Table 2.
Age and age-adjusted baseline characteristics of 710 control subjects by quartile of 12-hour overnight urinary 6-sulfatoxymelatonin level*
| Characteristic | Q1 (n = 177) | Q2 (n = 178) | Q3 (n = 178) | Q4 (n = 177) |
| Urinary aMT6s output per 12 h, μg | <6.5 | 6.5–10.8 | 10.8–16.5 | ≥16.5 |
| Age, y | 59.2 | 57.3 | 58.1 | 56.0 |
| Age at menarche, y | 13.4 | 13.2 | 13.3 | 13.3 |
| Age at menopause, y | 48.4 | 49.0 | 48.8 | 48.4 |
| Parity (among parous women only), No. of children | 2.5 | 2.4 | 2.1 | 2.3 |
| Age at first child's birth (among parous women only), y | 26.0 | 26.5 | 27.3 | 26.6 |
| Positive family history of breast cancer, % | 5.1 | 6.2 | 7.3 | 10.1 |
| HRT past use, % | 21.5 | 17.0 | 14.6 | 14.0 |
| OC use, % | 12.1 | 15.8 | 16.4 | 9.7 |
| BMI, kg/m2 | 26.4 | 26.2 | 26.2 | 25.8 |
| Alcohol consumption, g/day | 9.1 | 10.1 | 12.8 | 9.7 |
| Positive history of benign breast disease, % | 6.8 | 6.8 | 6.7 | 12.4 |
| Education beyond 8 y, % | 9.0 | 16.4 | 15.2 | 19.1 |
| Smoking history | ||||
| Current smoker, % | 16.9 | 16.8 | 12.0 | 12.5 |
| Past smoker, % | 8.1 | 12.9 | 10.7 | 10.0 |
| Never smoker, % | 75.0 | 70.3 | 77.3 | 77.5 |
| Pack-years among ever smokers | 13.6 | 10.2 | 10.4 | 9.3 |
| Sex hormone levels | ||||
| Sex hormone–binding globulin, nmol/L | 97.4 | 99.8 | 95.8 | 109.2 |
| Testosterone, ng/mL | 0.24 | 0.26 | 0.27 | 0.27 |
| Free testosterone, pg/mL | 0.53 | 0.62 | 0.62 | 0.65 |
| Estradiol, pg/mL | 10.3 | 11.1 | 9.6 | 11.7 |
*
Data are the mean. Q = quartile; aMT6s = 6-sulfatoxymelatonin; HRT = hormone replacement therapy; OC = oral contraceptive; BMI = body mass index.