The objective of the dietary interview component is to obtain detailed dietary intake information from NHANES participants. The dietary intake data are used to estimate the types and amounts of foods and beverages consumed during the 24-hour period prior to the interview (midnight to midnight), and to estimate intakes of energy, nutrients, and other food components from those foods and beverages. Following the dietary recall, participants are asked questions on water consumption during the previous 24 hours, salt use, whether the person’s intake on the previous day was usual or unusual, and whether the respondent is on any kind of diet. Selected population subgroups are asked questions on frequency of fish and shellfish consumed during the past 30 days.
The dietary interview component, called What We Eat in America (WWEIA), is conducted as a partnership between the U.S. Department of Agriculture (USDA) and the U.S. Department of Health and Human Services (DHHS). Under this partnership, DHHS’ National Center for Health Statistics is responsible for the sample design and data collection and USDA’s Food Surveys Research Group (FSRG) is responsible for the dietary data collection methodology, maintenance of the databases used to code and process the data, and data review and processing.
What’s New with the 2003-2004 WWEIA Release: A number of additions and changes have occurred since the release of the WWEIA 2001-2002 data. The table below summarizes these changes. One of the most important changes is the release of two days of intake data for each participant. The first day (Day 1) is collected in the Mobile Examination Center (MEC) and the second day (Day 2) is collected by telephone 3 to 10 days later. Most MEC participants (87 percent) have 2 days of complete and reliable intakes. The release of 2 days of data will permit the estimation of usual (long-run average) nutrient intakes in order to assess diets in the U.S. The Institute of Medicine recommends that assessment of the diets of population groups in relation to Dietary Reference Intakes be based on usual intake distributions of nutrients (Institute of Medicine, 2000). A minimum of two nonconsecutive days of dietary intake data for at least a subsample of the individuals is necessary for a more accurate estimation of the usual intake of nutrients.
Table 1. Changes between WWEIA 2001-2002 and WWEIA 2003-2004
|Number of days of intake data released
||Food energy and 60 nutrients/food components
||Food energy and 62 nutrients/food components. Added vitamin E and added vitamin B12 included.
|Food source (Where food obtained)
||Collected only in 2002; not publicly released.
||Collected and released.
|Combination food types
||Values for 14 combination types
||Values for 15 combination types; added “chips with additions”.
|Eating occasion names
||20 values; 3 existing values modified and 2 new values.
|Special diet variables
||Collected only in 2002; not publicly released.
||Collected and released.
Dietary Interview Data Files: Four data files were produced from the information collected in the dietary interview: two Total Nutrient Intake files and two Individual Foods files. Each file includes one day of intake data. The number “1” or “2” in the file name identifies the day (and mode) of the interview: 1 = first day (MEC), 2 = second day (phone). File names are the following:
File Names for Dietary Interview Data
|Individual Foods File
|Total Nutrient Intakes File
The nutrient amounts in these files reflect only nutrients obtained from foods and beverages, including sweetened water beverages. They do not include nutrients obtained from dietary supplements, medications, or plain drinking water.
Individual Foods Files (DR1IFF_C and DR2IFF_C): Contain detailed information about the types and amounts of individual foods reported by each participant, as well as amounts of nutrients from each food.
Two supporting files are also included with the Individual Foods Files: the Food Code Description file (DRXFCD_C) and the Modification Code Description file (DRXMCD_C). The DRXFCD_C file includes abbreviated descriptions (up to 60 characters) and complete descriptions (up to 200 characters) associated with each USDA food code identified in the Individual Foods Files. The DRXMCD_C file includes descriptions (up to 200 characters) associated with each modification code identified in the Individual Foods Files. Modification codes represent adjustments to predefined recipe ingredients that reflect more closely the food as described by the respondent. An appendix to the Individual Foods Files documentation provides SAS code examples that may be used to link the food code or the modification code description to the Individual Foods File.
