PMC:4137988 / 9290-19030 JSONTXT

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results\nOf 1,514 references identified, 1,484 were excluded based on review of title (n=1,395), language (n=84), and study type, abstract, and full text (n=5), as shown in Figure 1. One reference was added on cross-referencing. Thirty-one studies were eligible for systematic review, including 12 in adults, 13 in children, five in pregnant woman–child pairs, and one in both children alone and pregnant woman–child pairs. Six studies in adults were excluded from meta-analysis because asthma prevalence was not measured (three studies assessed asthma control, quality of life, lung function, and/or inflammatory markers) or was not a primary outcome (three studies assessed chronic obstructive pulmonary disease or persistent cough with phlegm as a primary outcome).\n\nScientific quality\nOne19 of the 31 studies was published as a conference abstract only and was not included in our quality review. Twenty-eight studies were rated positive (scoring 8 or 9 out of 10; Tables 2–4). One20 of the two studies rated as neutral was a case-control study that received a quality score of 8, but did not clearly describe whether potential confounding factors were comparable for the cases and controls, although the potential confounding variables were adjusted for in the analyses. The other study21 was a randomized controlled trial (RCT) that did not clearly describe the method of randomization or the amount of exposure to intervention. No study was excluded from meta-analysis based on quality ratings.\n\nAdult dietary patterns and asthma\n\nSystematic review\nTable 2 shows the main characteristics and results of the 12 studies in adults, published between 2006 and 2013. These papers included four cross-sectional studies,22–25 six cohort studies,26–31 one case-control study,20 and one RCT.21 Six of them were conducted in Europe,20,23–27 two in Australia,21,22 two in the USA,28,29 and two in Asia.30,31 Sample sizes ranged from 38 in the RCT to 156,035 in the cross-sectional studies. Two studies27,28 included female subjects only and one29 male subjects only, while the remaining examined both sexes, with one22 examining men and women separately.\nAll studies used food frequency questionnaires (FFQs) to measure dietary intakes, with the number of food items or groups ranging from 12 to over 200. Ten studies20,22–24,26–31 derived at least two dietary patterns a posteriori using PCA (n=9) or factor analysis (n=1). Two studies21,25 calculated a Mediterranean diet score defined a priori.\nAsthma outcomes evaluated in these studies included prevalence of ever or current asthma, asthma-related quality of life, asthma symptoms, lung function (forced expiratory volume in one second, forced vital capacity), frequency of asthma attacks, asthma control (Asthma Control Questionnaire alone or plus fractional exhaled nitric oxide), and asthma-related inflammatory markers. The findings were mixed. Among the 12 studies, six reported significant association between dietary patterns and ever asthma,22,31 forced expiratory volume in one second,24,26 frequency of asthma attacks,27 and risk of uncontrolled asthma.25 Although the asthma outcomes varied across these studies, potentially protective dietary patterns tended to include cheese/brown bread, nuts and wine, a prudent pattern diet (fruit, vegetables, oily fish, and wholemeal cereals), and the Mediterranean diet. At the same time, potentially risky dietary patterns tended to include meats/cheese, Chinese traditional pattern (rice and fresh vegetables), the Netherlands traditional diet (meat and potatoes), and Western pattern (pizza, salty pies, desserts, and cured meat). In contrast, one cross-sectional,23 one case-control,20 and three cohort studies28–30 reported no association. An RCT with 38 adults who had symptomatic asthma showed no effect of two Mediterranean diet interventions on asthma control, lung function, asthma-related quality of life, or inflammatory markers compared with no-intervention control.21\n\nResults of meta-analysis\nThe meta-analysis included three cohort studies,26,27,31 two cross-sectional studies,22,23 and one case-control study.20 Figures 2–4 show that there was no evidence of association between the prevalence of current or ever asthma and healthy (OR 1.01, 95% CI 0.78–1.31), unhealthy (OR 1.04, 95% CI 0.93–1.16), or neutral (OR 1.01, 95% CI 0.73–1.41) dietary patterns. The mixed model results show that the random effect estimates were zero, suggesting very small or negligible variance between versus within studies.\n\nPublication bias and heterogeneity\nFigure 5 shows funnel plots of studies examining the association between healthy, unhealthy, and neutral dietary patterns separately and the prevalence of current or ever asthma. The plots were roughly symmetrical, suggesting little evidence of publication bias, and almost all the studies lay within the diagonal lines indicating 95% CIs, suggesting negligible between-study heterogeneity, which is consistent with the model-based results described above.