Factors related to changes in the asthma male: female ratio by age in 403 Mexican counties.

Background: Asthma is more frequent in males during childhood and in females after adolescence, which has been attributed to changes in sexual hormones levels.

Objective: We explored changes of the asthma male: female ratio (AMFR) by age group in a large population (nationwide), and its ecological association (at county level) with some medical, geographical, or sociodemographic factors.

Methods: Registries of the largest medical institution in Mexico (∼37.5 million subjects assigned to a family physician) were analyzed and the AMFR calculated using asthma incidences.

Results: In boys, asthma incidence peaked at 0 to 4 years and progressively decreased, reaching a plateau in adulthood. In girls, asthma incidence showed a bimodal pattern, with maximal rates at 0 to 4 years old, and again at 50 to 54 years old. In the ecological analysis performed in more than 400 counties, the AMFR in adults (≥15 years old) inversely correlated with population density (r = -0.256) and altitude (r = -0.144), and directly correlated with acute respiratory tract infections (ARTI, r = 0.215), diabetes (r = 0.186), marginalization (r = 0.179), pneumonias (r = 0.166), and mean maximal temperature (r = 0.142), all with P < .01. In the multiple linear regression, only population density (P < .001) and ARTI (P = .006) remained statistically significant in the final model.

Conclusion: Asthma incidence in males and females did not match the expected sexual hormones variations, and other factors such as population density and ARTI also influenced the AMFR. These findings challenge the traditional belief that sexual hormones are major determinants of the AMFR.