NHANES National Youth Fitness Survey

The objective of the dietary interview component is to obtain detailed dietary intake information from NHANES National Youth Fitness Survey (NNYFS) participants. The dietary intake data are used to estimate the types and amounts of foods and beverages (including all types of water) 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 salt use, whether the person’s overall intake on the previous day was much more than usual, usual or much less than usual, and whether the respondent is on any type of special diet. The NNYFS dietary interview was conducted in the same manner as the What We Eat in America (WWEIA), NHANES 2011-2012 dietary interview; the format of the dietary intake files are consistent with WWEIA data files.

The NNYFS dietary interview component (like WWEIA, NHANES), 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 (NCHS) is responsible for the survey sample design and all aspects of 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.

All NNYFS participants are eligible for one 24-hour dietary recall interview. The dietary recall interview is conducted in-person in the NNYFS Mobile Examination Center (MEC).

The foods and beverages reported in the NNYFS are detailed in the Food and Nutrient Database for Dietary Studies 2011-2012 (FNDDS 2011-2012). The FNDDS 2011-2012 also contains all foods and beverages reported in the WWEIA, NHANES 2011-2012 dietary interview. Two types of dietary intake files are available for NNYFS, 2012: Individual Foods file and Total Nutrient Intakes file.

The What We Eat in America Food Categories, available on the FSRG website (www.ars.usda.gov/ba/bhnrc/fsrg), are a grouping scheme that combines foods and beverages together that have similar usage and nutrient content with the emphasis on how they are commonly consumed in the American diet. The Food Categories include foods reported in WWEIA, NHANES, and NNYFS (“WWEIA Food Categories”). There are approximately 150 unique categories and each is assigned a 4-digit number and description. The WWEIA Food Categories contain discrete food items and are not disaggregated (e.g., pizza vs. grain, cheese, tomatoes, etc.). Designed to be flexible, the categories can be combined as needed to address specific research questions. A new version of the WWEIA Food Categories is produced for each 2-year release cycle of WWEIA, NHANES.

Distinguishing Between Foods/Beverages and Dietary Supplements in NNYFS

The 24-hour dietary supplement use component is administered after the 24-hour dietary recall. All NNYFS participants responding to the 24-hour dietary recall interview are eligible for the dietary supplement and non-prescription antacid use questions. Information is obtained on all vitamins, minerals, herbals, and other dietary supplements as well as non-prescription antacids that were consumed during a 24-hour time period (midnight to midnight), including the name and the amount of supplement or antacid taken.

Distinguishing between foods/beverages and supplements can be challenging. NCHS and FSRG review questionable items reported in the dietary supplement and dietary recall components to resolve disposition of these items into the appropriate component. Products that are labeled as a dietary supplement, that have a supplement facts panel on the label, and are in tablets, capsules, softgels, gelcaps, or other pill forms, are considered dietary supplements. Items that are powders or liquids can be hard to distinguish. General guidelines used state that if powders and liquid concentrates have product directions stating that they be added to a liquid, they are classified as beverages. Examples are teas and protein powders. An exception is made for fiber products, which are classified as dietary supplements. Along this same guideline, energy drinks are considered beverages, but “energy shot” type products are considered dietary supplements.

It is best to refer to the two databases that detail every food/beverage and dietary supplement reported in NNYFS to identify exact determination used. The food and beverage database used for NNYFS is the same as that used for WWEIA, NHANES 2011-2012; the NNYFS dietary supplements database is the same as that used for NHANES dietary supplements use. The databases are:

• 2011-2012 Food and Nutrient Database for Dietary Studies 
• NHANES Dietary Supplement Database 

Dietary Interview Data Files: Two data files were produced from the information collected in the dietary interview: an Individual Foods file and a Total Nutrient Intakes file. The naming convention of the file and variables names is the same as WWEIA, NHANES 2011-2012. The number “1” in the file name identifies the day (and mode) of the interview: 1 = first day (in-person). One day of dietary intake was collected in NNYFS. The file names are as follows:

All NNYFS participants are eligible for the dietary interview component.