\n\nChild dietary patterns and asthma\nThe main characteristics and results of the 14 studies in children, published between 2006 and 2013, are shown in Table 3. Twelve studies were cross-sectional and two32,33 were cohort studies. Nine of them were conducted in Europe,19,32,34–40 three in America (one in Brazil41 and two in Mexico33,42), and one in Asia.43 One other study44 was an international study conducted in 20 countries. All studies examined both boys and girls with sample sizes ranging from 158 (cohort) to 50,004 (cross-sectional).\nEight of these studies (shown in the footnote to Table 2) were included in a recent meta-analysis14 to investigate whether the Mediterranean diet has a protective effect on ever asthma and current wheeze and whether these relationships were specific to the Mediterranean regions. The meta-analytic results showed adherence to the Mediterranean diet was negatively associated with current wheeze (OR 0.79, 95% CI 0.66–0.94; P=0.009) and current severe wheeze (OR 0.66, 95% CI 0.48–0.90; P=0.008) in Mediterranean regions, and with ever asthma (OR 0.86, 95% CI 0.75–0.98; P=0.027) in non-Mediterranean regions. When considering all regions together, the authors concluded adherence to the Mediterranean diet tended to have a protective effect on current wheeze and ever asthma but not on current severe wheeze.\nThe meta-analysis14 excluded six studies for the following reasons: three studies derived dietary patterns a posteriori,32,41,43 one measured lung function as the lone outcome,33 and two34,35 reported data published in other studies that were included in the meta-analysis. The limited additional studies on dietary patterns and child asthma or wheeze precluded another meta-analysis. Among these six studies, five found either a beneficial effect of the Mediterranean diet or a detrimental effect of an unhealthy (eg, Western) pattern. Interestingly, one study reported that the Mediterranean diet was a risk factor for severe asthma in girls aged 6–7 years.35 The authors speculated this could be due to a reverse causal effect (families of children with severe asthma may improve their diet) and residual confounding.\nOverall, the meta-analysis and six additional studies suggest that the Mediterranean diet is potentially protective against child asthma.\n\nMaternal dietary patterns and child asthma\nTable 4 shows the main characteristics and results of the six studies reporting an association between maternal dietary patterns and asthma prevalence in children. All were cohort studies published between 2008 and 2013, with four conducted in Europe,38,45–47 one in the USA,48 and one in Asia.49 Sample sizes ranged from 460 to 14,062.\nAll studies used FFQs to measure dietary intakes, with number of food items or groups ranging from 42 to 166. Three studies38,45,46 calculated a Mediterranean diet score defined a priori. Two studies47,49 derived dietary patterns using PCA or factor analysis. One study48 used a combination of both a posteriori and a priori approaches to measure dietary patterns.\nThese studies examined the association between maternal dietary patterns and wheezing prevalence in children between the ages of one and 7 years. The heterogeneity of the study populations and outcomes precluded a meta-analysis. However, four of the six studies did not find any association between maternal dietary patterns and prevalence of wheezing. One study38 reported a protective effect of maternal Mediterranean diet pattern on persistent wheeze (OR 0.22, 95% CI 0.08–0.58) and atopic wheeze (OR 0.30, 95% CI 0.10–0.90) in offspring at age 6.5 years. Another study49 conducted in Japan found a beneficial effect of maternal “Western” dietary pattern on wheeze (OR 0.59, 95% CI 0.35–0.98; P=0.02) in toddlers aged 16–24 months. The authors noted that this “Western” dietary pattern in Japan might be comparatively healthier than the typical Western dietary pattern in the USA, because it was characterized by low intake of soft drinks, confectionery, and fruit, in addition to high intake of vegetable oil, salt-containing seasonings, beef, pork, processed meat, eggs, chicken, and white vegetables. Adherence to this “Western” dietary pattern was actually positively associated with a high intake of α-linolenic acid, vitamin E, and β-carotene, which were shown in some studies to have a beneficial effect on asthma and wheezing.7,9,50,51 The Japanese study suggests that dietary patterns are region-specific and population-specific, and that caution is necessary when interpreting the results of studies in diverse populations. Overall, these studies show weak evidence of any association between maternal dietary pattern and child wheezing."}