Notes:
a  The first six studies were included in meta-analysis because all examined ever or current asthma as the primary outcome
b  quality was scored and rated independently using the American Dietetic Association Quality Criteria Checklist. Pack-years was defined as the number of packs smoked per day multiplied by the number of years smoked. The authors adjusted for both pack-years and the square of pack-years (pack-years2).
Abbreviations: ACQ, asthma control questionnaire; BMI, body mass index; CI, confidence interval; FFQ, food frequency questionnaire; OR, odds ratio; PCA, principal component analysis; FVC, forced vital capacity; FEV1, forced expiratory volume in one second; MSG, monosodium glutamate; RCT, randomized controlled trial.