Daily, Weekly, Seasonal, Within- and Between-individual Variation in Nutrient Intake According to Four Season Consecutive 7 Day Weighed Diet Records in Japanese Female Dietitians

Objective:To study daily, weekly, seasonal, within and between-individual variance in intake of selected nutrients and minimal days necessary for assessing true intake with a specified degree of error based on four season consecutive 7 day weighed diet records (WDRs).Subjects and Methods:We evaluated consumption of energy and 30 nutrients based on four season consecutive 7 day WDRs from 80 Japanese female dietitians in 1996-1997.We examined daily, weekly, seasonal, within- and between-individual variation in nutrient intake, relative contributions of their variances to total variance, and minimal days required to estimate a person's nutrient intake within 10% and 20% of their true mean with 95% confidence intervals.Results:The relative contributions of variation for all nutrients by person were larger than those by day, week and season.Within-individual variances were greater than the between-individual variances.The ratios of within- vs.between-individual variances thus ranged from 1.3 - 26.9.Minimal days necessary for estimating nutrient consumption per person within 10% (20%) of the true mean with 95% confidence intervals ranged from 10-35 (3-9) days for energy and major nutrients and 15-640 (4-160) days for micro-nutrients.Conclusions:The relative contributions of variability by person were largest for all nutrients, followed by those due to sequence of days, season and day of week.Within-individual variation was greater than between-individual variation.Minimal days necessary for ascertaining major nutrients were in general fewer than micro-nutrients. J Epidemiol, 2002;12:85-92