Appendix X - Misreporting – doubly labelled water study NI Report NDNS 2019-2023
Source: data.gov.uk (2025-10-10)

National Diet and Nutrition Survey 2017 to 2023 Report for Northern Ireland. This report presents results from 2017-2023 of the National Diet and Nutrition Survey (NDNS) for Northern Ireland. NDNS is a continuous cross-sectional survey, designed to assess the diet, nutrient intake and nutritional status of the general population aged 18 months and over living in private households in the UK.

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
Appendix X Misreporting in the 
National Diet and Nutrition Survey 
2019 to 2023: summary of results 
and their interpretation  
X.1 Introduction 
This appendix presents an overview of methods and results from the doubly labelled 
water (DLW) sub-study of the National Diet and Nutrition Survey (NDNS) carried out 
during years 12 to 14 (2019 to 2022) along with a summary of considerations 
relevant to the interpretation of these results.  
If a healthy adult participant is in energy balance their habitual energy intake (EI) 
equals their habitual total energy expenditure (TEE). The DLW method is an 
established method widely agreed to be the most accurate way of measuring TEE in 
free-living individuals over one to 2 weeks and hence providing the ability to assess 
misreporting of EI (Barrie and Coward, 1985; Bluck, 2008). Even though growing 
children, and adults losing or gaining weight intentionally or unintentionally, are by 
definition, not in energy balance, the DLW method can still be used to assess TEE in 
such individuals. 
 
When growth rates are not extremely rapid, for example in older children, correcting 
for weight change during DLW measurement has been found to make only a very 
small difference to calculated CO2 production rate (and therefore TEE) (IDECG 
Working Group, 1990). 
 
