Fig. 2

Associations of quartiles of caffeine and its metabolites with sex hormones in prepubertal boys of 6–19 years old in NHANES 2013–2014. Puberty status was defined as “pubertal” if TT ≥ 30 ng/dL in males, E2 ≥ 20 pg/ml in females, otherwise puberty status was defined as “prepubertal”. Estimates were adjusted for urinary creatinine (continuous), age (continuous), race/ethnicity (categorical), BMI category, ratio of family income to poverty (continuous), cotinine (categorical), time of sample collection (categorical) and time period of examination. Analyses for E2 were not conducted in children due to the detection frequency of E2 being less than 70% in that group. The quartiles of caffeine and its metabolites were denoted as “Q1” through “Q4”, representing the 1st to 4th quartiles, respectively. In the figures, red error bars indicate statistically significant estimates (P < 0.05). The significance of the trend test was indicated as follows: * for P < 0.05, ** for P < 0.01, and *** for P < 0.001.