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# Usual Dietary Intakes: The NCI Method

In collaboration with staff at the US Department of Agriculture (USDA)'s Center for Nutrition Policy and Promotion, NCI has developed a method to estimate usual dietary intakes of foods and nutrients. This method can be used for a variety of applications, including estimating:

• the distribution of usual food or nutrient intake for a population or subpopulation;
• individual food or nutrient intake for use in a disease model; and
• the effects of individual covariates on food or nutrient consumption.

The premise of the NCI method is that usual intake is equal to the probability of consumption on a given day times the average amount consumed on a "consumption day." The exact methods used for dietary components that are consumed nearly every day by nearly everyone differ slightly from those used for dietary components that are not. In general, the former category refers to nutrients and the latter category refers to foods.

For foods, a two-part model with correlated person-specific effects is used to model these components. The first part of the model estimates the probability of consuming a food using logistic regression with a person-specific random effect. The second part of the model specifies the consumption-day amount using linear regression on a transformed scale, also with a person- specific effect. Parts I and II are then linked by allowing the two person- specific effects to be correlated and by including common covariates in both parts of the model. Intake data from 24-hour recalls provide the values for the dependent variable, while average daily intake estimated from a food frequency questionnaire (FFQ) may be incorporated as one of the covariates. The resulting estimated model parameters can then be used to estimate the final products, depending on the application of interest.

For nutrients, the process is the same, except that the probability part of the model is not needed because the probability is assumed to be 1. Details of the NCI method for both foods and nutrients are available.

## Has the NCI method been validated?

Evidence for the validity of the NCI method, as it relates to estimating the distribution of usual intakes of foods, has been published through a series of papers in the Journal of the American Dietetic Association. Analyses establishing the validity of the method to estimate the distribution of usual intakes of nutrients and to estimate usual intakes for use in a regression analysis -- for example, to examine relationships between diet and health -- are being finalized, and publications are planned.

## What part does the frequency instrument play in the NCI method? Under what circumstances is it helpful?

The NCI method involves using two or more 24-hour recalls as well as covariates, which may include data from an FFQ such as the NHANES 2003-2006 Food Frequency Questionnaire (formerly called Food Propensity Questionnaire). A frequency instrument can substantially improve the power to detect relationships between dietary intakes and other variables. The magnitude of improvement depends on the specific food or nutrient in question and the population under study. For some dietary constituents and some populations, improvement may be modest. We are planning to explore those situations in which the frequency instrument provides the most benefit.

When applying the NCI method to estimate usual intake distributions, satisfactory results can generally be obtained without the FFQ as a covariate. However, there are conditions under which the FFQ may help -- particularly for estimating the tails of the distributions -- but how much improvement that covariate would yield has not been characterized yet.

## How does the NCI method compare to other methods?

Other methods have been proposed to estimate either the distribution of usual intakes or individual-level usual nutrient intake for use in epidemiologic analyses. These previous methods have some limitations, and they have used different assumptions, depending on whether they were designed for surveillance or epidemiological applications. To date, no other unified methods that estimate usual food intake from 24-hour recall data have been available that are:

• appropriate for use in both surveillance and epidemiologic analyses; and
• designed to address previous limitations.

Two methods have been used to estimate the distribution of usual intake of foods with a few days of 24-hour recalls. The simplest of these -- using the within-person mean -- usually leads to biased estimates of the prevalence of either inadequate or excess food intake. This is because it does not:

• account for reported days without consumption of a food group or for consumption-day amounts that are positively skewed;
• distinguish within-person from between-person variation;
• allow for the correlation between the probability of consuming a food and the consumption-day amount; or
• relate covariate information to usual intake.

The other method for estimating the distribution of foods -- developed at Iowa State University, and using modeling -- meets most of the challenges noted in the previous paragraph. However, it does not allow for correlation between probability and amount, and cannot incorporate covariate information regarding usual intake.

The NCI method was designed to meet all of these challenges. As established by Tooze et al., the NCI method is an improvement over the ISU method in two ways: it is applicable in situations in which the ISU method is not, and it allows efficient estimation of usual food intake distributions for subpopulations.

Analyses relating food intake to health outcomes in regression analyses have generally used reported intakes in place of estimated usual intakes. This approach, although simple to execute, is known to lead to biased estimation of risks. The NCI method is expected to improve on this approach.

Forthcoming papers will:

1. demonstrate that the NCI method works as well as other methods that have been used for estimating usual nutrient intake distributions; and
2. describe how to estimate usual nutrient intakes for epidemiologic analyses.

## What are the assumptions of the NCI method?

The NCI method assumes that the 24-hour recall is an unbiased instrument for measuring usual food intake -- in other words, that it does not misclassify the respondent's food intake and that it provides an unbiased measure of the amount of food consumed on a consumption day.