PMC:4137988 / 19032-27670 JSONTXT

{"project":"2_test","denotations":[{"id":"25143747-11790957-55576550","span":{"begin":1776,"end":1778},"obj":"11790957"},{"id":"25143747-9663398-55576551","span":{"begin":2168,"end":2170},"obj":"9663398"},{"id":"25143747-7560680-55576552","span":{"begin":2892,"end":2894},"obj":"7560680"},{"id":"25143747-11790957-55576553","span":{"begin":3377,"end":3379},"obj":"11790957"},{"id":"25143747-23865797-55576554","span":{"begin":3604,"end":3606},"obj":"23865797"},{"id":"25143747-24496398-55576555","span":{"begin":3607,"end":3609},"obj":"24496398"},{"id":"25143747-20383235-55576556","span":{"begin":4720,"end":4722},"obj":"20383235"},{"id":"25143747-22712878-55576557","span":{"begin":5086,"end":5088},"obj":"22712878"},{"id":"25143747-17947786-55576558","span":{"begin":6097,"end":6099},"obj":"17947786"},{"id":"25143747-23157561-55576559","span":{"begin":7122,"end":7124},"obj":"23157561"},{"id":"25143747-23648395-55576560","span":{"begin":7245,"end":7247},"obj":"23648395"},{"id":"25143747-16522216-55576561","span":{"begin":7770,"end":7772},"obj":"16522216"}],"text":"Discussion and conclusion\nTaken together, studies in adults and pregnant woman–child pairs failed to show that dietary patterns were associated with asthma outcomes. Only studies in children suggested a protective effect of the Mediterranean diet on current wheeze and ever asthma.\nCompared with studying individual foods or nutrients, an evaluation of dietary patterns may shed light on the combinatorial effects of foods and/or nutrients on the health outcome of interest. Studying the overall effect of dietary patterns on asthma is an emerging literature; however, the findings so far have been inconsistent. We offer several possible explanations for the inconsistency and suggestions for future research.\nFirst, the heterogeneous results may be partially explained by the notable variation in FFQs used for measuring dietary intakes and statistical approaches for deriving dietary patterns. For example, in adult studies, the FFQs included between 12 and over 200 food items or groups, which could influence the selection of foods loaded on the dietary patterns. Although most of the FFQs were validated, the dietary patterns derived from them explained only a small to medium percentage of total variance, ranging from 11% to 58% in the five adult studies that reported this information.\nTwo approaches, a priori and a posteriori, have been used to generate dietary patterns. Both approaches have strengths and weaknesses. The a priori approach focuses on a predefined dietary pattern based on prior knowledge of a specific diet (eg, Mediterranean) and its relationship to disease. Therefore, this approach is limited by current knowledge and could involve uncertainties in selecting individual components of the diet index and subjective decisions of defining cutoffs.52 In contrast, the a posteriori approach provides opportunities to open up new areas of diet-disease research and detect dietary patterns specific to the region and/or population of interest. However, it involves important but arbitrary decisions, including the number of components to extract, the method of rotation, consolidation of food items into groups, and labeling of the components.53 Researchers should choose the appropriate method according to the study objectives and dietary characteristics of the study population.\nMost studies in children (eleven of 14) and pregnant woman–child pairs (four of six) used an a priori approach and defined the dietary pattern using a Mediterranean diet index, whereas ten of 12 studies in adults utilized an a posteriori approach to derive dietary patterns statistically (eg, with PCA). In addition to the Mediterranean diet, future studies may also examine the association between healthy dietary patterns in other regions (eg, the widely promoted Dietary Approaches to Stop Hypertension in the USA54) or alternative diet quality indices (eg, the Healthy Eating Index55) and asthma outcomes. If using the a posteriori approach, the reliability and validity of the dietary patterns generated from FFQs can be examined using a different source of dietary data (eg, dietary records). To examine the reproducibility of the dietary patterns, sensitivity analyses can be performed to test whether the arbitrary choices made during PCA or factor analysis influence the results and whether similar dietary patterns can be obtained using randomly split samples.52\nStatistical approaches used to derive dietary patterns a posteriori have included PCA, factor analysis, cluster analysis, and to a lesser extent, reduced rank regression. Some review papers detailed each of these approaches.56,57 Different from the exploratory approaches (eg, PCA, factory analysis, and cluster analysis), reduced rank regression defines linear combinations of food