RESULTS
Table 1 summarizes some relevant characteristics and potential confounders for the study population of cases and controls. The mean age of cases was 41.9 years and that of controls was 42.5 years. When cases and controls were compared, controls were found to have a lower occurrence of family history of breast cancer, greater increase in body mass index between ages 20 and 30 years, more full-term pregnancies, longer durations of breastfeeding, and lower daily average alcohol consumption. Data on age at menarche, oral contraceptive use, age at first birth, educational level, body mass index and weight at age 20 years, height, energy intake, and smoking status were similarly distributed between cases and controls. Further classification of the population controls according to their total physical activity level during each age period (table 1) showed that women who reported low total physical activity were more likely to be nulliparous and have a higher educational level, a higher age at first birth, and a lower energy intake.
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TABLE 1.
Characteristics and potential breast cancer risk factors for cases and controls and for physically inactive controls in a population-based case-control study (n = 1,246) of premenopausal breast cancer in Germany, 1999–2000
Physical activity is a multidimensional exposure variable. For each type of physical activity, table 2 describes the distribution of the summary measure for physical activity in MET-hours/week. At age 12–19 years, the highest contribution to total physical activity was from walking, followed by sports and cycling. Household activities were less relevant, and occupational activities had no impact at that age. At age 20–30 years, the highest activity levels stemmed from household and walking, followed by occupational activities. Combining both age intervals resulted in the main contributions resulting from walking and household, followed by occupational activities. During adolescence, 96 percent of all women participated in any sports, mainly at school (92 percent), followed by swimming (19 percent) and gymnastics (11 percent). At age 20–30 years, only 54 percent of all women reported any sports, mainly gymnastics (17 percent) and swimming (15 percent) (data not shown). In general, this crude comparison showed only minor differences between cases and controls.