The examination protocol and data collection methods are fully documented in the NNYFS dietary interviewer procedures manual (https://www.cdc.gov/nchs/data/nnyfs/dietary_interview.pdf).

Interviews were conducted for survey participants less than six years of age with a proxy (who was generally the person most knowledgeable about the survey participant’s intake). Interviews of children ages 6 to 8, were conducted with a proxy and with the child present to assist in reporting intake information. Interviews of children ages 9-11, were conducted with the child and the assistance of an adult familiar with the child’s intake. Participants 12-15 years answered for themselves. Dietary interviewers conducted interviews 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 NNYFS mobile examination center (MEC). A set of measuring guides (various glasses, bowls, mugs, drink boxes and bottles, 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 (NNYFS Measuring Guides for the Dietary Recall Interview).

Dietary interview data for the NNYFS were collected using USDA’s dietary data collection instrument, the Automated Multiple Pass Method (AMPM) (http://www.ars.usda.gov/ba/bhnrc/fsrg). The AMPM was designed to provide an efficient and accurate means of collecting intakes for large-scale national surveys. The AMPM, which is interviewer-administered, is a fully computerized recall method that uses a 5-step interview outlined below:

1. Quick List - Participant recalls all foods and beverages consumed the day before the interview (midnight to midnight).
2. Forgotten Foods - Participant is asked about consumption of foods commonly forgotten during the Quick List step.
3. Time and Occasion - Time and eating occasion are collected for each food.
4. 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.
5. 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 for each 2-year collection of WWEIA 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 et al., 2004).

The AMPM was validated in a large study and shown to be an effective method for collecting accurate group energy intake of adults. Completed in 2004, this extensive research project included 524 healthy, weight-stable volunteers, aged 30-69 years. The accuracy of the AMPM was evaluated by comparing reported energy intake (EI) to total energy expenditure (TEE) using the doubly labeled water technique (Moshfegh et al., 2008). Among the findings were that EI compared to TEE was under-reported by 11% overall, by less than 3% for normal weight subjects with body mass index (BMI) < 25 and 16% for overweight subjects with BMI ≥ 25.

Additional studies provide evidence that the AMPM accurately measures group energy intake. Blanton (Blanton et al., 2006) reported that EI was not significantly different from TEE for a sample of 20 adult females. Rumpler and colleagues found that mean EIs were accurately reported for a sample of 12 adult males (Rumpler et al., 2008).

Additional evidence for the accuracy of AMPM has been provided by analysis of the 24-hour urinary sodium data collected in the AMPM Validation Study, which suggests the AMPM is a valid measure for estimating sodium intake in adults. Dietary sodium intake calculated from 24-hour recall data of 465 subjects collected via AMPM was compared with sodium values from 24-hour urine collections measured during the same 24-hour period. The AMPM-derived mean dietary sodium estimates reflected over 90% of the biomarker-based estimates (Rhodes et al., 2013).

For additional information about the dietary interview component and related survey protocols, please go to NNYFS Manuals site at:
https://www.cdc.gov/nchs/nnyfs/manuals.htm.

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.

Interviewers were monitored throughout the data collection period. Monitoring consisted of the following:

• Reviews of audio recorded 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.

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 (Raper et al., 2004).

USDA's Food and Nutrient Database for Dietary Studies (FNDDS) 2011-2012 was used for processing the 2012 NNYFS and WWEIA, NHANES intakes (http://www.ars.usda.gov/ba/bhnrc/fsrg). 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 2011-2012 were based on values in the USDA National Nutrient Database for Standard Reference, release 26, produced by USDA’s Nutrient Data Lab (http://www.ars.usda.gov/nutrientdata). FNDDS values are updated for every 2-year WWEIA, NHANES release cycle.

FNDDS 2011-2012, which corresponds with WWEIA 2011-2012, now includes the addition of enhanced characterization of food and beverages indicating if the item is fortified or contains fortified ingredients. Another enhancement for FNDDS 2011-2012 is the incorporation of the Recipe Protocol Project, which provides a framework for the selection of ingredients and amounts across similar foods. The protocols standardize and modernize new and existing recipes for about 2,500 items in FNDDS 2011-2012. Additional information about the FNDDS and related tools is available on the Food Surveys Research Group website (Raper et al., 2004; Anand et al., 2006; Ahuja et al., 2008).

