National Health and Nutrition Examination Survey

The objective of the dietary supplement component of the 24-hour dietary recall interviews is to obtain information from NHANES participants on the types and amounts of dietary supplements (DS) consumed during the 24-hour period prior to the interview (midnight to midnight). From 1999-2006, the dietary interviews only asked about food and beverages consumed. Beginning in 2007, data is now collected on the usage of all vitamins, minerals, herbals and other dietary supplements as well as non-prescription antacids, as part of the dietary interviews.

The continuous NHANES has been collecting data on study participants’ use of dietary supplements, in the past 30 days, during the Dietary Supplements Section (DSQ) in the household interview since 1999. However, prior to this data release, estimating total nutrient intakes from all sources (foods, beverages and DS), has been difficult because of the different data collection methods, referent time frames, and data formats. Now with these data collected using similar methods over the same time frame, nutrients from all sources can be easily combined. This will allow researchers to improve the total nutrient intake estimations as well as examine associations by nutrient source such as from foods versus supplements. The improved nutrient estimates will increase researchers’ ability to examine nutrient-disease or nutrient-health associations and to estimate percentages of the population with intakes over or under various nutrient standards.

All NHANES examinees are eligible for two 24-hour dietary recall interviews. The first dietary recall interview is collected in-person in the Mobile Examination Center (MEC) and the second interview is collected by telephone 3 to 10 days later.

Similar to the format of food intake files, two types of supplement intake data are available from 24-hour dietary interview for the 2007-2008 survey cycle: Individual Dietary Supplements files and Total Dietary Supplements files. Each dietary supplement data file is accompanied by a PDF file containing its documentation, codebook, and frequencies. The documentation section of the file for each of the two types of dietary supplement data files is identical.

24-hour Dietary Supplements Data Files: Four data files were produced from the dietary supplement component of the 24-hour dietary recall interviews: two Individual Dietary Supplements files and two Total Dietary Supplements 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:

The nutrient amounts in these files reflect only nutrients obtained from non-prescription and prescription dietary supplements as well as non-prescription antacids that contain calcium and/or magnesium. They do not include nutrients obtained from foods, beverages, and water.

Individual Dietary Supplements File (DS1IDS_E and DS2IDS_E): Contain detailed information about the types and amounts of individual dietary supplements and non-prescription antacids (that contain calcium and/or magnesium) reported by each participant. The names for both Day 1 and Day 2 variables are listed in Appendix 1.

The Individual Dietary Supplements Files include, for each interview day, one record for each dietary supplement or antacid consumed by a survey participant. Only participants’ that had reported taking a dietary supplement or antacid are included in these files. Each dietary supplement or antacid is identified by a supplement ID number (DSDSUPID) and each record contains the information listed below: 

• Number of days of complete intake obtained from participant 
• Day of the week of the intake 
• Name and ID number of the supplement 
• Amount of dietary supplement consumed in serving size, which is calculated as the reported amount consumed divided by the serving size from the product label
• Amounts of 33 nutrients/dietary components (listed in Appendix 2) from each dietary supplement and antacid, as calculated using the NHANES Dietary Supplement Database (NHANES-DSD)

This file only includes intake for a select group of nutrients. These records can be linked to the NHANES-DSD, using supplement ID numbers (DSDSUPID), to obtain more detailed information on reported products. The NHANES-DSD datasets provide information on nutrients in the dietary supplement as reported on the product label. Botanical ingredients would be an example of nutrients not released in the Individual Dietary Supplements files, but can be obtained from the NHANES-DSD files.

Total Dietary Supplements Files: (DS1TOT_E and DS2TOT_E): Contain, for each participant, daily total nutrient intakes from dietary supplements and antacids. The names for both Day 1 and Day 2 variables are listed in Appendix 3.

The Total Dietary Supplement Files provide a summary record of total nutrient intakes from dietary supplements and antacids for each individual. All participants that had a complete dietary interview for foods and beverages are included in this file. This includes users and non-users of supplements and antacids. Each total intake record contains the following information: 

• Number of days of complete intake obtained from participant 
• Whether a dietary supplement or an antacid was consumed in the past 24 hours 
• Total number of supplements and antacids reported for that participant 
• Daily aggregates of 33 nutrients/dietary components (listed in Appendix 2) from all supplements and antacids, as calculated using the NHANES-DSD. 

The 24-hour food dietary data are released in separate files available on the NHANES website. They can be linked to the dietary supplement files by the respondent sequence number (SEQN). 

All NHANES examinees are eligible for the dietary supplement component of the 24-hour dietary recall interview.

The examination protocol and data collection methods for the dietary interview are fully documented in the NHANES Dietary Interviewers Procedures Manuals (1, 2).

