We focused on hypothesized POAG risk factors: African heritage,
13 glaucoma family history,
13 body mass index (BMI),
14 mean arterial blood pressure,
15 diabetes mellitus,
16,17 physical activity,
18 cigarette smoking,
19,20 caffeine,
21 and alcohol intake.
22 All risk factors were assessed by self-report on questionnaires; validation studies have found a high degree of reliability and accuracy of information.
11,23–27 Age was calculated as years from birthdate until the return of each questionnaire. Weight was assessed biennially and, along with height (assessed in 1976 in NHS and 1986 in HPFS), was used to calculate updated cumulatively averaged BMI (kg/m
2). With cumulative averaging every two years, the average of all available information was used (e.g., in 1986, the 1986 BMI values were used; in 1988, the average of 1986 and 1988 values was used; in 1990, the average of 1986, 1988, and 1990 values was used). This approach was used because glaucoma is a chronic disease, and cumulative averages represent participants' long-term exposure; also, with this approach, no participants had missing data. Cigarette smoking details were assessed biennially and pack-years of smoking was updated. Participants' reported time spent per week on eight to ten selected activities was multiplied by each activity's energy expenditure requirements (metabolic equivalents [METs]) and summed to yield MET hours per week.
28,29 Diet was assessed in 1986 (also in 1980 and 1984 in NHS) and every 4 years thereafter using SFFQs. Participants reported the average consumption frequency of a portion size of specific foods/beverages containing alcohol (beer, wine, spirits) or caffeine (coffee, tea, cola, chocolate)
30 during the previous year. Nutrient values were calculated by multiplying the consumption frequency of each food/beverage portion by the nutrient content, summing these products across all items, and then adjusting for total energy intake
31 and cumulatively averaged. Diabetes, systolic and diastolic blood pressure, hypertension, and various types of blood pressure–lowering medications were repeatedly asked in questionnaires. Mean arterial blood pressure was derived by using the updated cumulatively averaged values for systolic blood pressure and diastolic blood pressure in the following equation: (1/3 × systolic blood pressure) + (2/3 × diastolic blood pressure). To reduce the influence of outliers, we excluded outliers identified using the extreme studentized deviate many-outlier detection approach, assuming an upper limit of 2% of values being outliers.
32 For each variable, at most 1.5% of person-time and cases with extreme values were excluded as outliers.