Total Nutrient Intakes Files (DR1TOT_C and DR2TOT_C): Contain, for each participant, daily total energy and nutrient intakes from foods and beverages; the daily amount of water consumed; whether the amount of food consumed was usual, much more than usual, or much less than usual. The Day 1 file also includes information on salt use in cooking and at the table; whether the participant is currently on any kind of diet to lose weight or for another health-related reason and, if so, the type of diet; and for subsets of participants, information on frequency of fish and shellfish consumption. The names for both Day 1 and Day 2 variables are listed in Table 2.
This document (NHANES 2003-2004 Data Documentation for Total Nutrient Intakes Files) provides additional details important to understanding the content of the Total Nutrient Intakes Files (DR1TOT_C and DR2TOT_C). The Total Nutrient Intakes Files provide a summary record of total nutrient intakes for each individual. Each total intake record contains the following information:
- Number of days of complete intake
- Day of week of intake
- Daily aggregates of food energy and 62 nutrients/food components (listed in Table 3) from all foods, as calculated using USDA’s Food and Nutrient Database for Dietary Studies 2.0 (FNDDS 2.0)
- Total number of foods reported for that participant
- Whether the amount of food consumed was usual, much more than usual, or much less than usual
- Daily amount of water consumed (total plain water, total home tap water and source of tap water, total bottled water, and plain carbonated water)
- Type of salt used and how often added at the table and in food preparation (Day 1 file only)
- Whether the participant is currently on any kind of diet to lose weight or for another health-related reason and, if so, the type of diet (Day 1 file only)
- Frequency of fish and shellfish consumption in the past 30 days (children 1-5 years and women 16-49 years, Day 1 file only)
All NHANES participants are eligible for the dietary interview component. However, only children 1-5 years and women 16-49 years of age are eligible for the frequency of fish and shellfish consumption questions following the 24-hour recall and thus, their responses are collected.
Protocol and Procedure
The examination protocol and data collection methods are fully documented in the NHANES Dietary Interviewers Procedures Manuals (Centers for Disease Control and Prevention, 2002, Centers for Disease Control and Prevention, 2004).
Proxy interviews were conducted for survey participants less than six years of age. Assisted interviews were conducted with survey participants 6 to 11 years of age. Dietary interviews were conducted in English and Spanish. Translators were used to conduct interviews in other languages.
The in-person interview was conducted in a private room in the NHANES mobile examination center (MEC). A set of measuring guides (various glasses, bowls, mugs, household spoons, measuring cups and spoons, a ruler, thickness sticks, bean bags, and circles) was available in the MEC dietary interview room for the participant to use for reporting amounts of foods. Upon completion of the in-person interview, participants were given measuring cups, spoons, a ruler, and a food model booklet, which contained two-dimensional drawings of the various measuring guides available in the MEC, to use for reporting food amounts during the telephone interview. Telephone dietary interviews were collected 3 to 10 days following the MEC dietary interview but not on the same day of the week as the MEC interview. Any participant who did not have a telephone was given a toll-free number to call so that the recall could be conducted.
What We Eat in America data were collected using USDA’s dietary data collection instrument, the Automated Multiple Pass Method (AMPM) (Agricultural Research Service, USDA Automated Multiple-Pass Method for Dietary Recalls). The AMPM was designed to provide an efficient and accurate means of collecting intakes for large-scale national surveys. The AMPM is a fully computerized recall method that uses a 5-step interview outlined below:
- Quick List - Participant recalls all foods and beverages consumed the day before the interview (midnight to midnight).
- Forgotten Foods - Participant is probed for foods forgotten during the Quick List step.
- Time and Occasion - Time and eating occasion are collected for each food.
- Detail Cycle - For each food, a detailed description, amount eaten, and additions to the food are collected. Eating occasions and times between eating occasions are reviewed to elicit forgotten foods.
- Final Probe - Additional foods not remembered earlier are collected.
The AMPM includes an extensive compilation of standardized food-specific questions and possible response options. Routing of questions is based on previous responses. The AMPM is updated yearly to reflect the changing food supply and to address research needs from the data user community. Additional information about the AMPM is provided in Raper et al. (Raper, 2004).
