What We've Learned: Measuring Intakes
To accurately monitor food and nutrient intakes and identify those at risk as well as
those who are meeting recommendations, it is necessary to precisely estimate intakes. That
has consistently posed a challenge because people don't always report accurately and
because dietary assessment instruments contain some degree of error. In addressing these
problems through our methodologic research, we've learned a few things:
- Underreporting is a common problem in dietary surveys and in
epidemiological studies. Survey respondents whose reported energy
intakes are implausibly low -- "low energy reporters" -- are less likely to report a broad
range of foods, but when they do, are more likely to report them less frequently and in
- Our Food Frequency Questionnaire (FFQ), the Diet
History Questionnaire, performed best overall in a 1997-1998 comparison to two other
FFQs in wide use at that time (the 1995 NCI-Block Health Habits and History Questionnaire
and the Willett, purple version) using four 24-hour recalls as the reference instrument.
The DHQ was developed with attention to cognitive ease and nutrient database enhancements.
- Analyses using our NHANES Food Frequency Questionnaire
(formerly called Food Propensity Questionnaire) show that food
frequency data can be used to boost the predictive power of 24-hour recall instruments,
thereby providing a way to estimate the distributions of usual intake of episodically
- Cognitive testing of diet questions can make such questions easier for
respondents to answer and enhance the accuracy of the responses. The formatting of diet
questions affects the rate of unanswered questions. For FFQs, the consecutive question
format is superior to the grid format.
- Short questionnaires estimate group mean dietary
intakes fairly well among highly educated study populations, but additional work is needed
to adapt these instruments to diverse populations.
- Short questionnaires and complete food frequency questionnaires are
generally moderately correlated with true intakes. However, studies using these
instruments to examine the association between diet and disease would require many more
subjects than previously thought in order to attain adequate statistical power.
- The use of biomarkers as reference instruments in dietary intake
validation studies suggests a large degree of measurement error and bias in both 24-hour
dietary recalls and FFQs. These errors and biases result in attenuation of estimated
relative risks in nutritional epidemiology studies. This attenuation is more severe than
previously thought. Thus, FFQs and 24-hour recalls may not be able to detect moderate but
important associations in diet and cancer studies.