1 Division of Epidemiology, School of Public Health, University of Minnesota, Minneapolis, MN.
2 Department of Nutrition, School of Public Health and Health Sciences, University of Massachusetts, Amherst, MA.
3 Prevention Research Center, Health Sciences Center, University of New Mexico, Albuquerque, NM.
4 Department of International Health, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD.
5 Division of Epidemiology and Clinical Applications, National Heart, Lung, and Blood Institute, Bethesda, MD.
6 Department of Biostatistics, Collaborative Studies Coordinating Center, University of North Carolina, Chapel Hill, NC.
7 Office of Native American Diabetes Programs, Health Sciences Center, University of New Mexico, Albuquerque, NM.
8 Department of Pediatrics/Center for Applied Research and Evaluation, University of Arkansas for Medical Sciences, Little Rock, AR.
Received for publication March 11, 2004; accepted for publication June 25, 2004.
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ABSTRACT |
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bias (epidemiology); intervention studies; nutrition assessment
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INTRODUCTION |
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Pathways Study results showed that the intervention program produced significant reductions in total energy intake. Average total energy intake was 263 kcal/day lower in the intervention group than in the control group (1). However, there were no differences in average percentage of body fat between the intervention and control groups (2), raising concerns over possible intervention-related bias in reporting of food intake with the 24-hour dietary recall method (3). More specifically, it is possible that children in the intervention schools may have been more likely than children in the control schools to underreport their food intake. Several other nutrition intervention studies in children have found changes in self-reported dietary intake in the absence of changes in adiposity (46), which raises further suspicion of intervention-related bias in reporting of dietary intake.
Data collected as part of the Pathways Study included both subjective (24-hour dietary recall) and objective (direct observation at school lunch) measures of participants dietary intake. This presented us with a unique opportunity to examine possible intervention-related bias in reporting of food intake. To our knowledge, no previous investigations have examined bias in food intake reporting by children in clinical trials. We analyzed data collected as part of the Pathways Study to test the hypothesis that bias in reporting of dietary intake occurred with the record-assisted 24-hour dietary recall method, with children in the intervention schools systematically underreporting their dietary intake relative to children in the control schools.
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MATERIALS AND METHODS |
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The institutional review boards at each participating university and tribe approved the study protocol, as well as the manner in which parental informed consent and childrens assent were obtained.
Measurements
Children were enrolled in the study and baseline measurements were completed at the end of the second grade, with selected measurements being conducted each spring through the fifth grade (follow-up). The impact of the Pathways intervention on diet was assessed in two ways: direct observation of children eating school lunches and record-assisted 24-hour dietary recalls. School lunch observation was chosen because it provides an objective estimate of intake unbiased by self-reporting (9, 10). Record-assisted 24-hour dietary recalls were used because they provide assessments of dietary intake for the entire day (11).
School lunch observation was conducted each spring on a sample of approximately 15 children in each school. At baseline, the children for the lunch observations were randomly chosen within each school. In subsequent years, as many of the same children were observed as possible, with additional children being chosen at random when the cohort children were not available. Lunch observations were made on at least two separate days in each school.
The approach of Gittelsohn et al. (9) served as the model for lunch observations. Observers evaluated each childs initial serving relative to a standard tray with expected foods and serving sizes. Each data collector observed up to three children at a time as inconspicuously as possible, recording portion sizes, food choices, food brought from home, food trading, food spillage, and second servings. Following lunch, amounts of food remaining on the childrens plates were measured using standardized utensils and recorded, thus allowing for calculation of food consumed. Data collectors were centrally trained and certified by two of the authors. To ensure a high skill level, observers were trained until they could visually estimate portions with less than 20 percent error, and they were graded against observations of the same children by expert observers.
Record-assisted 24-hour dietary recalls were administered during the spring of the fifth grade. The decision to use this posttest-only design for the recalls was based on anticipated difficulties in obtaining valid recalls from second-grade children at baseline (12, 13) and because of additional expense. Record-assisted dietary recalls were collected from random samples of approximately 15 children per school.
The recalls were collected on the day following the observation of the childs eating; hence, the recall information included observed lunch intake. Prior to the 24-hour recall period, children were trained to complete an abbreviated food record for use as a memory prompt during their recall interviews the next day.
Recalls were conducted in person using the Nutrition Data System for Research (version 4.02_30), a computer-based software application that allows for direct entry of dietary data in a standardized fashion (14). The multiple-pass interview technique was used to prompt children for complete food recall and descriptions. A variety of food-portion visual aids were available for use by participants in reporting portion size. Centralized training, certification of data collectors, and quality control were provided by staff at the University of Minnesota Nutrition Coordinating Center. To minimize the likelihood of bias in recording or reporting of food intake, intervention staff were not allowed to serve as data collectors.
Vendor products, menus, and recipes for food served in each school were collected for 5 days, including those days on which lunch observations and 24-hour dietary recalls were conducted. The menus and recipes were entered into the Nutrition Data System for Research and used as appropriate in the lunch observations and dietary recalls. Thus, nutrient intake calculations for the lunch observations and record-assisted 24-hour dietary recalls reflect as closely as possible the actual recipes and food preparation practices used at the childs school, rather than fixed default values from the food database. This aspect was important because of the school food-service intervention activities and the varying recipes and food preparation practices among schools (15).
Data analysis
The cohort included in the analyses presented in this paper comprised study participants for whom 24-hour dietary recall and lunch observation measurements were collected concurrently during the final follow-up measurement period (n = 608).
Because of the sampling frame and study design, appropriate estimates of mean values, variances, and tests of differences required restricted maximum likelihood solutions for mixed models. Conceptually, the models allowed for comparisons between nutrient estimates derived from the two types of intake measures (observed and recall) in the same children. Children were nested within the 41 schools, schools within intervention conditions, and conditions within the four field sites. This structure allowed for correlations within nested groups and accounted for differences among groups. The resulting mixed models were constructed with PROC MIXED in the Statistical Analysis System (version 6.12; SAS Institute, Inc., Cary, North Carolina).
Regression parameters were derived from mixed models accommodating the study design features as described above. Of particular interest was the association of intervention conditions with systematic bias in reporting of nutrient intake, as estimated from recalled intake adjusted for observed intake in multiple regression analysis. For these regression analyses, nutrient intakes from recalls were the dependent variables; nutrient intakes from observations, child age, child gender, and intervention condition were fixed effects; and school and field site were random effects.
Because the distributions of intakes for specific nutrients departed substantially from a Gaussian distribution, power transformations were applied to approximate normality, and tests of significance were conducted on the transformed variables. For the analyses, intakes of saturated fatty acids, iron, vitamin C, and ß-carotene were log-transformed, and intakes of carbohydrate, calcium, and vitamin A were square-root-transformed.
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RESULTS |
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DISCUSSION |
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Several limitations of this study should be noted. Because participants in this study were American Indian children from seven American Indian communities, strictly interpreted, these findings may be generalized only to similar children. Second, because participants provided a 24-hour dietary recall subsequent to being observed eating lunch, reporting may have been influenced by the observation, with participants providing more accurate reporting of intake because of increased attention to their diet.
In conclusion, these findings suggest that investigators should consider bias in reporting of dietary intake by intervention condition when conducting diet-focused intervention studies, in addition to the equally valid concerns about cognitive and social desirability biases in dietary reporting in epidemiologic studies. Intervention-related bias in reporting of dietary intake could potentially yield artifactual study results. In designing intervention studies, enhancement of measures that rely on self-reports with objective measures of dietary intake would help investigators to evaluate whether differential reporting by treatment group has occurred.
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NOTES |
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REFERENCES |
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