The accuracy of the AMPM is currently being assessed in the USDA AMPM Validation Study using biomarker data. The data collection phase of the study, which includes 525 participants, has been completed. The extent of misreporting of energy and protein intakes will be determined by comparing estimated energy intake with total energy expenditure, and estimated protein intake with urinary nitrogen excretion. Total energy expenditure has been measured by the doubly-labeled water method. Preliminary findings show that the use of the AMPM resulted in a mean energy intake for the first cohort of 100 subjects that was within 2 percent of their total energy expenditure, as estimated by the doubly-labeled water technique, and suggest acceptable accuracy of reported intakes (Moshfegh, 2003, Rhodes, 2004).
For the procedures relevant to this component, please go to Survey Operations Manual, Consent Documents, Brochures at: http://www.cdc.gov/nchs/about/major/nhanes/nhanes2003-2004/current_nhanes_03_04.htm.
Quality Assurance & Quality Control
All dietary interviewers were required to complete an intensive one-week training course and to conduct supervised practice interviews before working independently in the field. Retraining sessions were conducted periodically and annually to reinforce the proper protocols and technique.
Interviewers were monitored throughout the data collection period. Monitoring consisted of the following:
- Reviews of data transmittal sheets were used to verify receipt of data files.
- Reviews of audiotaped interviews or in-person observations were conducted for approximately 5% of each interviewer’s work.
- Interviews were checked for completeness of the recalls, missing information, inconsistent reports, and unclear notes. Written notification and feedback were provided to the interviewers.
Data Processing and Editing
Interview data files were sent electronically from the field and were imported into Survey Net, a computer-assisted food coding and data management system developed by USDA (Agricultural Research Service, USDA Automated Multiple-Pass Method for Dietary Recalls).
USDA's Food and Nutrient Database for Dietary Studies, 2.0 (FNDDS 2.0), was used for processing the 2003-2004 intakes (Agricultural Research Service, USDA Food and Nutrient Database for Dietary Studies 2.0). The FNDDS includes comprehensive information that can be used to code individual foods and portion sizes reported by participants and also includes nutrient values for calculating nutrient intakes. The underlying nutrient values for FNDDS 2.0 were based on values in the USDA National Nutrient Database for Standard Reference, release 18, produced by USDA’s Nutrient Data Lab (Agricultural Research Service, USDA National Nutrient Database for Standard Reference, Release 18). The FNDDS may be used in research projects using the NHANES 2003-2004 dietary intake data and also in other food intake studies. Additional information about the FNDDS and related tools is available on the Food Survey Research Group website (Agricultural Research Service, USDA Food and Nutrient Database for Dietary Studies 2.0).
Coders were required to pass a certification test after the initial training. They were routinely monitored to ensure quality and completeness of their work. Approximately 10 percent of the coder’s work was double-coded and adjudicated, if necessary.
After intake data were coded, various types of reviews were conducted to ensure the quality of the data. An overview of quality assurance procedures conducted during the data processing stage is available in Anand and Raper (Anand, 2006). Examples of reviews include the following:
- Overall acceptability of each recall. This review determined if the recall met minimum criteria listed below. A recall was considered unacceptable if it failed to meet either of these criteria:
- The first 4 steps of the 5-step AMPM are completed. Failure to meet this criterion occurs infrequently and is usually due to the participant stopping the interview before completion of the fourth step. This step collects the details (description and amount consumed) for each reported food.
- Foods consumed for each reported meal must be identified.
- Interviewers’ and coders’ questions and comments are reviewed to ensure that they have been accounted for in coding.
- Decisions are made about how to code new or unusual foods or food quantities reported by participants.
Foods or portions that could not be matched to items in the database are resolved by FSRG scientists. New food items and new portion sizes are added as needed to the FNDDS. Information about new foods and package sizes are collected using internet resources, direct contact with manufacturers, or food labels. Unusual food mixtures are coded using multiple food codes to represent the mixture, linking those foods with a combination food number, and specifying the type of food mixture (such as a salad or soup).
- Specific data integrity checks for reasonableness, consistency, and logic.
Many quality control features are built into the AMPM software, including automated routing based on the participant's previous answers and extensive checks which prevent illogical responses. Nevertheless, over 50 unique checks are conducted across all dietary data. Examples are meals reported at unusual times (e.g., school lunch at 12:30 a.m.), foods not usually consumed by certain age groups (e.g., baby foods consumed by respondents over 2 years of age), and extremely large quantities of foods.
