Hence, the aim of this study was to determine whether the consumption of fast foods is related to asthma and other allergic diseases. We further aimed to explore the dose–response of the consumption of fast foods to asthma/wheeze, the differential effects of specific types of fast foods consumption, such as hamburgers, carbonated soft drink and takeaway foods on asthma/wheeze and the differential effects of fast food consumption on asthma and wheeze in countries according to income level. 24

With adoption of westernized lifestyles, consumption of ‘fast foods’ has increased. 3 Fast foods, first popularized in the 1950s in America, are typically foods sold in restaurants or stores with preheated or precooked ingredients and served to customers in a packaged form for takeaway. 4 Fast foods are typically calorically dense, high in refined carbohydrates, sodium, sugar, cholesterol, additives such as preservatives and colourants, with high concentrations of saturated fat. 5 Hence, it has been hypothesized that the consumption of fast foods may exacerbate the development and progression of asthma and allergic diseases. 6 However, the data available to date are heterogeneous, with some, but not all studies, reporting that asthma and allergic diseases are associated with the consumption of fast foods. 7 - 23

Subgroup analyses were undertaken to assess the dose‐dependent response of the consumption of fast foods to asthma/wheeze and other allergic diseases, with consumption frequency of fast food described as ‘<1 time per week, occasionally and never’, ‘1–2 times per week’ or ‘≥3 times per week’. In addition, differential effects of consumption of specific types of fast foods such as hamburgers, carbonated soft drink and takeaways on asthma/wheeze were examined. Furthermore, effects of the consumption of fast foods on asthma/wheeze in countries of different income level were determined, with countries classified according to World Bank criteria. 24 Statistical analysis was conducted with Stata 11.0 (Stata Corp. LP, College Station, TX, USA) and a two‐sided P < 0.05 was considered as significant.

If adjusted odds ratios (aOR) were indicated in the original studies, adjusted estimates were pooled using the generic inverse variance method. OR in each study were converted to natural logarithms. Standard errors (SE) from these logarithmic numbers and their corresponding 95% CI were calculated. We conducted the metan command in Stata to pool the ln OR across studies, used random‐effects model as described by DerSimonian and Laird 34 with the estimate of heterogeneity being taken from the Mantel–Haenszel model, considering variation in effects due to differences in study populations and methods, and calculated the summary OR estimates with 95% CI.

Primary and secondary outcomes were treated as dichotomous variables and presented as OR with 95% CI. All OR with 95% CI were pooled based on study design and outcomes. In order to avoid counting participants more than one time, the method described by Greenland and Longnecker 32 was used if the raw data were available. Otherwise, the pooled effects of the consumption of fast foods on asthma and allergic diseases were estimated in accordance with the method described by Dong et al . 33

Fast foods are mass‐produced foods prepared and served very quickly, with poor nutritional quality. In general, any foods with less preparation time can be regarded as fast foods, especially foods sold in a restaurant or store with preheated ingredients, and served to the customer in a packaged form for takeout. Furthermore, fast foods typically fall into the category of foods high in calories, total fat, saturated and trans fat, sugar, simple carbohydrates and sodium (salt). In this study, we did not set a pre‐specified definition for fast foods.

We extracted the details, where available, from included studies, including author, publication year, study design, geographical location of the study, gender, age, sample size, frequency and the consumption of different kinds of fast foods, outcomes and OR related to these outcomes, and adjusted confounders for these OR. aOR of outcomes with 95% confidence interval (CI) were also extracted and the adjusted confounding factors were indicated. Otherwise, OR was calculated with the number of the exposed to the non‐exposed ratio in the case group divided by the same ratio in the control group if absent in original studies.

This study was carried out in OVID by searching databases, including MEDLINE (1946 to February week 2, 2018), EMBASE (1974 to 16 February 2018) and Cochrane Central Register of Controlled Trials (CENTRAL) (January 2018). Medical Subject Headings (MeSH) was used to construct the search terms, together with keywords from related literature. 26 - 29 The databases were conducted by search strategy related to fast foods, asthma/wheeze and allergic diseases and the search criteria can be found in Appendix S1 in Supplementary Information. Unpublished studies were also investigated through searching abstracts in the databases mentioned above. Reference lists of review articles were searched to identify other potentially eligible studies. Only English articles were included and there was no limitation on year of publication or publication status. Studies were included if they were cohort, cross‐sectional or case–control studies exploring whether fast food consumption was related to asthma or other allergic diseases with available data presented as odds ratios (OR). After exclusion of duplicates, two researchers (C.S.W. and J.W.) reviewed the full text of all citations with titles and abstracts that seemed to fit the criteria for inclusion. Disagreements were solved by a third reviewer (G.W.). The number of studies rejected and the reasons for rejection were tracked. Funnel plots were used to appraise risk of bias across studies for some of the primary outcomes. The symmetry in the funnel plots was assessed visually.

