Explanatory variable: dietary vitamin K intake

Explanatory variable: dietary vitamin K intake
The dietary vitamin K intake was estimated from a semiquantitative
food frequency questionnaire (FFQ) [13]. FFQs are
considered the only practical and affordable method to assess
dietary usual intake in relation to development of various diseases
in large epidemiological studies [13]. The 50-item FFQ used here
was specifically designed to determine the daily dietary vitamin K
intake during the previous 12 months. It comprises 50 food items
identified as important contributors to phylloquinone intake (e.g.,
spinach, iceberg lettuce, collards, and broccoli) and items with very
high phylloquinone content (≥500 g/usual portion). The FFQ was
interviewer-administered in 30 min by questioning the patients
and/or their relatives when applicable. Estimated vitamin K intake
was calculated for each food item by multiplying the amount of
phylloquinone for that food item by the selected frequency and
serving size (calculated as 0.5 for smaller than, and 1.5 for larger
than the suggested portion), and all values were added to provide