- Intakes with extreme levels for individual nutrients. Nutrient intakes are reviewed separately for various age and sex groups. Unusual values are examined and corrected when necessary.
During data processing, the following edits were made to ensure the logical consistency and analytic usefulness of the data:
- Adjusted sodium values for certain foods.
Sodium values for home-prepared foods are based on the sodium values of recipe ingredients in the FNDDS. In some cases, the amount of salt in recipes was reduced or eliminated if the participant answered dietary interview questions about salt use in cooking or preparing foods with the response "occasionally", "rarely” or “never," respectively.
- Nutrient values for some food mixtures modified.
During the food coding process, predefined recipes for some food mixtures are modified to match more closely the food as described by the respondent. Nutrients are modified by substituting ingredients in a predefined recipe for the mixture. An example of a modified recipe is an egg fried in butter instead of margarine. Each modification is assigned a unique 6-digit identification number. Recipe modification numbers appear in the variable DR1MC in the DR1IFF_C file and in the variable DR2MC in the DR2IFF_C file. Descriptions for each modification are provided in a separate file called DRXMCD.
Each Total Nutrient Intakes File (Day 1 and Day 2) contains one record for each participant. These files can be linked with other NHANES files by the respondent sequence number (SEQN).
Variable names: For data collected on both Day 1 and Day 2, variable names are differentiated by having the number “1” or “2” in the third position of the variable name to identify the collection day. For example, the name for the intake day of week is DR1DAY in the Day 1 file and DR2DAY in the Day 2 file. Table 2 lists the Day 1 and Day 2 variable names.
Names for the following variables are the same for both days:
|Day 1 and Day 2 variable name
||Respondent sequence number
||Dietary day one sample weight
||Dietary two-day sample weight
||Breast-fed infant (either day)
||Number of days of intake
Number of Intake Days Variable: Because two days of data are included in the 2003-2004 release, a variable has been added to indicate the number of days of intake available for each participant. The variable name is DRDINT.
Dietary Recall Status Code: A status code (DR1DRSTZ or DR2DRSTZ) is used in the file to indicate the quality and completeness of a survey participant's response to the dietary recall section. The codes are the following:
1 = Reliable and met the minimum criteria
For individuals with a code 1, all relevant variables associated with the 24-hour dietary recall contain a value.
2 = Not reliable or did not meet the minimum criteria
Individuals with a code 2 have incomplete records. No data on total nutrient intakes and the total number of foods are provided for these cases.
4 = Reported consuming breast milk
In the Individual Foods Files, records containing reports of human milk have missing values for the amount consumed and for the amounts of energy and nutrients from human milk. Because of the missing nutrient information for human milk, no total nutrient intakes were derived for participants with a code 4. Records for any other foods and beverages consumed by breast-fed infants and children are included in the Individual Foods Files along with their amounts and nutrient information.
A variable that identifies breast-fed children, DRABF, is included with the 2003-2004 release. This variable has a code of 1 if a child consumed breast milk in either intake day.
5 = Not done
This code is assigned when the dietary recall section of the interview did not take place due to various reasons (such as arrived late/left early, refusal, illness, emergency, or equipment failure). These individuals have a record in the Total Nutrients File with values only in the following variables: the respondent sequence number (SEQN), the dietary recall status code (DR1DRSTZ or DR2DRSTZ) and for children 1-5 and females 16-49 years old, the fish and shellfish questions in the DR1TOT_C file (DRD340, DRD350A-K, DRD350AQ-JQ, DRD360, DRD370A-V, and DRD370AQ-UQ)
In addition to the status code described above, the variable DR1_300 and DR2_300 denotes the participant’s assessment of whether the amount of food consumed on the recall day was usual, much more than usual, or much less than usual.
Participants Reported Fasting: Three participants reported fasting during one of their intake collection days. Their dietary recall status for the fasting day is coded as “1” (complete and reliable). The total number of foods reported and all total nutrient intakes variables are coded as “0”. Values are present for other variables collected after the dietary recall, such as water consumption. By definition, no individual food consumption is reported in these cases, therefore, no records were included in the Individual Foods File for these individuals for the specific fasting day.
