The incidence of childhood asthma and bronchial hyperreactivity (BHR) has been progressively increasing in developed countries, reaching nearly one-third of the population.1 This disease causes educational, economic and health problems for patients, their families, and society, which requires the adoption of preventive and therapeutic measures. The degree of asthma varies from patients who experience only a few episodes of wheezing, coughing with mild bronchospasm to patients with repeated episodes and persistent symptoms. The diagnosis of BHR in children is primarily clinical and can be complicated due to the subjective symptoms, which are sometimes difficult for their parents to assess.2 There is no universal test to determine the exact diagnosis. It has been demonstrated that the fundamental pathophysiological basis is bronchial inflammation, which triggers hypersensitivity to a variety of stimuli such as upper respiratory tract infections, air pollution, tobacco, smoke, exercise, central heating, seasonal pollination and obesity.

Current treatment is based on the use of bronchodilator β2-agonists (which reduce bronchial muscle spasticity), anti-inflammatory corticosteroids and anti-leukotrienes (which reduce BHR and clinical symptoms). All are effective upon acute application, but prolonged use is more questionable due to the limited evidence supporting efficacy and the emergence of adverse effects such as abnormal growth in children.3 It has been suggested that dietary interventions could be of potential use based on a number of publications that have linked the disease with the loss of traditional dietary patterns. Some of these results, however, are controversial.4–7 One of our recent studies on overweight and obese children supports this idea because patients with BHR improved considerably after controlling the quality of their diet.8 We therefore hypothesise that restoring traditional, healthy diets such as the Mediterranean diet could help decrease bronchial hyperreactivity significantly. In accordance with this idea, a meta-analysis has recently been published supporting the positive influence of Mediterranean diets on respiratory outcomes in children.9 The present study aims to extend knowledge in this direction by studying the effects of a traditional Mediterranean diet (TMD), on patients diagnosed with childhood asthma through a food re-education programme developed in the family sphere. The study diet was based on the Decalogue proposed by the Mediterranean Diet Foundation.10 The Mediterranean diet was proclaimed an Intangible Cultural Heritage of Humanity by UNESCO in 2010 and is characterised by a high intake of unrefined plant-based foods such as fruits and fresh vegetables, whole grains, legumes, olive oil and nuts; low to moderate consumption of foods of animal origin such as fermented milk, fish, eggs and lean meats; and by a low intake of sugar, refined flour and fast food.

According to the sample size calculation, the families of the 137 patients who met the inclusion criteria were invited to participate in the programme “Learning to Eat from the Mediterranean”, of whom 24 families refused to participate. Of the 113 patients initially enrolled in the programme, 6 gave up the project after the first session and 3 after the fifth session. Five of these patients left due to social or personal difficulties in implementing the diet and four due to disagreement with the limitation of certain foods. Therefore, a total of 104 patients completed the study, of whom 50 were girls and 54 were boys, with a mean age of 2.74 years (Table 1). The results were similar for both sexes and are therefore presented together. Fig. 1 shows the evolution of the indicators studied in symptomatic patients (number and severity of attacks, respiratory infections, emergency and hospital admissions). All of them showed a positive and statistically significant change in the BHR from the first weeks of the intervention onwards. This clinical improvement was maintained until the end of the treatment year. There were no episodes of BHR in 32.2% of children during the year of treatment, 35.3% had only one attack during the year and 24.9% had two episodes, compared to 4.73 episodes on average in the previous year. The intensity of the crisis was assessed by the paediatrician and decreased from mild-moderate to low-mild. This positive evolution of symptoms was accompanied by a significant decrease in the URTIs, bacterial complications, ER admissions and emergency assistance. Moreover, the need for anti-asthma therapy, antibiotic use and other symptomatic medications was also dramatically reduced (Table 2). In the year of study, none of the patients required treatment with anti-leukotrienes, or oral corticosteroids. The degree of family satisfaction was very high, as seen in the questionnaire regarding the improvement observed (Table 3). Air pollution and smoke snuff was not object of study, so we interpret that the results are mainly due to nutritional changes. The anthropometric variables measured before the intervention, at 4 months, and after 1 year revealed an adequate evolution of parameters related to growth and development, with a significant increase of lean mass (Table 4). The patient's dietary habits had also improved in the entire sample by the end of the programme (Table 5). As a result, there were an increased number of patients who had breakfast and consumed fruits, vegetables, fish, whole grains and fermented milk, while the proportion of those who ate sweets daily and bakery products for breakfast decreased. According to these data, the KIDMED index positively evolved from an upper-middle score at the beginning of the programme to a good-optimal value by the end.

