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Abstract
Sources of error in diet assessment studies include inaccurate estimation of portion
sizes, incorrect description of food items, incomplete recall of food items eaten
and inclusion of extraneous foods. Data from free-living subjects on self-selected
diets usually cannot separate the sources of error because of uncertainties about
actual food intake. The present study was conducted to identify the contribution of
portion size estimations of different food groups to the estimation of food group
nutrient intake. Because subjects in the Dietary Effects on Lipoproteins and Thrombogenic
Activity (DELTA) Study were fed well-controlled experimental diets, this was an excellent
opportunity to compare self-reported portion sizes with weighed portions provided
by the research kitchen.
Subjects were normal weight males (n=25) and females (n=18) from three DELTA field
centers (Columbia University, Penn State University and University of Minnesota) Twenty-four
hour recalls on three randomly selected days were collected for each subject at the
beginning of the feeding period. The recalls were conducted by telephone interview
at the Penn State Diet Interview Center using the University of Minnesota's microcomputer
Nutrient Data Systems. Research kitchen staff observed subjects consume portions.
Portion sizes recalled by subjects were converted to gram weights. Individual foods
in the experimental menus were grouped as major food groups and subgroups. Self-reported
and actual portion sizes for all food groups were compared.
Self-reported estimates of portion size versus actual quantities provided by the research
kitchen ranged from –43% (underestimation in gram quantities) for condiments to +156%
(overestimation) for pasta. In only 5 subgroups (i.e. juices, bread, cookies, milk/milk
products and condiments) were portion sizes underestimated while intake from 14 subgroups
was overestimated. The overestimation of food groups that resulted in the greatest
relative overestimation of protein were pasta, nuts and mixed dishes. The overestimation
of food groups that resulted in the greatest relative overestimation of fat were meats,
baked beans, nuts, gravy and fat spreads. The overestimation of fruits, pasta, nee,
mixed dishes and puddings resulted in the greatest relative overestimation of carbohydrate.
This study establishes that subjects overestimate portion sizes of foods they were
given to eat and recalled having eaten. In addition, portion sizes of certain food
groups are more likely to be overestimated than others. This error in portion size
estimation is one important contributor to the errors associated with dietary assessment
studies. However, the extent of under- or overestimation of total nutrient intake
is also affected by other sources of error.
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Publication history
OBJECTIVE: To examine the contribution of portion size estimation to error in diet assessment studies.
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© 1995 American Dietetic Association. Published by Elsevier Inc. All rights reserved.
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