2.4. Subject Grouping

The subjects were divided into the milk intake group (M_yes group: milk intake >0 g/day) and the no milk intake group (M_no group: milk intake = 0 g/day). For physical activity grouping, the median of the weekly participation time of moderate-to-vigorous physical activity was calculated by converting vigorous physical activity times to moderate physical activity time. Subjects with a value below the median were classified as the low-level physical activity (P_low) group. Those with a value equivalent or higher than median were classified as the high-level physical activity (P_high) group. Groups can also be classified according to the satisfaction of the physical activity guidelines of 60 min of moderate-to-vigorous activities every day. However, only 5.1% of men and 1.9% of women actually meet these criteria (420 min per week), making it impossible to compare the groups using statistical analysis. Therefore, in this study, groups were classified using the median of converted physical activity time per week. The physical activity questions of the 2009–2011 KNHANES were based on the Korean version of the International Physical Activity Questionnaire (IPAQ) short form.

By combining these two classifications, the subjects were finally classified into four groups according to milk intake and physical activity level: no milk intake + low-level physical activity group (M_noP_low), no milk intake + high-level physical activity group (M_noP_high), milk intake + low-level physical activity group (M_yesP_low), and milk intake + high-level physical activity group (M_yesP_high).