Duplicate diets are assessments of dietary intake by retaining a duplicate portion of all food and drinks consumed during the study period. Assessment of dietary intake by duplicate diets is considered the gold standard method of assessing nutrient intakes, in particular minerals, at an individual level . Dietary dimensions assessed by duplicate diets are summarised in table D.3.5.
Table D.3.5 Dietary dimensions assessed by duplicate diets.
|Dietary dimension||Possible to assess?|
|Energy and nutrient intake of total diet||Yes|
|Intake of specific nutrients or food||Yes|
|Infrequently consumed foods||Yes|
|Frequency of eating/meal occasions||Yes|
|Adult report of diet at younger age||No
A duplicate portion of all food and drink prepared, served and consumed is retained, normally over a 24-hour period. These identical portions are weighed by the respondent or fieldworker and usually a concurrent weighed food diary is maintained. All food is then chemically analysed.
Asking respondents to keep a weighed food diary is useful to act as a check on the completeness of the duplicate portions. The weighed diary also serves as a way of checking portion sizes of the duplicate - one study used an arbitrary 10% difference between weights as an acceptability cut-off .
Regular fieldworker visits every couple of days are required to collect food and monitor compliance. At the end of the assessment the diet diary should be reviewed in person with the respondent for clarification.
Assessment can be done without concurrent duplication of meals. De Filippis et al. collected meals from fast food restaurants as reported by study participants, assayed dioxin contents in meals, and considered them as dietary exposure . Despite the lack of complete meals on a day and possible differences between meals actually consumed and meals collected on later days, the approach embraced the advantage of the method to assess individuals’ dietary exposure, free from recall bias and need for a food composition table . In addition, the approach is likely to serve well to inform possible exposure to dioxins in a given population through a certain dietary occasion.
The method has several uses:
Estimates of dietary intake are derived directly from laboratory calorimetric or biochemical assays of the duplicate food portions. Sometimes, multiple days of assessment may be combined into a single composite, and then homogenised before analysis; this does not provide data on within-person day-to-day variation.
Key characteristics of duplicate diet methods are summarised in Table D.3.6.
Table D.3.6 Characteristics of duplicate diet methods*.
|Number of participants||Typically small|
|Cost of development||High|
|Cost of use||High|
|Researcher burden of data collection||Low|
|Researcher burden of coding and data analysis||High|
|Risk of reactivity bias||Yes|
|Risk of recall bias||Minimised if duplicate saved at time of consumption|
|Risk of social desirability bias||Yes|
|Risk of observer bias||Minimised|
|Participant literacy required||If diary is also completed|
|Suitable for use in free living||Yes|
|Requires individual portion size estimation||No|
* Degrees of bias vary by approaches to collect duplicate meals. If an investigator mimics meals on the basis of self-reported data from participants (24-hour recall), participant burdens and bias could be identical to the self-report method.
Considerations relating to the use of duplicate diets for assessing diet are summarised by population in Table D.3.7.
Table D.3.7 Use of diet assessment by duplicate diets in different populations.
|Infancy and lactation|
|Toddlers and young children||Requires proxy to save duplicate portions and if necessary record diary.|
|Adolescents||Requires proxy to save duplicate portions and if necessary record diary.|
|Older Adults||Potentially difficult task for those with cognitive impairment.|
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