Is affluence a risk factor for bronchial asthma and type 1 diabetes?

In the last decades, an increase in bronchial asthma and type 1 diabetes occurrence has been observed in affluent countries, and a positive association between the two disorders has been demonstrated at the population level. This association could be explained by common risk factors predisposing to both disorders. Altered environmental and lifestyle conditions, possibly related to socio-economic status, might account for the rising trend of the two disorders. To test this hypothesis, we calculated the correlation between the occurrence of type 1 diabetes and asthma, the gross national product (GNP) and the infant mortality rate, in several European and extra-European countries. GNP was positively correlated with the incidence of type 1 diabetes and with symptoms of asthma in European (r(sp): 0.53 and 0.69; p = 0.001 and p < 0.0001, respectively) and extra-European countries (r(sp): 0.44 and 0.46; p = 0.04 for both diseases). Infant mortality rate was inversely correlated with GNP and with the occurrences of the two diseases in Europe (r(sp): -0.66, p < 0.0001 for type 1 diabetes; r(sp):- 0.51, p = 0.01 for asthma). In extra-European countries, a significant relationship was found between infant mortality and asthma (r(sp): -0.46; p = 0.03); a trend towards a negative correlation between infant mortality and type 1 diabetes was also found, although no statistical significance was reached (r(sp): -0.21; p = 0.31). This analysis indicates that type 1 diabetes and asthma are positively associated with the GNP at the population level. Similarly, countries with low infant mortality rates tend to have a higher incidence of these immune-mediated diseases. Although GNP reflects many societal and lifestyle differences, it is notable that a high socio-economic status implies a reduced or delayed exposure to infectious agents. The reduced pressure of infectious agents on the immune system throughout life might contribute to increase the susceptibility to bronchial asthma and type 1 diabetes.