Duplicate diets

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 [1]. Dietary dimensions assessed by duplicate diets are summarised in table D.3.5.

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 [5].

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 fast-food meals from fast-food restaurants as reported by study participants, assayed dioxin contents in meals, and considered them as dietary exposure [2]. Despite lack of complete meals on a day and possible differences between meals actually consumed and meals collected on later days, the approach embraced advantage of the method to assess individuals’ dietary exposure, free from recall bias and need for a food composition table [2]. 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:

• Duplicate diets are particularly useful for the assessments of minerals, because elements do not degrade overtime and therefore can be assayed any time after collection of duplicated meals.
• Direct chemical analysis of trace elements such as sodium, selenium, iodine, and zinc in plant based staples may be necessary where there are no local food composition data due to the wide variation of these elements in the soil and the effect of agronomy practices. Norwegian investigators reported major differences when Finnish rather than Norwegian selenium food composition data were used to calculate selenium intakes [3].
• Duplicate diets can be the only method available to measure dietary exposure to some contaminants, phytochemicals, and others that are not available in a food composition table [7, 8].
• The method can be used to validate biomarkers [8] and estimation of nutrient intakes from any dietary assessment methods.
• Duplicate diets are a suitable method for metabolic or energy balance studies [1].
• Duplicate diets are useful in an experimental research in which investigators provide meals to participants. When a food composition table is not reliable because of lack of information or known variability, duplicate diet measures can inform food compositions delivered to study participants. In combination with assessments of leftovers of provided meals, nutritional exposure can be measured precisely.

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.

Strengths

• The method provides accurate nutrient intake data which is not subject to the errors inherent in data processing including the limitations of food composition data
• Does not depend upon composition data from other sources

Limitations

• The method is expensive and elaborate to execute
• It imposes a high individual burden
• Individuals must be highly motivated
• Unsuitable for large-scale studies
• It is unlikely to capture habitual diet unless adequate days are assessed, unless repeated measurements are implemented
• There is possibility that this method altering food intake to ease the burden
• Relies on the respondent providing a complete duplicate of consumption
• May not account for leftover portions
• Requires analysis using specialist laboratory equipment

Considerations relating to the use of duplicate diets for assessing diet are summarised by population in Table D.3.7.

• Availability of laboratory apparatuses for specific nutrients and other chemicals determines whether or not duplicate diet measurements are useful.
• Ideally assays should be implemented just after collection of duplicate diets. This requirement depends on nutrients or chemicals on target. Contents of minerals are impossible to change over time and thus assays are not necessarily to be implemented just after collection of duplicate diets. However, chemicals subject to oxidation or degradation, such as vitamin C, need to be assessed quickly after the collection, if those nutrients are of interest.
• If a research interest is in dietary exposure via a certain dietary occasion (e.g. school lunch, fast-food restaurant), duplicate meals are needed from the occasion, not from meals over a day or days.

• This method requires sophisticated chemical analysis of foods which will usually be done on a consultancy basis by a specialist laboratory
• Individuals should be issued with a diet diary, large strong bags for food collection and leak-proof plastic containers for liquids, ice packs and insulated cooling bag and weighing scales
• Regular fieldworker visits every couple of days are required to collect food and monitor compliance

A method specific instrument library is being developed for this section. In the meantime, please refer to the overall instrument library page by clicking here to open in a new page.