Special Diet Variable: This is the first WWEIA NHANES data release to include information on whether the participant is currently on any kind of diet to lose weight or for another health-related reason and, if so, the type of diet. The variable DRQSDIET identifies whether a participant is on a special diet. The variables DRQSDT1 through DRQSDT8 and DRQSDT91 identify the type of diet(s) that the participant is following. This information was also collected in 2002 but was not publicly released because of confidentiality issues concerning single-year data.
Note that responses to the type of diet were collected as "code all that apply”. A participant could report more than one type of diet, and all responses were recorded. The variable DRQSDT1 denotes the type of diet the participant followed specifically for weight loss purposes, which includes a variety of low calorie diets, low carbohydrate diets and/or high protein diets. If the participant reported being on a high protein diet for the purpose of gaining weight or muscle building instead of weight loss, the response was coded in variable DRQSDT8 (Weight gain/Muscle building diet).
Sample weights for dietary intake data: The NHANES participants were selected on the basis of a national probability design. In order to increase the number of participants for specific demographic groups, a multi-stage, unequal probability of selection design was implemented. The NHANES oversamples blacks, Mexican Americans, low income whites, adolescents 12-19 years, and persons 60 years and older. Sample weights are constructed that encompass the unequal probabilities of selection, as well as adjustments for non-participation by selected sample persons. In order to produce national, representative estimates, the appropriate sample weights must be used.
For the 2003-2004 NHANES, there were 12,761 persons selected; of these 9,643 were considered respondents to the MEC examination and data collection. However, only 9,034 of the MEC respondents provided complete dietary intakes for Day 1. Furthermore, of those providing the Day 1 data, only 8,354 provided complete dietary intakes for Day 2.
Most analyses of NHANES data use data collected in the MEC and the variable WTMEC2YR should be used for the sample weights. However, for the dietary data, different sample weights are recommended for analysis. Although attempts are made to schedule MEC exams uniformly throughout the week, proportionally more exams occur on weekend days than on weekdays. Because food intake can vary by day of week, use of the MEC weights would disproportionately represent intakes on weekends.
A set of weights WTDRD1 is provided that should be used when an analysis uses the Day 1 dietary recall data (either alone or when Day 1 nutrient data are used in conjunction with MEC data). The set of weights WTDRD1 is applicable to the 9,034 respondents with Day 1 data. Day 1 weights were constructed by taking the MEC sample weights (WTMEC2YR) and further adjusting for (a) the additional non-response and (b) the differential allocation by day of the week for the dietary intake data collection. These Day 1 weights are more variable than the MEC weights, and the sample size is smaller, so estimated standard errors using Day 1 data and Day 1 weights are larger than standard errors for similar estimates based on MEC weights.
When analysis is based on both days of dietary intake, only the 8,354 sample persons have valid data. The NHANES protocol requires an attempt to collect the second day of dietary data at least 3 days after the first day, but the actual number of days between the two days is variable. A set of adjusted weights, WTDR2D, is to be used only when analysis uses both Day 1 and Day 2 dietary data. This two-day weight was constructed for the 8,354 respondents by taking the Day 1 weights (WTDRD1) and further adjusting for (a) the additional non-response for the second recall and (b) for the proportion of weekend-weekday combinations of Day 1 and Day 2 recalls.
Note that all sample weights are person-level weights and each set of weights should add to the same population control total as the MEC weights (WTMEC2YR). In addition, the MEC weights (WTMEC2YR) are appropriate for use in the analysis of the fish and shellfish consumption data (i.e., variables DRD340, DRD350A-K, DRD350AQ-JQ, DRD360, DRD370A-V, and DRD370AQ-UQ) located in the Day 1 Total Nutrient Intake File (DR1TOT_C), if no other dietary data are included in the analysis. Additional explanation of sample weights and appropriate uses are included in the NHANES Analytic Guidelines. Please also refer to the Analytic Guidelines for further details on other analytic issues at: