Fig. 4

Participant sex* and underlying phenotype. A Studies by percentage of female participants. B Studies by participant type. Studies that included healthy, non-asthmatic, non-atopic participants only were categorized under healthy. Studies involving healthy participants that were not screened for atopy were categorized under healthy. Studies that included at least some participants with positive skin prick testing, positive methacholine challenge (typically PC₂₀ ≤ 8 mg/mL), diagnosed asthma, exercise-induced bronchoconstriction, or other atopic diseases (even if healthy participants, with none of these conditions, were also included) were categorized under “asthmatic/atopic”. Metabolic syndrome for most studies was classified according to criteria outlined in [126]. See Additional file 1 for full dataset. *Most studies categorized participants by biological sex though some reported participant gender (self-identification as female or male). As the potential difference between sex and gender was not carefully elaborated in these studies, the term “sex” is used herein (recognizing that in some cases sex and gender may not correspond but that we do not have the data resolution to address this further)