Low intake of Vit A & C linked to Asthma & Wheeze

Diet Low in Vitamins A and C Linked to Asthma and Wheeze CME
News Author: Laurie Barclay, MD
CME Author: Charles Vega, MD, FAAFP
April 23, 2009

Relatively low dietary intakes of vitamins A and C are associated with statistically significant increased odds of asthma and wheeze, according to the results of a systematic review and meta-analysis reported in the April 16 Online First issue of Thorax.

"Epidemiological studies suggest that dietary intake of vitamins A, C and E may be associated with the occurrence of asthma," write S. Allen, from the University of Nottingham in Nottingham, United Kingdom, and colleagues. "However, randomized controlled trials of vitamin supplementation in asthma thus far have been inconclusive. In view of this apparent inconsistency between the observational and experimental data, we have attempted to determine the magnitude of the overall associations of these vitamins estimated by the observational studies by carrying out a systematic review and meta-analysis to provide pooled quantitative estimates of the likely magnitude of the effect of dietary intake and blood levels of antioxidant vitamins on a range of measures of asthma and asthma severity."

The reviewers searched MEDLINE, EMBASE, CINAHL, CAB abstracts, and AMED up to November 2007 for studies of asthma, wheeze, or airway responsiveness in association with dietary intakes and serum concentrations of vitamins A, C, and E. Conference proceedings and bibliographies of identified studies were also searched. Random-effects models allowed estimates of pooled odds ratios (ORs) or mean differences with 95% confidence intervals (CIs).

The review included 40 studies meeting the above criteria. Compared with people without asthma, those with asthma had significantly lower dietary vitamin A intake (mean difference, –182 µg/day; 95% CI, –288 to –75 µg/day; 3 studies). In addition, people with severe asthma had significantly lower dietary vitamin A intake vs those with mild asthma (mean difference, –344 µg/day; 2 studies).

The odds of asthma were also increased in patients with lower quantile dietary intakes (OR, 1.12; 95% CI, 1.04 - 1.21; 9 studies) and in those with lower serum levels of vitamin C. Although vitamin E intake was generally not associated with asthma status, it was significantly lower in patients with severe asthma vs those with mild asthma (mean difference, –1.20 µg/day; 95% CI, –2.3 to –0.1 µg/day; 2 studies).

"Relatively low dietary intakes of vitamins A and C are associated with statistically significant increased odds of asthma and wheeze," the study authors write. "Vitamin E intake does not appear to be related to asthma status."

Limitations of this study include methods used to determine levels of the antioxidant vitamins not consistent across studies, levels of antioxidant vitamins assessed after the onset of asthma in most studies, and most of the studies not reporting adjusted results.

"The epidemiological evidence thus suggests that vitamins A and C are linked to asthma," the study authors conclude. "Epidemiological studies and metaanalyses are useful for identifying association between exposures and diseases but cannot reliably establish causation. Further investigations are necessary to account for the observed associations using well-designed randomised controlled trials of vitamin supplementation in asthma."

This study was internally funded by the University of Nottingham. The study authors have disclosed no relevant financial relationships.

Thorax. Published online April 16, 2009.