Figure 1.

Evolution of parameters indicative of BHR symptomatology in the year before intervention (baseline) and during the year of intervention (1 year). Error bars.

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The overall results reflect a fast and positive evolution of patients enrolled in the programme, as shown by the various study variables. Thus, the number of attacks had notably decreased by the conclusion of the study, as had their intensity and associated complications. There was a very favourable start within weeks of dietary treatment, even in younger children and in those with more intense involvement response. Of particular importance is the marked decrease observed in the need for bronchodilators and inhaled corticosteroids, given that one of the main goals of the treatment according to the Global Initiative for Asthma is the prevention of exacerbations and the avoidance of adverse effects of antiasthma medication.15–17 There was a significant decrease in symptomatic drugs as a result of fewer illnesses and their lesser intensity degree. It is important to note that the use of anti-leukotrienes, and oral corticosteroids was not necessary during the study period. Significant improvements with respect to the previous year's situation were observed within the first 4 months of the intervention, which led to a high adherence to the programme and greatly enhanced monitoring. There was good tolerance of the proposed diet, with an easy adaptation and a lack of relevant culinary difficulties. The evolution of the height and weight of the patients was also satisfactory. There was a slight decrease in BMI and fat mass, and a significant increase in lean mass which happened in a similar way in our previous study of childhood obesity.8 The clinical outcome was assessed by a paediatrician when there was disease and by periodic inspections. The lack of a control group is the major weakness in our study and prevents us from categorically affirming that the dietary intervention was the direct cause of the observed improvements. However, the clinical history of the patients included in the study did not permit to expect a spontaneous favourable evolution of the symptoms18 and therefore the therapeutic efficacy of the diet seems obvious. Furthermore, our results agree with those of other studies that tend to show the beneficial effects of adherence to the TMD and the increased consumption of fruits, vegetables, legumes, oily fish and whole grains.4,6,19,20 In contrast, the consumption of fast food and sugar has been associated with childhood asthma and counteracts the protective effect of breastfeeding. Junk food21 and excessive salt consumption22 have also been linked with asthma. Despite these findings, a number of other studies do not support the idea that healthy food intake and/or avoidance of certain foods can play an important role in the control of childhood asthma.23 Thus, a recent review and meta-analysis concluded that nutritional factors responsible for the increase in these diseases are unclear.24,25 Of note is the cross-sectional study of 14,700 Spanish children that found no protective effect of the TMD on the prevalence and severity of asthma.5 Among the possible causes that could explain the lack of effectiveness of these studies related to the dietary intervention, we highlight the lack of an exhaustive follow up and inadequate diet control. We consider that the beneficial effect of our intervention is due to the implementation of a very specific and well-controlled TDM. The positive results of interventions such as ours on childhood asthma could depend on the effects of the TMD; we highlight the implementation of a very specific and well-controlled TMD on inflammation and the immune processes underlying the disease. This was previously proposed after a prospective study of children with asthma in Mexico City in which the strict adherence to the TMD appeared to have a beneficial effect on airway inflammation and lung function.7 In a recent clinical trial in adults with asthma, consistent improvements were seen in the clinic and spirometry in those who adhered to the TMD.26 In patients with mild asthma, it has been shown that inflammation remains asymptomatic for long periods from the start of the disease.27 The bronchial mucus inflammation will condition bronchial hyperresponse to stimuli, causing a major airflow limitation due to smooth muscle contraction, mucous oedema and mucus hypersecretion. The putative anti-inflammatory properties of the TMD could be partially attributed to a proper intake of carbohydrates. By limiting the intake of white bread, refined flour, sugar and bakery products, our dietary intervention decreases the amount of carbohydrates with a high glycaemic index and glycaemic load, which could prevent an excess of circulating insulin from interfering with the proper formation of eicosanoids. It is known that hyperinsulinaemia stimulates the delta-6-desaturase enzyme, which favours the formation of arachidonic acid and type II eicosanoids, which are strongly proinflammatory.28

