Structure of Dietary Measurement Error
Summary: An analysis of data from the Observing Protein and
Energy Nutrition (OPEN) Study finds that measurement error in a food frequency
questionnaire seriously reduces its ability to detect potential associations of
absolute protein or energy intake with disease relative risk. The impact of this
measurement error is somewhat less after adjusting for energy. Measurement
error in the 24-hour recall also casts doubt on its use as a reference
instrument for validating/calibrating of food frequency questionnaires for
nutritional epidemiology studies.
Reference: Kipnis V, Subar AF, Midthune D, Freedman SL,
Ballard-Barbash R, Troiano RP, Bingham S, Schoeller DA, Schatzkin A, Carroll RJ.
Structure of dietary measurement error: results of the OPEN biomarker study.
Am J Epidemiol 2003;158(1):14-21.
Food frequency questionnaires (FFQs) are commonly used in analytic
epidemiologic studies that examine the relationship between diet and cancer.
The fact that recent large cohort studies have failed to find a consistent
relationship may be due to a true lack of diet-cancer associations or to
methodologic limitations of the studies, especially measurement error in the
FFQ. FFQ measurement error is important because it often leads to
underestimates of disease relative risks, therefore reducing the statistical
power of the study to detect their significance. This underestimation is called
attenuation. To compensate for this problem, investigators have integrated
calibration substudies that include a presumably more accurate reference
instrument, such as food records or a 24-hour dietary recall (24HR) to estimate
attenuation factors and adjust for them. To apply this approach correctly
requires that any inherent error in the reference instrument be independent of
true intake and of error in the FFQ.
Substantial evidence indicates, however, that commonly used dietary reference
instruments are unlikely to meet these requirements. In this study, which
analyzed data from the Observing Protein and Energy
Nutrition (OPEN) Study, investigators evaluated absolute protein intake as
well as total energy and energy-adjusted protein intakes among a sample of 484
people aged 40-69 living in the Washington DC suburb of Montgomery County, MD.
The use of an FFQ, 24-HR, and reference biomarkers for protein (urinary
nitrogen) and energy (doubly labeled water), allowed the authors to evaluate the
structure of dietary measurement error in the FFQ and 24HR and to examine
whether adjusting for energy substantially reduced measurement error in reported
intake, permitting remaining error to be reliably corrected for by the commonly
used approach.
Results of this analysis showed that the impact of FFQ measurement error for
total energy and absolute protein intake was severe. The biomarker-based
attenuation factors were close to 0 (in nutritional studies, the attenuation
factor is generally between 1 and 0; values close to 0 lead to more serious
underestimations of relative risk). Correlations between the FFQ and true
intake also were very low. Even thought this impact seemed to be less severe
after adjusting for energy, the results suggest that measurement error is still
an important problem. Further, the investigators found that the 24HR was
seriously flawed in that it had both intake-related bias and person-specific
bias. This latter bias was correlated with person-specific bias in the FFQ. As
a result, it did not meet either requirement for a valid reference instrument
and misrepresented the impact of measurement error in the FFQ.
These results have several important implications for nutritional
epidemiology. First, FFQs are unable to provide sufficiently accurate reports
of absolute protein, nonprotein, and energy intakes, which reduces their ability
to detect moderate associations with disease. Second, FFQ-based energy-adjusted
nutrient intakes may be just accurate enough to use in large cohort studies to
detect moderate diet-disease associations. However, because this study was
restricted to energy, protein and nonprotein intakes, it is not clear whether
these results would also apply to other dietary components, especially
non-energy-contributing nutrients. Third, these results throw doubt on the use
of 24-hour recalls as a reference instrument for validation or calibration
studies of absolute protein and nonprotein intakes as well as energy-adjusted
intakes.
Learn more about the Risk Factor Monitoring and Methods
Branch's dietary assessment activities.
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