intakes that maximally explain the outcome variable (eg, an asthma outcome). In other words, in contrast with PCA and factor analysis, which derive dietary patterns to maximally explain the variance in food intake among participants, reduced rank regression identifies dietary patterns to maximally explain the outcome variable. Among the studies reviewed in this paper, most used PCA, two used factor analysis, and only one in children used reduced rank regression. This may be another reason why many studies of dietary patterns using PCA or factor analysis found no associations with asthma outcomes. The appropriate statistical method should be chosen based on study objectives. In doing so, one must keep in mind that PCA, factor analysis, and cluster analysis identify existing dietary patterns while reduced rank regression is likely to yield useful information for hypothesis generation but may not describe actual intake patterns in the population.58\nSecond, the heterogeneity of reported results is also possibly attributable to the varied number of confounders controlled for in the studies. For example, the number of confounders controlled for ranged from five to 19 in the observational studies among adults. Confounding could pose challenges for interpretation of the diet–asthma relationship. Nurmatov et al59 have proposed a comprehensive list of primary and secondary confounders that should be considered in future epidemiologic studies examining the early-life diet and asthma relationship in children. The authors suggested that the primary confounders should account for maternal and child characteristics, socioeconomic status, environmental exposures, and dietary factors, while the secondary confounders could be confirmed using appropriate statistical tests. Confounders in the diet–asthma relationship are different between children and adults; therefore, further research is needed to investigate confounders in the adult population. Future observational studies should select confounders based on existing knowledge of the causal mechanism in the diet– asthma relationship and suggestive evidence from statistical analysis. The criteria for selection of confounders should also be reported in observational studies, so readers can be well informed to reach a valid and reliable interpretation of findings.60 In addition, very few population-based studies have been conducted to investigate the association between dietary patterns and asthma outcomes. National survey data (eg, the National Health and Nutrition Examination Survey) may be leveraged to examine the diet–asthma association and identify potential covariates.\nLastly, the inconsistent findings underline the importance of prospective studies and RCTs in helping to better understand the role of dietary patterns in the etiology and disease course of asthma. People’s dietary patterns often change over time through the lifespan and because of changes in socioeconomic and/or health status. Most of the studies reviewed were cross-sectional, precluding investigation of a temporal or causal relationship between dietary patterns and asthma. The cumulative effects of diet on asthma warrant prospective studies. Also, to date, only two RCTs have been designed to evaluate the impact of a healthy dietary pattern on asthma. One is a Mediterranean diet intervention study21 recently completed in New Zealand and the other is an ongoing Dietary Approaches to Stop Hypertension intervention study61 in the USA. More experimental studies like these are needed to elucidate the causal relationship.\nThis systematic review and meta-analysis has a number of strengths and limitations. This is a comprehensive review of the literature on dietary patterns and asthma from 1950 to 2014; however, it was limited to studies published in English. Because of the institutional subscription limitation, we did not include Embase as one of the databases searched for this review. Although Scopus overlaps substantially with Embase,62,63 any studies only indexed in Embase would have been missed. The funnel plots suggested no evidence of publication bias. Nonetheless, the meta-analysis was limited by the abovementioned inherent limitations of individual studies, including a low percentage of total variance explained by the dietary patterns, and inconsistent and possibly incomplete adjustment for potential confounders. Regardless of the limitations, this paper shows that the results of existing studies do not reveal a clear and consistent relationship between dietary patterns and asthma outcomes. Although higher adherence to the Mediterranean diet may be associated with reduced asthma risk in children, more well designed and controlled studies are needed to provide solid evidence and explore whether other healthy dietary patterns are associated with asthma outcomes in children and adults."}