The methodology is objective and robust and demands relatively little from the 
participant. The method uses an oral dose of DLW, that is, water enriched in two 
naturally occurring stable isotopes, hydrogen (2H, deuterium) and oxygen (18O). By 
following the excretion of these isotopes from the body, through analysis of samples 
of body water (typically urine) over the subsequent 7 to 14 days, a mean daily rate of 
CO2 production is obtained for the participant. From this average a daily TEE can be 
calculated which comprises the energy expended on basal metabolism, digestion 
and metabolism of food, and on physical activity. In brief, the method works as 
follows: the ingested DLW equilibrates with the total body pool of water, from which 
the rate of disappearance (r) of 2H from the body represents water (2H2O) lost, for 
example in urine, breath, sweat, and breast milk. The rate of disappearance of 
oxygen-18 (18O) represents the sum of both water (H218O) loss and carbon dioxide 
(C18O2) loss in breath. Rapid exchange and equilibrium of 18O between water, and 
carbon dioxide in body fluids, occurs via the action of the enzyme carbonic 
anhydrase in red blood cells and the lungs. The difference between these rates 
therefore equates to CO2 production (that is [rH2O+rCO2] – [rH2O] = rCO2). TEE can 
be calculated from CO2 production using standard respiratory equations because 
there is a known amount of heat (energy) associated with each litre of CO2 produced 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
during metabolism. The exact amount of CO2 produced depends on the composition 
of the diet; that is the mixture of carbohydrate, fat, protein and alcohol consumed. It 
should be noted that the DLW method gives an integrated estimate of TEE for the 
period of measurement and not data for individual days. 
A DLW sub-study has been an integral part of the UK NDNS since its inception and 
the NDNS is one of the few national large-scale population nutrition surveys to 
include this method. Since the start of the NDNS programme in 2008, a DLW sub-
study has taken place once every 5 year contract phase to assess the degree of 
misreporting of EI estimated from reported food consumption. Previous sub-studies 
carried out in years 1 and 3 (2008 to 2009 and 2010 to 2011) and years 6 and 7 
(2013 to 2014 and 2014 to 2015) compared TEE with EI from the 4-day paper diary. 
Appendix X of the NDNS years 1 to 4 report and appendix X of the UK years 1 to 9 
report provide the results for these sub-studies. 
With the change in dietary assessment method in year 12 of the NDNS from the 
paper diary to the online 24-hour recall tool, Intake24, the DLW sub-study in years 
12 to 14 assesses for the first time the degree of misreporting of EI using Intake24.  
X.2 DLW sub-study sample 
In previous surveys, the aim was to obtain valid samples for approximately 10% of 
the core survey participants, aged 4 years and over, who completed a minimum of 3 
of the four food diary days. For years 1 and 3 this was completed with the aim of 
obtaining valid samples for 20 participants per age and sex groups per year. As the 
degree of misreporting was observed to be greatest in the 16 to 49 years groups, the 
strategy was amended for Years 6 and 7 as follows: 4 to 10 years (n=60), 11 to 15 
years (n=80), 16 to 49 years (n=100), 50 to 64 years (n=80) and 65 years and over 
(n=60) with equal numbers within group for each sex. In order to achieve the valid 
sample target, all age and sex groups were over-recruited with sample analysis 
occurring simultaneously where possible. 
Due to the high cost of the DLW protocol and increasing budget pressures within the 
survey, the participant number targets for valid samples for years 12 to 14 were 
reduced compared to previous years to enable the sub-study to run within the 
available funding. A power calculation was undertaken to assess (i) the level of 
concordance (EI:TEE) that could be detected as statistically significant, and (ii) the 
degree of change in concordance possible to detect as statistically significant 
between the years 12 to 14 DLW sub-study and the NDNS DLW sub-studies in 
earlier years. This informed the sample size for achieved valid DLW samples for 
years 12 to 14 that were feasible within budget constraints. A sample size of 280 
was selected as this enabled the DLW sub-study to detect a EI:TEE of 0.83 and a 
change in EI:TEE between Intake24 and the previous paper diary method of 0.14 
within each age and sex group as statistically significant. Recruitment targets to 
achieve the required number of valid DLW samples were set as follows: 4 to 10 
years (n=40), 11 to 15 years (n=60), 16 to 49 years (n=80), 50 to 64 years (n=60) 
and 65 years and over (n=40), with equal numbers within group for each sex.  
In previous surveys, recruitment for the DLW sub-study was carried out 
approximately 2 to 3 weeks following the completion of the food diary. In years 1 and 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
3, the interviewer selected those participants they thought would fully complete the 
DLW protocol. In years 6 and 7, this changed such that participants were simply 
asked on a ‘first come first served’ basis, as long as their age and sex group cell had 
not been filled. For the present DLW sub-study, participants were recruited as in 
years 6 and 7; however, they were recruited following completion of the first dietary 
recall using Intake24 to increase overlap between the DLW measurement and 
dietary assessment. 
Interviewers invited participants who had completed the first dietary recall; had 
height and weight measured and were not actively trying to lose or gain weight, to 
take part in the DLW protocol until the quota for each age and sex group was filled. 
In year 12 height and weight were measured at the interviewer visit. For years 13 
and 14 when interviewing took place remotely due to COVID-19 protocols, height 
and weight were self-reported. However, it was stipulated that height and weight had 
to have been measured during the last month and not estimated. 
Each age and sex group was slightly over-recruited to allow for drop out and 
unusable samples.  
X.3 Overview of DLW methods in the NDNS   
X.3.1 Isotope dosing and sampling 
Each participant was asked to provide a baseline urine sample before receiving a 
weighed oral dose of 2H218O (Day 0). The dose was equivalent to 80 mg·kg-1 body 
mass deuterium oxide (D2O, 99.9atom%, Cambridge Isotope Laboratories, Inc., MA, 
USA) and 150 mg·kg-1 body weight of H218O (H218O, 10atom%, Taiyo Nippon Sanso 
Corporation, Tokyo, Japan).  
Participants were asked to collect a single sample of their urine every day for a total 
of 10 days following the day of dosing and were asked not to collect samples from 
the first void of the day. The date and time of sample collection was noted by the 
participant in a log sheet. Urine samples were stored in 7ml glass bijou vials in the 
participants’ fridge, until the end of the 10-day collection. They were then collected 
by the interviewer and posted back to MRC Epidemiology Unit where they were 
frozen at -20°C pending analysis. Isotopic enrichments of the dose provided and of 
the urine samples were analysed using continuous flow isotope-ratio mass 
spectrometry (IRMS), described in section X.4.2. 
 
