Eui-Hyun Yoon, et al: BMD and Food-frequency (hamburger, pizza, and fried foods). Reported intake frequency of food items was calculated to obtain the food intake per year for each person. The annual food- frequency of all food items included in the groups was summed to obtain the food-frequency of each food group in this study. 4. Measurement of Confounders In addition to diet, factors reported to affect BMD include body mass index (BMI, in weight [kg]/height2 [m2]), physical activity,18) smoking,9) estrogen use by women,19) and use of calcium or vitamin D supplements or both.20) A comprehensive questionnaire was used to collect each participant’s demographic characteristics (e.g., age, sex), anthropometric measurements (height and weight), physical activity, current smoking status, alcohol drinking, disease, history of hypertension, rheumatoid arthritis, thyroid diseases, cerebrovascular accident, coronary Table 1. General characteristics of the study population. Variables Men (n = 18,069,970) Women (n = 18,433,574) P-value* Age (y) 43.8 ± 0.0 45.8 ± 0.0 <0.001 Body mass index (kg/m2)† 24.1 ± 0.0 23.1 ± 0.0 <0.001 Physical activity (IPAQ)‡ Low 12,707,249 (70.7) 13,912,243 (75.9) <0.001 Moderate 1,495,308 (8.3) 1,694,646 (9.2) High 3,763,613 (20.9) 2,730,589 (14.9) Current smoker 8,443,052 (46.9) 1,283,164 (7.0) <0.001 Alcohol use (gm/y) 5517.2 ± 1.7 1219.0 ± 0.8 <0.001 History of other diseases Hypertension 3,072,802 (17.1) 3,075,795 (16.7) <0.001 Rheumatoid arthritis 171,397 (1.0) 597,908 (3.3) <0.001 Thyroid disease 160,384 (0.9) 931,559 (5.1) <0.001 Cerebrovascular accident 306,011 (1.7) 236,087 (1.3) <0.001 Coronary artery disease 319,522 (1.8) 270,069 (1.5) <0.001 Diabetes mellitus 1,152,409 (6.4) 1,174,106 (6.4) 0.180 Bone mineral density (gm/cm2)§ Femoral neck 0.83 ± 0.0 0.71 ± 0.0 <0.001 Lumbar spine 0.97 ± 0.0 0.92 ± 0.0 <0.001 Vitamin or mineral supplements 1,688,533 (9.3) 2,209,462 (12.0) <0.001 Reproductive information in women Age of menarche (y) - 14.5 ± 0.0 - Postmenopausal period (y) - 14.0 ± 0.0 - Total HRT period (mo) - 1.8 ± 0.0 - Total period of oral contraceptives (mo) - 2.3 ± 0.0 - Values are presented as mean ± SD or number (%). HRT: hormone replacement therapy. *P-value by Student t-test or χ2 test after weighting by wt_tot. †Calculated as weight in kilograms divided by height in meters squared. ‡Calculated by the International Physical Activity Questionnaire (IPAQ) score. Non response rate was 0.6% (n = 103,800) in male, 0.5% (n = 96,097) in female. §Measured by dual-energy X-ray absorptiometry. Korean J Fam Med Vol. 33, No. 5 Sep 2012 | 289