According to the Th1/Th2 paradigm, there would be an inverse relationship between the prevalence of T1DM and asthma [11]. This would occur since Th1 and Th2 cells can inhibit each other by secreting cytokines, so that asthma and allergic diseases are supposed to appear less frequently in patients with Th1 mediated autoimmune diseases [10]. Among the articles that were included in this review, seven conformed with this paradigm, five showed that the prevalence of asthma is similar between patients with and without T1DM, and two found a direct relationship between asthma and T1DM, suggesting that the interaction between Th1 and patterns Th2 is more complex than initially proposed.

Of the articles that showed an inverse relationship between the two conditions, A1 showed that the existence of T1DM was inversely associated with asthma and hypersensitivity to allergens, compared to individuals in the control group. A2 observed a lower prevalence of asthma and no association with the presence of atopic diseases in patients with T1DM compared to controls. A3 found that the relationship between the two diseases depends on their order of appearance, with a previous diagnosis of asthma increasing the risk of T1DM by 41%, while a prior diagnosis of T1DM decreased the risk of asthma by 18%. In the same way, A8 pointed out that children with asthma had an increased risk of T1DM later on, however, the subsequent risk of asthma did not differ substantially between children with T1DM and controls. A4 pointed that the prevalence of asthma in patients with T1DM is approximately half that found in the general population (5.7% vs 12.3%). Some studies show that the prevalence of asthma and rhinitis is lower in individuals with T1DM than in control groups (A10, A13 and A14). These examples of inverse relationship indicate the protective role of Th1 cells for allergic diseases [10] and suggest that, overall, when the dominant immunologic response is enhanced by cytokines of Th1 cells, this diminishes the effect of cytokines from Th2 cells in the same host, so that the end result is that in patients with Th1 driven diseases, Th2 driven diseases are usually not found [9].

However, some studies have shown that the Th1/Th2 paradigm does not seem to actually occur, so that there may be coexistence of cytokines from both patterns in the development of both diseases, with complex interactions that have not yet been fully elucidated. Thus, some analyzed articles showed similar frequencies of asthma in individuals with and without T1DM (A5, A6, A9, A16). Other studies, despite not specifically establishing the relationship between T1DM and asthma, point out that the prevalence of atopy is similar between individuals with and without T1DM (A7, A15). These examples of studies are in contrast with the “traditional” concept of an inverse association between atopy and autoimmunity, and some evidences have shown that autoimmune Th1 diseases such as T1DM, thyroiditis and psoriasis in both adults and children could coexist with Th2 mediated diseases, suggesting that the Th1/Th2 paradigm is oversimplified [3].

There was also an article (A11) that found the presence of a direct relationship between T1DM and asthma, since it was found that children with T1DM are more likely to have asthma, however, A17 reported a similar frequency of sensitization to allergens in children with and without T1DM.

With regard to body mass index (BMI), it was found that patients with T1DM and concomitant asthma had a higher BMI than those who had asthma alone (A9, A12). This association can be explained by the existing inflammatory process in obesity that would precipitate the onset of asthma in individuals with T1DM (A12).

Regarding sex, males were the most affected among patients who had both conditions concomitantly or T1DM alone (A3, A4, A8, A9). However, in A17, gender did not influence the development of allergic symptoms or the prevalence of allergic diseases in those individuals with T1DM.

A complex familial relationship was found between asthma and T1DM. A1 showed that the frequency of T1DM in relatives was inversely associated with that of asthma. A8 showed that relatives of individuals with asthma or T1DM have an increased risk of developing both diseases, with this risk being greater among siblings of the same father and mother and more attenuated among cousins and half-siblings. This suggests that there are shared genetic and/or environmental factors that contribute to the development of both diseases.

Articles A4 and A12 showed that individuals with asthma and T1DM had worse glycemic control when compared to patients with only T1DM. The A4 study that was carried out with Iranian children with T1DM classified glycemic control, through the measurement of glycated hemoglobin, as good in 33.9%, moderate in 53.5% and poor in 12.6%. Those individuals who had asthma and T1DM, glycemic control was worse, with the prevalence of good control in 25%, moderate in 50% and poor in 25%, respectively. The same was found in study A12, where among young people with T1DM, asthma was associated with poor glycemic control, especially if glycemic control was inadequate (approximately 31%). However, there does not seem to be unanimity regarding this proposition. Study A9, carried out with children and adolescents from Germany and Austria, did not find differences in glycemic control between groups with T1DM and asthma and only with T1DM. However, the insulin doses used by individuals with concomitant asthma and T1DM were higher.

Study A9 identified that patients with T1DM and asthma had higher occurrence of diabetic ketoacidosis among those who used inhaled sympathomimetics compared to those who used inhaled corticosteroids. In this way, asthma management seems to have an influence on the outcomes of T1DM. Furthermore, it is worth noting that, in this study, no difference was found comparing diabetes-related complications in individuals using all other asthma medications.

Articles A4 and A12 showed a significant correlation between parental education and diabetes control. Studies A4, A9 and A12 verified that the development or not of asthma is not influenced by the educational level of the parents.

Although following an established methodology, both for the search and for the identification of published literature, this scoping review has some limitations that should be mentioned, since some information may have been omitted, as articles that were not written in English, Spanish or Portuguese were not included. Gray literature was also not accessed, and PubMed, Scopus, Embase, Web of Science (WoS) and LILACS were the only databases consulted. There was also a great difference between the studied populations, the number of participants, the age groups and the methodology of the analyzed articles.