The 24-hour dietary supplement component was administered after the 24-hour dietary recall for food and beverage, using a similar protocol. All NHANES examinees responding to the food 24-hour dietary recall interview are eligible for the dietary supplement and non-prescription antacid use questions. Information was 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 taken. Non-prescription antacids were collected since a high percentage of these products contain calcium and/or magnesium. Participants’ were shown a hand card that lists examples of supplements (Appendix 4). For participants who reported taking a supplement or antacid in the last 30 days during the household interview, the supplements or antacids were imported from the household questionnaire. The Interviewer verified that each dietary supplement and non-prescription antacid reported during the household interview was consumed the day before the interview (midnight to midnight). Participants were also asked how much was taken. Then the interviewers collected any additional supplement or antacid that was taken that previous day that were not reported during the household interview.

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). 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.

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 annually to reinforce the proper protocols and technique.

Interviewers were monitored throughout the data collection period. Interviews were checked for completeness of the recalls, missing information, inconsistent reports, and unclear notes. Written notification and feedback were provided to the interviewers.

Trained nutritionists reviewed incoming data and matched reported dietary supplement entries to known supplements from the in-house NCHS Product Label Database (PLD) where possible; sought additional product labels if feasible; assigned generic or default supplements as appropriate; transferred or removed products that were not considered dietary supplement (i.e. prescription medication); and assigned matching codes/confidence codes to indicate the degree of matching certainty. All coding was reviewed by the NCHS project officer.

Matching reported dietary supplements to known products

Trained nutritionists, at NCHS, match the product names entered by the interviewer (including prescription dietary supplement, non-prescription dietary supplement and non-prescription antacid) to a known product when possible. NCHS nutritionists first determine if the product is in our in-house PLD. If it is, then the nutritionist verify whether the product label was entered into the system within the 2-year cycle of data collection. If the product label has not been entered into PLD, or the product label was entered before the beginning of the 2-year data collection cycle, NCHS nutritionists attempt to obtain a new product label. NCHS obtains labels for each dietary supplement or non-prescription antacid reported by participants from sources such as the manufacturer, distributor or retailer, the Internet, company catalogs, and the Physician’s Desk Reference (PDR). Nutritionists determine during processing whether or not the non-prescription antacids collected contain calcium and/or magnesium. Only non-prescription antacids containing calcium or magnesium are included in the present data files.

NCHS communicates with many major manufacturing company representatives to determine when various product re-formulations become available. Based upon manufacturer advice, we have used a lag time of 5 months after the new re-formulated product has hit the market and matched products to participants’ accordingly. Despite these precautions, there is no guarantee that the product taken by participants’ is matched to the correct formulation in our release files.

NCHS nutritionists attempt to find the exact product(s) reported by participants, however this is done with varying degrees of precision. A “confidence code” (DS1MTCH and DS2MTCH, located in files DS1IDS_E and DS2IDS_E, respectively) accompanies each record to indicate the degree of matching certainty. The matches are: 1) Exact or near exact match; 2) Probable match; 3) Generic match; 4) Reasonable match; or 5) Default match. In some cases no match can be made with any certainty. These products are coded as “6, No match”. Products whose names were reported as “Refused” (DSDSUPP=7777) or “Don’t know” (DSDSUPP=9999) have matching codes of 7 and 9, respectively.

NCHS created generic and default dietary supplements and non-prescription antacids, which are also maintained in the PLD database. Generics are created in the database when we have information about a reported supplement such as strength of all ingredients, i.e. one which had no brand name. These were generally single ingredient supplements which included a strength (e.g. vitamin C 500 mg) or multiple vitamins and/or mineral supplements made by a private label manufacturer that was known to us and for which we had a label with identical ingredients and strengths (e.g. brand X all-purpose multivitamin was reported, and we had a label for brand Y, made by the same manufacturer). When all ingredient strengths were not known, a default supplement was created in the database. Defaults were created for single ingredient and multiple ingredient supplements based on our own data of most frequently reported supplements of that type, based on the survey cycle in which the data was collected. Created default products and the actual products or strengths that were assigned to these defaults are listed in Appendix 5.

After the dietary supplement data is coded and matched to a product in our in-house database, various types of reviews were conducted to ensure the quality of the data. Interviewers’ comments are reviewed to ensure that they have been accounted for in coding. Decisions are made about how to code new or unusual dietary supplement products or unusual quantities or units reported by participants. Dietary supplements that could not be matched to items in the database are resolved by NCHS nutritionists.

Product information is released from the in-house PLD as the NHANES Dietary Supplement Database (NHANES-DSD) which contains detailed information on the dietary supplements and antacids reported by survey participants since NHANES 1999. The NHANES-DSD release consists of three datasets which contain information on products:
Dietary Supplement Product Information (DSPI)
Dietary Supplement Ingredient Information (DSII)
Dietary Supplement Blend Information (DSBI)
The supplement ID numbers (DSDSUPID) located in files DS1IDS_E and DS2IDS_E of this release can be used to merge with the NHANES-DSD files. For more information on the NHANES-DSD, please refer to the documentation and release files located on the NHANES website:
https://wwwn.cdc.gov/nchs/nhanes/search/datapage.aspx?Component=Questionnaire&CycleBeginYear=2007.