Analysis of the relationship between different types of fast food consumption and asthma/wheeze (Table S4 in Supplementary Information) revealed that the consumption of hamburgers was associated with current asthma 15 (aOR: 1.59; 95% CI: 1.13–2.25), severe asthma 22 (aOR: 1.34; 95% CI: 1.23–1.46), asthma ever 19 , 35 (aOR: 1.47; 95% CI: 1.13–1.92) and current wheeze 7 , 11 , 22 , 35 (aOR: 1.22; 95% CI: 1.16–1.28). The consumption of hamburgers ≥3 times/week was associated with an increased risk of severe asthma (aOR: 1.34; 95% CI: 1.23–1.46 vs aOR: 1.09; 95% CI: 1.04–1.13; P < 0.001) and current wheeze (aOR: 1.22; 95% CI: 1.16–1.28 vs aOR: 1.07; 95% CI: 1.03–1.11; P < 0.001) in comparison to consumption of 1–2 times/week. In terms of high or middle‐income countries, the consumption of fast foods was associated with current asthma in case–control studies 13 , 15 , 20 and cross‐sectional studies 7 , 12 , 17 , 18 (aOR: 1.58; 95% CI: 1.17–2.13 and aOR: 1.44; 95% CI: 1.00–2.11, respectively), severe asthma 12 (aOR: 1.09; 95% CI: 1.04–1.13) and wheeze ever 14 (aOR: 1.65; 95% CI: 1.07–2.52) in high‐income populations (Table S5 in Supplementary Information). In middle‐income countries, the consumption of fast foods was associated with current asthma 21 (aOR: 2.89; 95% CI: 1.40–5.96) and current wheeze 11 (aOR: 1.74; 95% CI: 1.30–2.34).

The effect of different levels of fast food consumption on asthma/wheeze is shown in Table 4 . The consumption of fast foods 1–2 times/week was associated with severe asthma 22 (aOR: 1.09; 95% CI: 1.04–1.13) and current wheeze 11 , 22 , 35 (aOR: 1.07; 95% CI: 1.03–1.11). The consumption of fast foods ≥3 times/week was associated with asthma ever 35 (aOR: 1.42; 95% CI: 1.08–1.87), severe asthma 22 (aOR: 1.34; 95% CI: 1.23–1.46) and current wheeze 11 , 22 , 35 (aOR: 1.22; 95% CI: 1.16–1.28). The consumption of fast foods ≥3 times/week was associated with an increased risk of severe asthma (aOR: 1.34; 95% CI: 1.23–1.46 vs aOR: 1.09; 95% CI: 1.04–1.13; P < 0.001) and current wheeze (aOR: 1.22; 95% CI: 1.16–1.28 vs aOR: 1.07; 95% CI: 1.03–1.11; P < 0.001) in comparison to consumption 1–2 times/week (Fig. 2 ).

Primary and secondary outcomes were pooled using the highest category in fast food consumption described as following. Associations between the consumption of fast foods and asthma/wheeze are shown in Figure 1 . The pooled aOR from three case–control 13 , 15 , 20 and six cross‐sectional studies 7 , 12 , 17 - 19 , 21 indicated the consumption of fast foods was significantly related to current asthma (aOR: 1.58; 95% CI: 1.17–2.13 and aOR: 1.58; 95% CI: 1.10–2.26, respectively). Furthermore, severe asthma was associated with the consumption of fast foods 22 (aOR: 1.34; 95% CI: 1.23–1.46). The consumption of fast foods was associated with asthma ever (aOR: 1.36; 95% CI: 1.06–1.75) in three cross‐sectional studies. 14 , 19 , 35 The consumption of fast foods increased the risk of current wheeze in five cross‐sectional studies 7 , 9 , 14 , 22 , 35 (aOR: 1.21; 95% CI: 1.16–1.27) and wheeze ever in one cross‐sectional study 14 (aOR: 1.65; 95% CI: 1.07–2.52).