 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
X.3.2 Isotopic analyses 
Measurements of deuterium enrichment of the samples were made using a Sercon 
ABCA-Hydra 20-22 instrument (Sercon Ltd, Crewe, Cheshire, UK).1  This was done 
by equilibration of a 400 µL aliquot of urine with hydrogen gas over a platinum 
catalyst. A 500 µL aliquot of the sample and equilibration with 5% CO2 (Roether 
1970) was used to determine the oxygen isotopic composition of the urine samples. 
Analysis was completed using a Sercon ABCA continuous flow IRMS (Sercon Ltd, 
Crewe, Cheshire, UK). In all cases analytical standards prepared in house and 
traceable to the international standards Vienna Standard Mean Ocean Water (V-
SMOW2) and Standard Light Arctic Precipitation (V-SLAP2) were included in each 
batch of samples analysed.  
For the DLW sub-studies in years 1 and 3 and years 6 and 7, the isotope enrichment 
analysis took place at MRC Elsie Widdowson Laboratory (EWL; formerly MRC 
Human Nutrition Research). During 2018, the stable isotope facility at EWL was 
transferred to the Nutritional Biomarker Laboratory, MRC Epidemiology Unit, 
University of Cambridge. In order to ensure continuity of measurement in the NDNS 
and comparison of TEE measurements between survey years, a partial re-validation 
and cross validation of the stable isotope analysis methods was conducted. The aim 
of this work was to determine the precision and accuracy of the deuterium and 18O 
analysis, and the reproducibility of the results with respect to previously measured 
NDNS year 6 (2013 to 2014) samples. Accuracy was demonstrated using 
international reference material, and re-analysis of samples demonstrated precision 
and consistency of analysis from earlier years. Further details on the experimental 
approach to the validation study and its results are available on request.  
 
X.3.3 Energy expenditure calculations 
TEE was calculated as described in the SACN dietary reference values for energy 
report (SACN, 2011) from slopes and intercepts of the isotope disappearance curves 
based on urine samples collected on days one to 10. Basal metabolic rate (BMR) for 
each individual was estimated using the Schofield equations (Schofield, 1985).  
Physical activity level (PAL) was expressed as TEE divided by BMR (Schofield 
1985). This ratio removes virtually all the differences between individuals due to sex, 
age and body size. 
X.3.4 DLW QC criteria 
On receipt, all samples were checked for appropriate time and date information. 
Each participant sample set was required to include a pre-dose sample and a 
minimum of 6 post-dose samples to be included for analysis.  
 
X.3.4.1 Analytical accuracy and precision: 
 
 
1 Deuterium analysis of Y12 urine samples was performed at Iso-Analytical Ltd, Crewe, UK using a 
Europa Scientific ANCA-GSL and 20-20 IRMS. Analysis of Years 13 and 14 samples was performed 
at the Nutritional Biochemistry Laboratory (NBL), MRC Epidemiology Unit, University of Cambridge, 
Cambridge, UK. 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
All samples were measured alongside 2 levels of secondary reference standards 
previously calibrated against the primary international standards Vienna-Standard 
Mean Ocean Water (vSMOW2) and Vienna-Standard Light Antarctic Precipitate 
(vSLAP2) (International Atomic Energy Agency, Vienna, Austria). Acceptable QC 
variation for deuterium is 5% and for 18-Oxygen 1%. All samples were measured in 
duplicate, with an acceptable difference of one ppm. Six discrete post-dose samples 
are required for modelling.  
 