Coders were required to pass a certification test after the initial training. They were routinely monitored to ensure the quality and completeness of their work. Approximately 10 percent of the coder's work was randomly selected to be independently coded by another coder. Results from the two codings were compared 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, et al. (Anand et al., 2006). Examples of reviews include the following:

• Overall acceptability of each recall. This review determined if the recall met the two minimum criteria listed below. A recall was considered unacceptable if it failed to meet either of these criteria:
  1. The first 4 steps of the 5-step AMPM were 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.
  2. Foods consumed for each reported eating occasion must be identified. A report of an eating occasion with missing foods occurs most often when an adult serving as a proxy, usually for a child, has reported that the survey participant ate a meal, usually at daycare or school, but does not know what was served or how much was consumed (Anand et al., 2006).
• 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/beverages or portions that could not be matched to items in the database were resolved by FSRG scientists. New items and new portion sizes are added as needed to the FNDDS. Information about new foods/beverages and package sizes were collected using internet resources, direct contact with manufacturers, or food labels. Unusual food mixtures were coded using multiple food codes to represent the mixture, linking those foods with a combination food number, and specifying the type of combination.
• 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 30 unique checks were conducted across all dietary data.
• Intakes with extreme levels for individual nutrients.
  Nutrient intakes were reviewed separately for various age and gender groups to determine if corrections were necessary.

During data processing, the following edit was made to ensure the logical consistency and analytic usefulness of the data:

• Modified nutrient values for some food mixtures.
  During the food coding process, predefined recipes for some food mixtures were modified to match more closely the food as described by the respondent. Nutrients were modified by substituting ingredients in a predefined recipe for the mixture. An example of a modified recipe is spinach cooked with butter instead of margarine. Each modification was assigned a unique 6-digit identification number. Recipe modification numbers appear in the variable DR1MC in the Y_DR1IFF file. Descriptions for each modification are provided in a separate file called DRXMCD_G.
  Starting in 2011, the following foods were not modified because new items are now included in FNDDS 2011-2012 to capture this detail: eggs; fish; macaroni and cheese; oatmeal; rice, select salads—such as potato salad, coleslaw, tuna salad, etc.; rice.

A data processing step called salt adjustment that was based on survey participants' responses to selected questions was discontinued in 2009-2010 (Sebastian et al. 2013; Sebastian et al. 2012). For participants whose households used salt in cooking occasionally or less often, some or all of the salt attributable to home preparation was removed from foods that typically have salt added during preparation and were obtained from the store.
Factors leading to the discontinuation of the data processing step of salt adjustment, include the following:

• The use of store purchase as a proxy indicator of home preparation is no longer appropriate, due to the proliferation of ready-to-eat, ready-to-heat, and other convenience foods that are available in stores.
• Results from the AMPM Validation Study demonstrate that the AMPM is a valid measure for estimating sodium intakes at the group level when the data processing step of salt adjustment is not applied. 

For more detail about the data processing step of salt adjustment and its discontinuation, see Sebastian et. al. (Sebastian et al. 2013; Sebastian et al. 2012).

The Individual Foods file is comprised of food records. For most survey participants, there are multiple records in the file. In the Total Nutrient Intakes file there is one record for each participant. These files can be linked with other NNYFS files by the respondent sequence number (SEQN).

Variable names: The variable naming convention is consistent with that of the WWEIA, NHANES dietary data files. The number "1" in the third position of the variable name identifies the collection day. One day of intake was collected for NNYFS, thus, variables contain the number "1". For example, the USDA FNDDS food code variable (in the Individual Foods file), which identifies the food reported by the participant, is named DR1IFDCD. Appendices 1 and 4 list the variable names for the Individual Foods file and the Total Nutrient Intakes file, respectively.

Number of days of intake: For NNYFS, only one day of dietary intake interview was conducted. In 2012, 1,518 NNYFS participants provided complete dietary intakes.

Dietary recall status code: A status code (DR1DRSTZ) is used in both the Individual Foods and Total Nutrient Intakes files 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 reported are provided for these cases. These individuals have no records in the Individual Foods file.