Figure S1 in Supplementary Information shows the flowchart for screening studies. The primary search strategy initially yielded 4138 citations from the OVID system. Of these, we identified 16 studies included in this systematic review and meta‐analysis. Table 2 indicates that definitions of fast foods varied across all included studies. Table 3 shows the key characteristics of the 16 studies included in this analysis, published from 2001 to 2015, including 13 cross‐sectional studies 7 - 9 , 11 , 12 , 14 , 16 - 19 , 21 , 22 , 35 and 3 case–control studies. 13 , 15 , 20 Sample size ranged from 144 to 500 827. One study included females only, while the remaining included both sexes. Two studies were undertaken in multi‐centres and others were conducted in Colombia, Canada, Japan, China (Mainland and Taiwan), New Zealand, India, Spain, Saudi Arabia, Sweden, Turkey and Australia. All studies had primary or secondary outcomes relevant to this systematic review. Three case–control studies, 13 , 15 , 20 with a total sample size of 1326, investigated the association between the consumption of fast foods and asthma/wheeze. Four cross‐sectional studies 9 , 17 , 21 , 35 with a total of 54 314 subjects researched the association between the consumption of fast foods and asthma/wheeze. Two cross‐sectional studies 8 , 16 (total of 23 028 participants) only studied the consumption of fast foods and atopy and the remaining seven cross‐sectional studies 7 , 11 , 12 , 14 , 18 , 19 , 22 (total of 530 678 participants) analysed the association between both asthma/wheeze and allergy and the consumption of fast foods. We intended to estimate heterogeneity and find publication bias with the use of funnel plots. However, considering the insufficient numbers of studies, this was not performed. The quality scores were 5.33 ± 1.16 in the case–control studies 13 , 15 , 20 and 5.69 ± 1.55 in the cross‐sectional studies 7 - 9 , 11 , 12 , 14 , 16 - 19 , 21 , 22 , 35 (Tables S1 and S2 in Supplementary Information).

DISCUSSION

To our knowledge, this is the first systematic review and meta‐analysis to investigate the relationship between the consumption of fast foods and asthma/wheeze and other allergic diseases. Our study indicated that the consumption of fast foods significantly correlates with current/severe/ever asthma, current/ever wheeze, physician‐diagnosed allergic rhinitis (pollen fever), (severe) rhino‐conjunctivitis and severe eczema. In terms of different types of fast food consumption, hamburger, but not takeaway or carbonated soft drink intake, was associated with severe/ever asthma, current wheeze, (severe) rhino‐conjunctivitis and severe eczema. Furthermore, we determined that the consumption of fast food, especially hamburgers, correlates to severe/current asthma in a dose–response pattern. In addition, these relationships exist in both high‐ or middle‐income populations.

In recent decades, fast foods have become an important component of the diet, especially in westernized, high‐income countries. Fast foods consumption is associated with poor diet quality, high caloric intake, overweight and obesity in children and adolescents.29, 36-40 Overweight–obesity is an independent risk for asthma and allergic sensitization. After performing the sensitivity analyses for included studies with adjustment for BMI, we found that the positive association between the consumption of fast foods and current asthma (aOR: 0.85; 95% CI: 0.48–1.52), ever asthma (aOR: 1.09; 95% CI: 0.7–1.69) and current wheeze (aOR: 1.08; 95% CI: 0.63–1.83) disappeared, while the association between the consumption of fast foods and current asthma (case–control design) (aOR: 1.58; 95% CI: 1.17–2.13) and ever wheeze (aOR: 1.65; 95% CI: 1.07–2.52) remained, which would be explained by BMI as a confounder or a reduced small sample size. It indicated that as one of the indispensable confounders, BMI might mediate the association between the consumption of fast foods and asthma.41 Furthermore, the poor quality diet that develops when fast foods are regularly included in the diet introduces nutrient deficits that are likely to independently contribute to asthma development and progression.42

Poor quality diet is likely to contribute to the development and progression of asthma/wheeze via multiple mechanisms. For example, saturated fatty acids can activate toll‐like receptors (TLR), leading to release of pro‐inflammatory cytokines (TNF‐α and IL‐6) and Nuclear factor (NF)‐κB‐mediated innate immune responses which can contribute to chronic inflammatory diseases of the airways.13, 43-45 Indeed, our previous study43 provided evidence that a high‐fat meal exacerbates airway inflammation in asthma, with increased airway neutrophilia and TLR4 expression in induced sputum. In addition, the consumption of fast foods reduces the consumption of foods that are rich in protective nutrients, such as fruits and vegetables. Fruit and vegetables contain many phytochemicals that have anti‐oxidative and anti‐inflammatory properties. Hence a reduction in fruit and vegetable intake is likely to have an unfavourable impact on asthma prevalence/management.46 Indeed, we have previously shown that consumption of a low fruit and vegetable diet leads to a reduced risk of exacerbation in adults with asthma.47 Hence, the associations that we have seen between the consumption of fast foods and asthma/wheeze are biologically plausible.