X.3.4.2 Modelling acceptance criteria: 
 
Post modelling, valid data must meet the following 6 criteria defined in the SOP: 
1. A pre-dose and 6 discrete post-dose samples analysed for both deuterium 
and 18-oxygen enrichment 
2. R2 of the isotope disappearance regression lines must be greater than 0.99. 
3. Final sample enrichments are within 3 half-lives and for hydrogen the final 
sample enrichment must be higher than 128 ‰ 
4. Dilution space ratio lies between 1.00 and 1.07 
5. Rate constant ratio lies between 1.1 and 1.7 
6. The error of the model is less than 8%. 
X.4 Results of DLW analysis in NDNS years 12 to 14 (2019 
to 2022) 
X.4.1 Participants 
In total 344 eligible participants were recruited in the years 12 to 14 DLW sub-study.2 
Of these, 279 (82%) participants provided valid data for both TEE measured by DLW 
and EI estimated using data collected via Intake24 (see table X.1). The valid sample 
target was generally met in all groups except in males aged 50 to 64 years which 
was under target by 6, males and females aged 11 to 15 years which were under 
target by 1 and 3 respectively and females aged 65 years and over which was under 
target by 1 (see table X.1).  
 
 
 
2 For drop-out and unusable data numbers see the stage 2 evaluation report Appendix B3 participant 
flow chart  
 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
Table X.1 Number of valid DLW participants in NDNS years 12 to 14 (2019 to 2022) 
Age group 
(years) 
Sex 
Target3 
Year 12 
Year 13 
Year 14 
Total 
4 to 10  
Male 
20 
15 
4 
1 
20 
4 to 10  
Female 
20 
15 
6 
0 
21 
11 to 15  
Male 
30 
7 
15 
7 
29 
11 to 15  
Female 
30 
10 
11 
6 
27 
16 to 49  
Male 
40 
14 
21 
8 
43 
16 to 49  
Female 
40 
22 
22 
0 
44 
50 to 64  
Male 
30 
11 
10 
3 
24 
50 to 64  
Female 
30 
10 
17 
3 
30 
65 and over 
Male 
20 
11 
11 
0 
22 
65 and over 
Female 
20 
7 
11 
1 
19 
Total 
Male 
140 
58 
61 
19 
138 
Total 
Female 
140 
64 
67 
10 
141 
 
Of the 279 participants with valid data, 109 (39%) completed recalls and DLW 
concurrently, whilst the remaining 170 participants had a mean lag of 10 days (min = 
0, max = 51) between recall 4 and DLW dosing day. Of the 170 non-concurrent 
participants, 24 completed recall 4 on the same day as DLW dosing took place. Of 
the 279 participants with valid data: 274 completed all 4 dietary recalls; 3 participants 
completed 3 recalls; and 2 participants completed two recalls. 
 
3 Target is valid sample target 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
For drop-out and unusable data numbers see the participant flow chart in appendix 
B3 of the stage 2 evaluation report. 
 
Representativeness 
Analyses have been carried out to assess the representativeness of the DLW sub-
sample in relation to the achieved core survey sample in years 12 to 14. The 
following variables were assessed: BMI (kg per m2), total EI (MJ per day), total fruit 
and vegetables consumption (g per day), free sugars intake (% of total energy) and 
saturated fatty acids intake (% of total energy). Figures X.1 to X.5 do not show any 
clear observed differences between the DLW sub-sample and the main survey 
sample responses, indicating that the DLW sub-sample is representative of the main 
NDNS sample with respect to these measures.  