In addition to limiting simple carbohydrates, reducing the intake of unhealthy fats appears to be a highly beneficial aspect of the TMD and could significantly contribute to limiting inflammation. Before participating in the study, most of our patients ate little in the way of vegetable fats and consumed significant amounts of animal or industrial fats. By limiting the intake of meat and animal derivatives, fatty and unfermented dairy, butter, margarine, bakery products and precooked meals, patients can reduce their intake of saturated fats, trans-fats and arachidonic acid, as well as reduce certain sensitising proteins29 that could interfere with the proper function of cell membranes that are more hyperreactive to triggers. The TMD also provides a correct balance of omega-3 and omega-6 fatty acids, which can decrease the levels of inflammatory cytokines (already high in patients with asthma)30 and affect the differentiation of T helper cells.4

Fresh and seasonal foods contain numerous vitamins, minerals, enzymes, yeasts and antioxidants, and many of these are important co-factors in the formation of the eicosanoids involved in bronchial inflammation.31 We have encouraged the use of raw and steamed foods and foods heated at low temperatures so as to avoid modification by excess heat. By promoting fresh, seasonal foods, many of the additives in processed foods have been eliminated, additives that might be involved in the enzymatic block of bronchial inflammatory mediators. The TMD is rich in antioxidants that can counteract the reduced intake of these substances, which have been associated with increased asthma prevalence.31,32 Additionally, plant foods, acting as symbiotic, favour intestinal flora fermentation, which has co-evolved with the human race since ancient times.33 Intestinal microbiota depends on the food we eat and have a critical role in modulating the immune system. The immune response also depends on the consumption of significant amounts of fibre,34 which is present in the TMD. The significant decrease of URTIs and their complications are remarkable in our study, which suggests a protective immune effect. Intestinal microbiota is essential for initiating individual IgA production in children younger than 1 year and influence the development of the entire IgA system.35 IgA is abundant in most mucous membranes, where harmless antigens are neutralised and prevented from entering the epithelium.36 Therefore, low levels of IgA in infancy might be the cause of the lack of a significant response to innocuous antigens, namely the induction of oral tolerance. This could be important in the prevention of allergic diseases. Microbiota have a symbiotic relationship with the host, and if this relationship is not adequate, it can cause immune dysregulation and pathological response to environmental antigens (atopy).37

It is noteworthy that the patients in our study showed a high score in the KIDMED test at the beginning of the intervention, before they had started undergoing dietary changes. In our opinion, the KIDMED test, which measures the quality of the Mediterranean diet, is not sensitive to all the nutritional factors proposed in the Decalogue of the Mediterranean diet.38,39 Thus, it does not take into account whether the cereals are whole grain or refined grain, the percentage of fresh or processed food, the kind of dairy products or the proportion of meat and sausages. A reformulation of this tool could be useful. In addition to the lack of a randomised control group, our study has a number of other limitations that should be mentioned. In the course of the study, it was impossible to make a clinical classification as proposed by the GEMA40 because the intervention began from the third crisis of BHR, and therefore the possible evolution of the patients could not be known. No additional systematic tests were conducted, which is why we have no asthma predictive index values. We did not use the Asthma Control Questionnaire (ACQ) in children proposed by the Spanish consensus because it does not include nutritional questions.

In conclusion, our study supports the hypothesis that a diet of high nutritional quality such as the Traditional Mediterranean Diet can contribute to a better control of asthma in children, possibly impacting the underlying immune and inflammatory pathophysiology. The study protocol presented here is easily reproducible in any primary care clinic and can significantly contribute to a better understanding of the therapeutic effects of diet on respiratory diseases, being thus complementary to the use of modern technologies in cross-sectional studies and clinical trials that also address childhood asthma but require much larger budgets.

The authors have no conflict of interest to declare.