Tolerable Upper Intake Level (UL) may be underestimated. The extent of under- or overestimation will depend on the dosages and frequency of use, and for groups, on the percentage of the group using supplements. Currently, the only national surveys available which quantify supplement usage along with dietary nutrient intakes are the 1987 National Health Interview Survey and the Third National Health and Nutrition Examination Survey.
Merging two different databases—one dealing with food use and the other dealing with supplement use —to estimate the distribution of usual total intakes is complex because supplements provide relatively high doses of specific nutrients but may be taken intermittently. More accurate methods for measuring nutrient supplement intake are needed.
When assessing adequacy of intake, it may be helpful to average supplement intake over time when the supplement is consumed intermittently (e.g., once per week or month). This will mask or smooth out the high intake associated with the day the supplement was actually consumed. This smoothing effect might be appropriate when assessing for chronic high intakes using the UL. However, if acute effects on health are possible from excessive intake of a nutrient, then a different approach to combining food and supplement intake needs to be proposed. An additional drawback of smoothing supplement intakes is that the day-to-day variability in nutrient intake cannot be estimated. This creates a problem when estimating the usual nutrient intake distribution in a group.
When dietary intakes of a population or a population subset (e.g., athletes in training) vary systematically, reasons for this variation must be understood and incorporated into data gathering. These techniques also are part of defining what is usual intake (for example, over a calendar year). If systematic variations are not considered, prevalence of inadequate intakes may be under- or overestimated.
Seasonal effects on dietary intakes are reflected in changing patterns of food availability and use. These effects are usually greater for food items than for energy or nutrients (Hartman et al., 1996; Joachim, 1997; Van Staveren et al., 1986). The season of collecting yearly dietary data may bias results because the data will selectively overemphasize items consumed during the season of the interview