Supply and demand: a framework for explaining variability in dietary intake and its impact on data.

Nutrition has an important relationship with health and illness. One difficulty in measuring intake is related to variability. The purpose of this paper is to examine 1) the impact of supply and demand on variability in data collected for dietary studies and 2) the relationship between data and estimates of usual intake. The forces of supply and demand over time generate a consumption curve for each food. Two types of consumption curves are identified. One curve is horizontal and represents staples that are steadily consumed. The other curve exhibits peaks and dips and is unique for each food whose consumption varies with time. The measurement of usual intake is discussed in light of these two types of curves. Usual intake of foods whose consumption curve is horizontal could be read at any time since consumption does not vary with time. For all other foods, measuring usual consumption presents problems since the data vary with time. This examination indicates that foods whose consumption varies with time have unique properties that must be considered when attempting to calculate consumption. Suggestions are given to enhance measurement of consumption of these foods. Although excellent methodology currently exists for the calculation of intake, attention to the force of supply and demand with only serve to strengthen existing methods.