Our analysis has considered the effects of the quality and heterogeneity of included studies. Given the relatively high heterogeneity and bias, evidence from cross‐sectional or case–control studies was graded as low quality. Study design of the cross‐sectional and case–control studies included in this meta‐analysis, based on questionnaire surveys and self‐report, can lead to recall bias and information bias giving rise to misclassification. This may weaken the quality of evidence and, thus, causal association between the consumption of fast foods and asthma/wheeze and other allergic diseases cannot be confirmed. In terms of the quality of the included studies, intriguingly, our subgroup analyses from relative high‐quality studies confirmed these associations except for current asthma. Furthermore, the heterogeneity in the included studies is due to potential confounding factors such as age, gender, exercise, television viewing, maternal education, current smoking, environmental tobacco smoke in the dwelling, gross national income, current urban residency, indoor painting, new floor materials, region, language, parental atopy, exercise, parental asthma/allergy rural childhood and number of siblings (Table 3). When these confounding factors were adjusted, the heterogeneity was significantly reduced.7, 9, 12, 14, 16-19, 21, 22, 35 Our sensitivity analyses with regards to studies adjusting for less than two common confounders including BMI, the state of social economy (SES), urbanization (URB), smoking and parental allergy, indicated that these confounders might modify the associations of consumption of fast foods with current asthma (aOR: 1.19; 95% CI: 0.72–1.98) and current wheeze (aOR: 0.77; 95% CI: 0.18–3.39). However, the statistical approaches used were not consistent enough to make definitive conclusions about the effect of each of these variables. In addition, eliminating the study by Ellwood et al.,22 which has a large sample size, did not change the associations observed, except for current wheeze.

This analysis is limited by the lack of standard definition for fast foods in the included studies,48 although a formal survey questionnaire was designed in most of included studies and investigators asked subjects to answer the detailed questions on fast food intake. Variation in the definition of fast foods would contribute to the heterogeneity across included studies. Therefore, we only explored the initial relationship between the consumption of fast foods and asthma. Another limitation is the deficiency of temporal data, a characteristic of cross‐sectional studies, which makes it impossible to demonstrate a causal effect of the consumption of fast foods on study outcomes. Furthermore, although the difference in dietary habits of adults versus children may exist, the association between the consumption of fast foods and diseases was not analysed by age group. In addition, the definitions of asthma/wheeze and other allergic diseases, as primary and secondary outcomes in included studies, were somewhat different, and in some cases, lacking objective measures. Finally, confounders such as BMI, SES, URB, smoking and parental allergy might modify the associations between the consumption of fast foods and asthma and allergic diseases, thus influencing our results.

While the current study has some limitations as described above, bias has been reduced by our analysis strategy. First, all OR with 95% CI in primary and secondary outcomes were separately pooled based on study design. Second, aOR were used to reduce the effects of confounding factors on outcomes. Third, we have strengthened our observations by demonstrating a dose‐dependent response of the consumption of fast foods on asthma/wheeze. Fourth, subgroup analysis was undertaken to consider the effects of different types of fast foods consumption, including hamburgers, carbonated soft drinks, takeaways and deep‐frying. Fifth, although the definitions of asthma and other allergic diseases are subjective, the repeatability and predictive validity have previously been demonstrated. Parameter estimates (95% CI) of specificity and sensitivity between response to questionnaires and physician diagnosis of current asthma were 0.97 (0.90, 0.99) and 0.80 (0.58, 0.93) for adult, 0.81(0.76, 0.86) and 0.85(0.73, 0.93) for children.49, 50 Lastly, results were reported based on the highest fast food consumption frequency, which gave rise to possibly overstating the association between the consumption of fast foods and asthma/allergic diseases. Nevertheless, it can be explained by the extent of exposure in terms of a dose–response effect.

In conclusion, our study, for the first time, has systematically assessed the relationship between the consumption of fast foods and asthma/wheeze and other allergic diseases. It has demonstrated that the consumption of fast foods, in particular hamburgers, correlates to asthma in a dose–response manner. Relationships between the consumption of fast foods and asthma/wheeze exist in high‐ or middle‐income populations. Given the quality of studies included, further longitudinal cohort and intervention studies are needed to confirm these relationships and identify causal associations between the consumption of fast foods and asthma/wheeze and other allergic diseases, which could in some degree explain the increasing prevalence of these diseases and offer a potential intervention strategy.