Usual dietary intake is the long-term average daily intake of a nutrient or food. The concept of usual intake is important because dietary recommendations are intended to be met over time and diet-health hypotheses are based on dietary intakes over the long term. Consequently, it is the usual intake that is often of most interest to policy makers — when they want to know the proportion of the population at or below a certain level of intake — or to researchers — when they want to examine relationships between diet and health. However, until recently, sophisticated efforts to capture this concept have been limited at best.
Difficulties Posed By Intra-Individual Variation
A single day represents only a “snapshot in time” and does not represent a person’s long-term average daily intake. In fact, for many dietary constituents (especially those consumed episodically rather than daily), there is greater variation in intake day-to-day within a single individual than there is person-to-person within a population. Nevertheless, 24-hour recalls are considered among the most precise methods of assessing diet. They ask people to report everything eaten and drunk during the previous 24 hours and are rich in details regarding every item consumed (when, how, how much, with what). Since 24-hour recalls attempt to capture a day’s intake, it follows that 24-hour recalls are also subject to excess intra-individual variation relative to usual intake.
About the difficulties posed by intra-individual variation
The NCI Method
Researchers at the National Cancer Institute (NCI) and elsewhere have developed a method to model particular aspects of usual dietary intakes of foods and nutrients using 24-hour recalls. This method can be used to:
- estimate the distribution of usual intake for a population or subpopulation;
- assess the effects of non-dietary covariates on usual consumption; and
- correct (at least partially) bias caused by measurement error in estimated associations between usual dietary intakes and health outcomes using the statistical technique of regression calibration. (Note: This modeling technique does not accurately estimate usual intake for individuals.)
About the NCI Method
National Data on Usual Intakes
We have applied the NCI Method for estimating distributions of usual intake to data from two recent cycles of the National Health and Nutrition Examination Survey (NHANES), a nationally representative sample, to estimate means and percentiles of the distributions of food intake and the percentage of persons meeting recommendations for a range of sex-age groups in the U.S. population. View the PDFs below to learn more about the methods for analyzing the NHANES data, results and selected findings, acknowledgements, and suggested citations.
These reports, originally published online as references for the 2010 and 2015 Dietary Guidelines Advisory Committees, can be viewed in an archive in their original format.
View our compilation of selected references for more information about the importance of estimating usual intake distributions, accounting for measurement error in estimating diet-health relationships, statistical methods, and combining self-report data with biomarker data.