Specific variables and edits:

DSDSUPID: Supplement ID

Supplement ID is a 10-digit identifier assigned to each product entered into the PLD. All identifiable products have a supplement ID beginning with the number ‘1’. The next 3 digits (positions 2-4) are: ‘888’ if the supplement was created by NCHS as a generic or default product; otherwise the digits in positions 2-4 are coded ‘000’. The next 4 digits (positions 5-8) are sequentially assigned by the system for each product. The last 2 digits (positions 9-10) indicate formulations of the same supplement: the first formulation entered into the database = 00, the first re-formulation = 01, the next = 02, etc. Note that these are reformulations of the same product. Different versions of products (e.g. liquid vs. tablet, with iron vs. without iron, regular vs. high potency) are considered as different products thus have different 4 digit numbers in positions 5 to 8. When a product name was entered as “no product information available”, “refused” or “don’t know”, its ID number starts with a string of 6’s. The NHANES-DSD Dietary Supplement Product Information (DSPI) file contains the text name of the dietary supplement.

DS1ANTA (DS1IDS_E) and DS2ANTA (DS2IDS_E): variable indicates whether a product was a dietary supplement or non-prescription antacid

This variable indicates whether the product reported was a non-prescription antacids (code=1) or a dietary supplement (code=0).

DS1MTCH (DS1IDS_E) and DS2MTCH (DS2IDS_E): Match code/confidence code

Supplements are recorded during the dietary interview with varying degrees of accuracy and completeness. NCHS has created a system to specify how certain we are with matching a supplement recorded during the interview with the actual product label. Below describes how matches are made:

Exact or near exact match (DSDMTCH=1) indicates that this is the only product that could match this entry.

Probable match (DSDMTCH=2) indicates that the match is not exact, but knowledge of the company’s products strongly suggests that this is the only possible match choice. For example the entry may not specify strength or include words such as timed release, but no other options are available for this brand according to manufacturer or retailer information.

Generic match (DSDMTCH=3) indicates we had information on the strength for all ingredients, either a) as part of name (e.g. vitamin C 500 mg) or b) because the manufacturer is known and NCHS has an identical product made by this manufacturer for a different distributor or retailer. Thus the ingredients and amounts are considered to be accurate despite an exact brand match.

Reasonable match (DSDMTCH=4) indicates that the product name may be incomplete or could be complete but other products of this brand also start with these same words so this cannot be assured. In these cases, the entered name is matched to either: a) the most frequently reported of these products in the NHANES 2007-2008 data, if this could be determined; b) the best selling product by this company that matches the entered name; or c) the most basic product by this company, as assessed by label wording.

Default match (DSDMTCH=5) indicates that the exact product could not be obtained because the name was imprecise or the exact brand product could not be located and no generic could be assigned. In these cases, the entered product was matched to a created default product based upon: a) the most commonly reported strengths for single ingredients; b) the most commonly reported brands for major multiple ingredient products such as multivitamins and multivitamin/multiminerals for children, seniors, or adults, if available; or c) products manufactured by a large, private-label manufacturer.

Finally, a match codes of No match (DSDMTCH=6) indicates that no product could be found and there was not enough detail in the name to assign a generic or default match with any confidence. The words “no product information available” are listed as the product name (DSDSUPP).

Analysts should be aware that for default matches and matches that chose between several similarly named supplements, there is less certainty that the ingredients and ingredient amounts in the supplement assigned exactly match those in the supplement actually taken. Additionally, NCHS cannot guarantee in any case that the matched product was the exact product taken or even that any product actually was taken, as these data are self-reported.

DS1ACTSS (DS1IDS_E) and DS2ACTSS (DS2IDS_E): Reported serving size/label serving size

This variable was derived using the information from the 'Supplement Information' file (DSPI) of the NHANES-DSD and the participants’ reported amount taken. File  DSPI provids information on the serving size from the product label’s supplement facts panel for which the nutrient amounts were based on. The participant were asked to report the amount taken during the dietary interview. The reported amount taken was divided by the serving size amount from DSPI (variable DSDSERVQ). Prior to calculating, the data needed to be edited to standardize the unit the participant used to report the amount taken and the unit used in the product serving size label. For example, if a respondent reported taking 1 tablespoon of a supplement and the label serving size was 3 teaspoons, than the reported amount taken was edited as 3 teaspoons (1 tablespoon = 3 teaspoons). There were about 230 records in which data was edited. In most of the cases, using the alternative serving size (information available on some product labels), using simple conversions (i.e. teaspoons to tablespoons) or contacting the manufacturer for information on label serving size (i.e. actual amount for a “capful”) provided enough information to make clear edits. However, in some cases the reported amount taken was very different from the product label serving size. For example, the respondent may have reported 1 tablet, but the label serving size is 1 tablespoon. This was assumed to be an error in reporting or an interviewer data entry error. In these cases the serving size was assumed to be the label serving size. There were 10 records in which this occurred and the data was edited to the label serving size. Additionally, all records that were assigned as a default product were edited and the label serving size was assigned.

