Table 2.
Baseline characteristics (at urine collection in 2000) of matched controls across quartiles of creatinine-standardized urinary melatonin level (n=799)
| Quartiles of urinary melatonin levels (in ng/mg creatinine) | ||||
|---|---|---|---|---|
| Characteristic | Quartile 1 | Quartile 2 | Quartile 3 | Quartile 4 |
| Mean melatonin level, in ng/mg creatinine (SD) | 6.9 (3.0) | 17.6 (3.4) | 30.3 (4.3) | 64.8 (44.2) |
| Mean age, in years (SD) | 67.9 (6.6) | 67.5 (6.6) | 66.1 (6.9) | 66.2 (6.9) |
| Mean age at menarche, in years (SD) | 12.6 (1.4) | 12.6 (1.3) | 12.6 (1.3) | 12.6 (1.4) |
| Mean age at menopause, in years (SD) | 49.4 (4.4) | 49.4 (5.3) | 49.7 (4.2) | 49.8 (4.9) |
| Mean number of births (SD) | 3.3 (1.6) | 3.2 (1.8) | 3.2 (1.5) | 3.3 (1.5) |
| Family history of breast cancer, % | 14 | 17 | 13 | 19 |
| Mean BMI, in kg/m2 (SD) | 27.6 (5.8) | 26.9 (5.4) | 25.9 (4.9) | 25.3 (4.1) |
| Mean alcohol intake, in g/day (SD) | 5.5 (9.0) | 4.9 (8.8) | 5.9 (8.7) | 4.9 (7.3) |
| Current smoker, % | 7 | 3 | 3 | 4 |
| History of benign breast disease, % | 61 | 61 | 50 | 61 |
| Recent PMH use, % | 54 | 58 | 56 | 58 |
| Duration of PMH use (estrogen only), % | ||||
| None | 58 | 57 | 57 | 59 |
| <5 years | 13 | 15 | 12 | 12 |
| ≥5 years | 29 | 28 | 31 | 29 |
| Duration of PMH use (estrogen + progesterone), % | ||||
| None | 62 | 61 | 62 | 63 |
| <5 years | 19 | 13 | 17 | 12 |
| ≥5 years | 19 | 26 | 21 | 25 |
| First morning urine, % | 94 | 97 | 96 | 95 |
SD=standard deviation