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.16.1.

Table D.16.1 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
Dietary pattern Yes
Habitual diet No
Within-individual comparison Yes
Between-individual comparison Yes
Meal composition Yes
Frequency of eating/meal occasions Yes
Eating environment 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 [3].

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 [2]. 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 [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:

  1. Duplicate diets are particularly useful for the assessments of minerals because elements do not degrade over time and therefore can be assayed any time after collection of duplicated meals.
  2. Direct chemical analysis of trace elements, such as sodium, arsenic, mercury, selenium, iodine, and zinc, in foods may be necessary where there are no local food composition data partly because of the wide variation of these elements in the soil, the effect of agronomy practices or a lack of resources [3,4].
  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 [5, 6].
  4. The method can be used to validate biomarkers [6] and estimation of nutrient intakes from any dietary assessment methods [7,9].
  5. Duplicate diets are a suitable method for metabolic or energy balance studies [1].
  6. Duplicate diets are useful in an experimental study in which investigators provide meals to participants. When a food composition table is not reliable because of a lack of information or known variability, duplicate diet measures can inform the food composition of meals 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.16.2.


  1. provides accurate nutrient intake data without the limitations of food composition data.
  2. Does not depend upon composition data from other sources.


  1. The method is expensive and complex to execute
  2. It imposes a high individual burden
  3. Individuals must be highly motivated
  4. Unsuitable for large-scale studies
  5. It is unlikely to capture habitual diet unless an adequate number of days are assessed
  6. There is possibility that this method may alter food intake to ease the burden [9]
  7. Relies on the respondent providing a complete duplicate of consumption
  8. May not account for leftover portions
  9. Requires analysis using specialist laboratory equipment

Table D.16.2 Characteristics of duplicate diet methods*.

Characteristic Comment
Number of participants Typically small
Cost of development High
Cost of use High
Participant burden 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.16.3.

Table D.16.3 Use of diet assessment by duplicate diets in different populations.

Population Comment
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.
Ethnic groups
  1. Availability of laboratory apparatuses is critical to do duplicate diet assessment for specific nutrients and other chemicals.
  2. Ideally, assays should be implemented just after collection of the duplicate diets. This requirement depends on the nutrients of interest. For example, minerals do not degrade over time, whereas other nutrients that are subject to oxidation such as vitamin C, need to be assessed quickly after collection.
  3. 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.
  1. This method requires sophisticated chemical analysis of foods which will usually be done on a consultancy basis by a specialist laboratory
  2. 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
  3. 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.

  1. Abdulla M, Andersson I, Asp NG, Berthelsen K, Birkhed D, Dencker I, et al. Nutrient intake and health status of vegans. Chemical analyses of diets using the duplicate portion sampling technique. Am J Clin Nutr. 1981;34(11):2464-77.
  2. De Filippis SP, Brambilla G, Dellatte E, Corrado F, and Esposito M, Exposure to polychlorinated dibenzo- p -dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), dioxin-like polychlorinated biphenyls (DL-PCBs) and polybrominated diphenyl ethers (PBDEs) through the consumption of prepared meals in Italy, Food Additives & Contaminants: Part A, 2014;(31(6): 1114-1126.
  3. ILightowler HJ, Davies GJ. Iodine intake and iodine deficiency in vegans as assessed by the duplicate-portion technique and urinary iodine excretion. Br J Nutr. 1998;80(6):529-35.
  4. Lightowler HJ, Davies GJ. Assessment of iodine intake in vegans: weighed dietary record vs duplicate portion technique. Eur J Clin Nutr. 2002;56(8):765-70.
  5. Pennington J. Proceedings of the workshop on food-consumption surveys in developing countries: general approaches to estimation of dietary exposure to contaminants. Food Nutr Bull. 2004;25(4):420-7.
  6. Ritchie MR, Morton MS, Deighton N, Blake A, Cummings JH. Plasma and urinary phyto-oestrogens as biomarkers of intake: validation by duplicate diet analysis. Br J Nutr. 2004;91(3):447-57.
  7. Trijsburg L, de Vries JH, Boshuizen HC, Hulshof PJ, Hollman PC, van 't Veer P, Geelen A, Comparison of duplicate portion and 24 h recall as reference methods for validating a FFQ using urinary markers as the estimate of true intake, Br J Nutr, 2015;114(8):1304-12, PMID: 26314241
  8. Trijsburg L, de Vries JH, Hollman PC, Hulshof PJ, van 't Veer P, Boshuizen HC, Geelen A, Validating fatty acid intake as estimated by an FFQ: how does the 24 h recall perform as reference method compared with the duplicate portion?, Public Health Nutr, 2018;21(14):2568-2574, PMID: 29734960
  9. Kim WW, Mertz W, Judd JT, Marshall MW, Kelsay JL, Prather ES, Effect of making duplicate food collections on nutrient intakes calculated from diet records, Am J Clin Nutr,1984;40(S6):1333-7, PMID: 6507354