Nutient variables from Individual Dietary Supplements files, DS1IDS_E and DS2IDS_E

These variables are created by using files from the NHANES_DSD that contain information on the serving size and the quantity of each nutrient from the product label’s supplement facts panel. The participants reported amount taken is divided by the serving size from the label in order to determine the actual amount of nutrient consumed. For example, a participant may report taking one tablet, but the serving size amount is 2 tablets. Therefore, only half of the nutrients on the label are being consumed. The variables DS1ACTSS (Day 1) and DS2ACTSS (Day 2) indicate the actual amount of product that was consumed. The actual amount of product consumed is then multiplied by the ingredient amount for each dietary supplement or antacid. In the example above, 0.5 would be multiplied by each ingredient/nutrient, to estimate the nutrient intake.

DS1IFDFE (DS1IDS_E) and DS1IFDFE (DS2IDS_E): Folate, DFE (mcg)
Dietary Folate Equivalents (DFE) were calculated by using a conversion factor of 1.7. This conversion is based on recommendations set by the Institute of Medicines Dietary Reference Intakes.(3)

Nutient variables from Total Dietary Supplements files, DS1TOT_E and DS2TOT_E

These variables are created by adding up each nutrient for each participant from the Individual Dietary Supplements Files (DS1IDS_E and DS2IDS_E).

In some cases it may appear as though a participant reported the same dietary supplement more than once. There are a several reasons for these duplications. Participants’ may have reported different brand name dietary supplements which when collected had the same generic ingredient (i.e. 2 different brands of calcium, but calcium is collected generically with only the strength recorded) or the dietary supplement may have been different forms or dosages of the same product. There were a few cases where the respondent did report the exact same dietary supplement, with the same length of use, frequency of use or amount used. In these cases, since the interviewer recorded the dietary supplement separately, both mentions were retained in the file.

During the data editing process, outlier values were examined. When there was insufficient information to conclude that values were invalid, they were left in the data set. Analysts should examine the distribution of the data and consider whether or not it is appropriate to include or exclude extreme values in a given analysis.

Each Individual Dietary Supplement File (DS1IDS_E and DS2IDS_E) is comprised of dietary supplement and non-prescription antacid records. Only participants who reported taking a dietary supplement or non-prescription antacid are inlcuded in these files. For many survey participants, there are multiple records in each file. For each Total Dietary Supplements File (DS1TOT_E and DS2TOT_E), there is one record for each participant and includes the entire dietary interview sample. 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 calcium variable in the Individual Dietary Supplements File, which identifies the amount of calcium in each supplement reported by the participant, is named DS1ICALC in the Day 1 file and DS2ICALC in the Day 2 file. Appendices 1 and 3 list the Day 1 and Day 2 variable names for the Individual Dietary Supplements files and the Total Dietary Supplements files, respectively.

Names for the following variables are the same for both days in the Individual Dietary Supplements files and the Total Dietary Supplements files:

Number of days of intake: Because two days of data are included in the 2007-2008 release, a variable has been included 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 both the Individual Dietary Supplements and Total Dietary Supplements files to indicate the quality and completeness of a survey participant's response to the dietary recall section. These codes are based on the food 24-hour dietary recall interview data. The codes are the following: 1. reliable and met the minimum criteria, 2. Not reliable or did not meet the minimum criteria, 4. Reported consuming breast milk, and 5. Not done. Although there are four possible values, only codes 1 and 4 appear in the Individual Dietary Supplements Files. For more information on this variable, refer to the documentations accompanying the 24-hour dietary interview datasets for food and beverage intake on the NHANES website.

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. In 2007-2008, the NHANES oversamples African Americans, Mexican Americans, all Hispanics, low income persons, 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 2007-2008 NHANES, there were 12,943 persons selected; of these 9,762 were considered respondents to the MEC examination and data collection. However, only 9,255 of the MEC respondents provided complete dietary intakes for Day 1. Furthermore, of those providing the Day 1 data, only 7,838 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. Dietary supplements are collected as part of the dietary interview. 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 9,255 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 7,838 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 interviews 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 7,838 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 dietary weights should sum to the same population control total as the MEC weights (WTMEC2YR). Additional explanation of sample weights and appropriate uses are included in the NHANES Analytic Guidelines.