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
Figure X.1 
Representativeness plot for BMI of valid DLW participants versus main NDNS participants in years 12 to 14 
(2019 to 2022)

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
Figure X.2 
Representativeness plot for total energy intake of valid DLW participants versus main NDNS participants in 
years 12 to 14 (2019 to 2022)

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
Figure X.3 
Representativeness plot for total fruit and vegetables consumption of valid DLW participants versus main 
NDNS participants in years 12 to 14 (2019 to 2022)

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
Figure X.4 
Representativeness plot for free sugars intake of valid DLW participants versus main NDNS participants in 
years 12 to 14 (2019 to 2022)

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
Figure X.5 
Representativeness plot for saturated fatty acids intake of valid DLW participants versus main NDNS 
participants in years 12 to 14 (2019 to 2022)

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
X.4.2 Misreporting 
Determination of adequacy of dietary reporting for a group of individuals is based on 
the ratio of reported EI and measured TEE. Because of the variability of EI and TEE, 
an individual may not be in perfect energy balance at any given time and EI:TEE will 
not equal 1.0. For some individuals their ratio at that time will be less than 1.0 and for 
some it will be greater than 1.0; but for a group, the expectation is that the mean 
ratio will be 1.0. Where the mean ratio for a particular group is lower than 1.0, this 
indicates a discrepancy between mean reported EI and estimated TEE, potentially 
due to underreporting of food intake or undereating during the dietary intake 
assessment period. 
Tables X.2 and X.3 present the mean values for reported EI and estimated TEE 
along with the ratio (EI:TEE) and mean difference (TEE-EI) for the DLW sub-study 
carried out in years 12 to 14 (2019 to 2022).  
The results of the analysis indicate good agreement between mean reported EI and 
mean estimated TEE in young children and less good agreement in older children 
and adults. Overall, in combined age and sex groups mean EI:TEE was 0.70. Mean 
EI:TEE was 0.63 for males and 0.67 for females aged 16 to 64 years (table X.2). 
Mean EI:TEE ranged from 0.60 for males aged 16 to 49 years at the lowest to 0.84 
for boys aged 4 to 10 years at the highest (table X.3). For all age and sex groups 
mean EI:TEE was significantly different to 1.0 (p<0.05). 
Table X.2 
Mean values of reported EI and estimated TEE (kcal) in the DLW 
sub-study years 12 to 14 (2019 to 2022): combined age groups 
Age group 
(years) 
Sex 
N 
EI (kcal) 
TEE (kcal) 
TEE-EI 
EI:TEE 
4 and over 
Males 
138 
1896 
2903 
1006 
0.67 
4 and over 
Females 
141 
1592 
2291 
699 
0.72 
4 and over 
Sex-
combined 
279 
1743 
2594 
851 
0.70 
16 and over 
Males 
89 
2003 
3200 
1197 
0.63 
16 and over 
Females 
93 
1631 
2442 
812 
0.68 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
16 and over 
Sex-
combined 
182 
1813 
2813 
1000 
0.66 
16 to 64  
Males 
67 
2073 
3348 
1275 
0.63 
16 to 64  
Females 
74 
xx-xxxx-xxxx 
2540 
856 
0.67 
16 to 64  
Sex-
combined 
141 
1869 
2924 
1055 
0.65 
Table X.3 
Mean values of reported EI and estimated TEE (kcal) in the DLW 
sub-study years 12 to 14 (2019 to 2022) 
Age group 
(years) 
Sex 
N 
EI (kcal) 
TEE (kcal) 
TEE-EI 
EI:TEE 
4 to 10  
Males 
20 
1518 
1908 
390 
0.84 
4 to 10  
Females 
21 
1340 
xx-xxxx-xxxx 
341 
0.82 
4 to 10  
Sex-
combined 
41 
1427 
1792 
365 
0.83 
11 to 15  
Males 
29 
1831 
2677 
845 
0.69 
11 to 15  
Females 
27 
1655 
2243 
589 
0.77 
11 to 15  
Sex-
combined 
56 
1746 
2468 
722 
0.73 
16 to 49  
Males 
43 
1995 
3369 
1373 
0.60 
16 to 49  
Females 
44 
1710 
2535 
826 
0.69 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
16 to 49  
Sex-
combined 
87 
1851 
2947 
1096 
0.64 
50 to 64  
Males 
24 
2213 
3312 
1099 
0.67 
50 to 64  
Females 
30 
xx-xxxx-xxxx 
2547 
901 
0.65 
50 to 64  
Sex-
combined 
54 
1898 
2887 
989 
0.66 
65 and over 
Males 
22 
1787 
2749 
962 
0.66 
65 and over 
Females 
19 
1425 
2062 
637 
0.70 
65 and over 
Sex-
combined 
41 
1619 
2430 
811 
0.67 
 