3   [Code 3 is not included in the current datasets. It was only used for data from the NHANES 1999-2000 survey cycle.]

4 = Reported consuming breast milk [Code 4 is not included in the current dataset. No one reported consuming human breast milk.]

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 no records in the Individual Foods file. These individuals have a record in the Total Nutrients file with values only for the following variables: the respondent sequence number (SEQN) and the dietary recall status code (DR1DRSTZ).

Although there are four possible values, only code 1 appears in the Individual Foods file. In addition to the status code described above, the variable DR1_300 and DR2_300 in the Total Nutrients file, 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 who reported consuming only water, no food or other beverages: Records are included in the Individual Foods file for participants who consumed only water. There are no such individuals in the NNYFS dataset.

Participants who reported consuming no water, food or other beverages: There can be survey participants whose intakes are determined to be complete even though they reported no water, food, or other beverage records for the day. For such participants there are no records in the Individual Foods file but their dietary recall status is coded as complete and reliable and the Total Nutrients file will include records with zero values for all nutrients. In the 2012 dataset, there are no such participants.

Number of days between the intake day and the day of family interview: The two intake files include a variable (DR1DBIH) to indicate the number of days between the intake day (i.e., the period covered by the 24-hour recall) and the day that the family questionnaire was administered in the household. A positive value in DR1BHIH indicates the family interview occurred prior to the intake day. In the survey, most of the family interviews were done before the participant came to the MEC and participated in the dietary interview. A value of “0” in DR1BHIH indicates the family interview occurred on the same date as the intake day. A negative value (i.e., DR1BHIH<0) means that the family interview occurred after the intake day.

Food source: The source from which each food was obtained (e.g., from a store, fast food restaurant, cafeteria) is identified by the variable DR1FS in the Individual Foods file.

The code descriptions for this variable are:

Eating occasion: The variables DR1_030Z and DR2_030Z are located in the Individual Foods file. The code descriptions for the eating occasion variables are shown in the table below.

Eating occasion was designated by the respondent. During the interview, a list of eating occasion names was available to the respondent for selection. However, eating occasion names were not defined for the respondent and the interpretation may differ from one person to another.

Foods and beverages coded as part of a combination: Thirty-seven percent of foods and beverages reported in the NNYFS, 2012 dietary interview were identified as items consumed together as combinations. Items consumed as a combination were identified by one of fifteen unique "combination food types". Foods and beverages not coded in combination have the code "0" for the combination food type variable.

The combination types provide a linkage for:

• Foods or beverages with additions, such as cereal with milk, coffee with cream;
• Multi-component foods that have specific protocol for collection such as some salads and sandwiches (primarily those that are not from fast food establishments); and
• Other combinations that do not have a unique code in the FNDDS.

All items given a combination food type are given an additional variable to identify each of the items within the combination. This variable is the "combination food number" that is unique to the combination food type within the individual intake.  

Special diet: Information on whether the participant is currently on any kind of diet to lose weight or for other health-related reason and, if so, the type of diet, was provided by the variable DRQSDIET identifies whether a participant was on a special diet. The variables DRQSDT1 through DRQSDT12 and DRQSDT91 identify the type of diet(s) that the participant was following. These variables can be found in the Total Nutrient Intakes file.

Note: A participant could report more than one type of diet, and all the responses were recorded.

Sample weights for NNYFS dietary intake data: 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. Please refer to the NNYFS Analytic Guidelines for details on the sample design and analytic guidance for the NNYFS data: https://www.cdc.gov/nchs/nnyfs/analytic_guidelines.htm.

For the 2012 NNYFS, there were 1,640 persons selected; of these 1,576 were interviewed and examined in the MEC. 1,518 of the MEC participants provided a complete dietary intake.

Most analyses of NNYFS data use data collected in the MEC and the variable WTMEC should be used for the sample weights. However, for the NNYFS 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 the 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 1,518 participants with Day 1 data. Day 1 weights were constructed by taking the MEC sample weights (WTMEC) 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.

Note that all sample weights are person-level weights and the set of dietary weights should sum to the same population control total as the MEC weights (WTMEC).