systematic review analysed data from 40 observational studies, in which researchers collected data from other studies rather than conducting experiments themselves. While this review was well conducted, its results are subject to the limitations and biases in the original research it combined. Also, the observational studies analysed cannot establish causation for a condition such as asthma, as they are only able to find how common a particular factor is among affected people. Given the weaknesses in the original studies, the most reasonable message is a call for further research, particularly for randomised trials of vitamin supplements.

....This systematic review and meta-analysis showed a consistent negative association between intake of antioxidant vitamins and asthma status, although the findings in relation to wheeze symptoms were less consistent.
There is a discrepancy between this observational data and the findings of randomised controlled trials (RCTs): a recent Cochrane review of RCTs looking at the effects of dietary vitamin C has, concluded that there is no appreciable effect. The researchers discuss three possible reasons for this discrepancy, including the fact that observational data may be flawed and subject to bias.
Alternatively, an explanation could be that the link between vitamins and asthma is not a causal one, rather one linked to other dietary or non-diet factors. Another possibility for the conflicting results is that dietary factors in early life are important, but this is yet to be tested in randomised trials.

Read the full review

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