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.
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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.
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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:
- demonstrate that the NCI method works as well as other methods that have been used for estimating usual nutrient intake distributions; and
- describe how to estimate usual nutrient intakes for epidemiologic analyses.
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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.
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What important caveats are associated with the NCI method?
- Many studies have found misreporting of energy intake on both 24-hour recalls and
food frequency instruments, almost always in the direction of underreporting; this
suggests that some foods are underreported.
- If only a limited number of repeated 24-hour recalls are available, reliable
separation between non-consumers, irregular consumers, and always-consumers is not
possible. Therefore, in the absence of extra information about ever- vs.
never-consumption, the NCI method does not estimate the proportion of
non-consumers/always-consumers of a given food.
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