X.5 Discrepancy between mean values of reported energy 
intake and estimated energy expenditure in the NDNS  
For combined age and sex groups within NDNS years 12 to14, underreporting of EI 
is approximately 30%. Previous NDNS DLW sub-studies, which used a paper-based 
diary method, have shown underreporting of EI of approximately 29%, very similar to 
that seen using Intake24. For further information and results regarding the 
comparison see the stage 2 evaluation report.  
Misreporting in self-reported dietary methods is a well-documented issue (Archer, 
Hand and Blair, 2013). A number of different factors may contribute to why mean 
reported EI is lower than estimated TEE in the NDNS, including conscious or 
unconscious participant underreporting. Misreporting is difficult to address due to a 
variety of factors, including: 
• memory, which is subject to recall bias 
• social desirability bias, where people may consciously or sub-consciously 
over- or under- report some foods (for example those perceived as healthy or 
unhealthy) 
• portion sizes, portion estimation can be difficult to report and its assessment 
can be affected by subjective and cognitive factors  
• food composition assumptions 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
• dietary complexity and range, including for foods and meals with many 
ingredients, and where eaten in different settings  
These factors have been discussed previously in the NDNS RP years 1 to 9 report.  
A report by Thane and Stephen (2006) has shown that reported EI is higher on 
Saturdays and to some extent on Fridays and Sundays in some age groups. Since 
the measurement of TEE by DLW always covered at least one weekend day and the 
estimate of dietary EI in the NDNS did not necessarily include weekend days, the 
question may be raised as to whether this might explain some of the difference 
between reported EI and estimated TEE. For the DLW sub-study, 30% of 
participants did not report EI for Saturday or Sunday. This was because the aim 
within NDNS years 12 to 15 was to collect EI data across an even spread of days 
which, if all participants completed 4 recalls, would mean 26% would not have a 
recall for Saturday or Sunday. Therefore, day of the week may be a factor 
influencing the difference between EI and TEE.   
A further factor affecting misreporting could be that EI and TEE data are not 
collected over the same time period. As has been reported here only 39% of the 
participants completed EI assessment and TEE measurements concurrently.  
 
X.6 Application of the DLW method in the NDNS  
Biases such as underreporting are inherent in self-reported dietary data but remain 
an area of ongoing concern and warranting further investigation. As TEE using DLW 
was only estimated in a sub-sample of the NDNS, self-reported energy intake has 
not been adjusted in this report.  
References 
Barrie A and Coward WA (1985) A rapid analytical technique for the determination of 
energy expenditure by the doubly labelled water method.Biomed Mass Spectrom 12, 
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Bluck L (2008) Doubly labelled water for the measurement of total energy 
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xx-xxxx-xxxx. 
 
The Doubly-labelled Water Method for Measuring Energy Expenditure. Technical 
recommendations for use in humans. A consensus Report by the IDECG Working 
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Roether W (1970) Water-CO2 exchange set-up for the routine 18oxygen assay of 
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Scientific Advisory Committee on Nutrition (2011). Dietary Reference Values for 
Energy. TSO: London.  
 
Schofield W (1985) Predicting basal metabolic rate, new standards and review of 
previous work. Human Nutrition: Clinical Nutrition 39C, 5-40. 

National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15) 
 
 
 
 
Archer, E, Hand GA, Blair, SN  Validity of U.S. Nutritional Surveillance: National 
Health and Nutrition Examination Survey Caloric Energy Intake Data, 1971-2010 
PLoS ONE, Vol 8, Issue 10, 9 Oct 2013. 
 
Thane CW, Stephen AM (2006). Day-to-day variation in food and nutrient intakes of 
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National Diet and Nutrition Survey: 2019 to 2023 (years 12 to 15)