The questionnaire assessed socio-demographic characteristics, including date of birth, nationality, body weight, height and education. Furthermore, participants were asked to comment on their consumption of food, beverages, physical activity and their smoking status. The food questionnaire of this study was partially adapted from the food questionnaire of the Million Women Study . The choice of the components was based on the hypothesis that a low consumption of “unhealthy” food, such as meat (contains saturated fatty acids)  and a high consumption of “healthy” food, such as fruits (contain antioxidants)  protect against a high mammographic density as indicator for higher breast cancer risk. Participants provided information on reproductive factors, such as age at menarche, menopausal status and parity, and family history of breast cancer. Data on hormone replacement therapy, biopsy of breast and past medical history was also included.
In this study, we collected information of the supplementary intake of vitamins and minerals both individually and combined. Due to the small numbers on single vitamin and mineral use, we investigated the association between intake of multivitamin-multimineral supplements and mammographic density. The corresponding question in our questionnaire was: “Do you take vitamins, minerals or dietary supplements regularly?”
Data on the consumption of wine, beer and spirits beverages in glasses per week has been collected to assess a drinking of alcoholic beverages. We calculated how much alcohol in g (gram) each woman consumed according to conversions described in other German studies [15
]. The following conversions were made:
wine: 1 glass = 0.25 liter .
beer: 1 glass = 0.3 liter.
spirits beverage: 1 glass = 0.02 liter.
1 liter wine = 100 g alcohol, and
1 liter beer = 40 g alcohol .
There were 25 g alcohol in 1 glass wine; 12 g alcohol in 1 glass bier and 6.2 g alcohol in 1 glass spirits beverage . On the basis of this data, we calculated the total alcohol consumption (wine, beer and spirits beverage) per day (d) in gram for each participant. We created quantitative (increase per 10 g/d) and qualitative variables to analyze alcohol consumption. Categorized variable included four classes: > 10.0 g/d total alcohol; 5.0 - 10.0 g/d total alcohol; 0.1 - 5.0 g/d total alcohol and 0 g/d total alcohol.
We selected the dietary components contributing to a score based on other studies of adherence to Mediterranean diet. Based on other studies on the adherence to Mediterranean dietary pattern [11, 17] we selected the dietary components in our study. The following foods were included: boiled and raw vegetables, fruit, fish, nuts and olive oil (pro-Mediterranean diet); butter, beef, pork, sausages, ham, hamburger and lemonade/soft drinks (contra-Mediterranean diet). Points were assigned for pro-Mediterranean food: low consumption = 1 point, regular consumption = 2 points, frequent consumption = 3 points. Points for contra-Mediterranean food were reversed: low consumption = 3 points, regular consumption = 2 points, frequent consumption = 1 point. In cases of no consumption or where no information was provided, 0 points were given. In total, a maximum of 29 points could be achieved. High score indicates adherence to Mediterranean diet.
As potential confounders were considered: physical activity at recruitment (yes/no), Body-Mass-Index (BMI) at recruitment (kg/m2), ever use of hormone replacement therapy (HRT) (yes/no), menopausal status at recruitment (proxy variable with premenopausal (<50) and postmenopausal (≥50) women), age at recruitment (years), mother with history of breast cancer (yes/no), school education (9 years of school/13 years of school), ever use of oral contraceptives (yes/no), age at menarche (years), smoker at recruitment (yes/no), ever breast feeding (yes/no), number of live births (quantitative variable; 0,1,2,3,4 or 5 live births), age at the first birth (quantitative variable; years), alcohol consumption at recruitment (yes/no) if not exposure variable.
2-test, the t-test and the Wilcoxon-test were used to analyze the significance of different frequencies and distribution, respectively. We defined the level of statistical significance as 0.05. We used these tests to investigate the significance of relationships between possible confounders (such as BMI, education, etc.) and either the target variable (mammographic density) or influencing variables (such as current intake of multivitamin-multimineral supplements, current alcohol consumption and Mediterranean dietary pattern). Tests for trend across quartiles were performed by assigning the mean level within specific quartiles of alcohol intake to all individuals in that quartile, and using this as a continuous variable in a linear regression. Logistic regression models were fitted to obtain Odds Ratios (ORs) with 95% confidence intervals (95%CIs) for high mammographic density. Additionally interactions between influencing variables and the target variable were tested. Variables, which were identified as potential confounders because they had a biological plausibility and statistically significant relationships with the target variable and/or influencing variables, were tested with manual step-by-step regression. A variable showed an effect and was identified as a confounder, if it had changed estimator more than 10%. The parsimonious models were adjusted for age and BMI based on results of the manual step-by-step regression and/or the biological plausibility. These models were computed stratified by smoking status in order to account for differential association according to hormonal and oxidative stress, respectively. Also the fully adjusted models were calculated, adjusted for physical activity, BMI, HRT use, menopausal status, age, mother with history of breast cancer, school education, ever use of oral contraceptives, age at menarche, smoker at recruitment, number of live births, alcohol consumption. Statistical analysis was assessed with the Analyst Interface of the SAS (Statistical Analysis System) Version 9.2.