The first step in individual assessment is to obtain the most accurate information possible on total dietary intake (food and supplements), recognizing that this is always a challenge because of the documented high incidence of underreporting (Johnson et al., 1998; Lichtman et al., 1992; Mertz et al., 1991), and the large day-to-day variation in intake (Beaton et al., 1979, 1983; Gibson, 1990; Sempos et al., 1985; Tarasuk and Beaton, 1991b, 1992; Van Staveren et al., 1982). Intake on one or even several days may give very inaccurate estimates of usual intake, especially if the individual's food choices vary greatly from one day to the next, which is a common occurrence. Following are some issues to consider when determining the magnitude of day-to-day variation:
Factors that affect day-to-day variation in nutrient intake include:
variety versus monotony in an individual's food choices (Basiotis et al., 1987; Sempos et al., 1985; Tarasuk and Beaton, 1991b, 1992)
day of the week (Beaton et al., 1979; Tarasuk and Beaton, 1992; Van Staveren et al., 1982)
holidays and special occasions
appetite (which may be related to changes in physical activity, the menstrual cycle, etc. [Barr et al., 1995; Tarasuk and Beaton, 1991a])
The number of days needed to estimate usual intake also varies according to the desired precision of the estimate (see examples in Box 3-1). Obtaining an estimate within ± 10 percent of the usual intake requires more days of intake data than obtaining an estimate within ± 20 percent of the usual intake (Basiotis et al., 1987).
BOX 3-1 The Number of Days Needed to Estimate Usual Intake Varies with the Specific Nutrient and the Desired Precision
Consider trying to estimate an individual's usual intake of niacin and vitamin C. In a study of 13 men over 1 year, it was estimated that determining mean niacin intake within ± 10 percent of their true usual intake required 53 days of intake data, whereas 249 days of intake data were needed to estimate usual vitamin C intake with the same precision. In a study of 16 adult women over 1 year, an average of 222 days of intake data was needed to estimate their vitamin C intake within ± 10 percent of true usual intake, while an estimate within ± 20 percent of true usual intake required only 55 days (Basiotis et al., 1987).