• Component Description
• Eligible Sample
• Description of Laboratory Methodology
• Laboratory Method Files
• Laboratory Quality Assurance and Monitoring
• Data Processing and Editing
• Analytic Notes
• References
• Codebook
  • SEQN - Respondent sequence number
  • WTTFA2YR - Trans Fatty Acid Subsample 2 Year Weight
  • LBXHDT - trans-9-Hexadecenoic acid (umol/L)
  • LBDHDTLC - Trans 9-hexadecenoic acid comment code
  • LBXOD1 - Trans 11-octadecenoic acid (umol/L)
  • LBDOD1LC - Trans 11-octadecenoic acid comment code
  • LBXOD9 - Trans 9-octadecenoic acid (umol/L)
  • LBDOD9LC - Trans 9-octadecenoic acid comment code
  • LBXOTT - trans,trans-9,12-Octadecadienoic(umol/L)
  • LBDOTTLC - trans,trans-9,12-Octadecadienoic comt

National Health and Nutrition Examination Survey

1999-2000 Data Documentation, Codebook, and Frequencies

Trans Fatty Acids (TFA_A)

Data File: TFA_A.xpt

First Published: May 2016

Last Revised: March 2019

Component Description

Trans fatty acids are unsaturated fatty acids that contain at least one double bond in the trans configuration. The three-dimensional structure of trans fatty acids is more similar to saturated fatty acids than to regular unsaturated fatty acids, which have their double bond in the cis configuration. The trans configuration substantially alters the physical properties of the fatty acids, and thus the properties of the oil containing these trans fatty acids for cooking and food manufacturing. Also, it substantially alters the biologic and health effects of the fatty acids when consumed.

A positive relationship between trans fatty acid intake, and total and LDL cholesterol concentration was established, which links elevated trans fatty acids in blood with increased risk of coronary heart diseases. The Institute of Medicine recommended that ‘trans fatty acid consumption be as low as possible while consuming a nutritionally adequate diet’.

In addition, the Dietary Guidelines for Americans, 2000 and guidelines from the National Cholesterol Education Program (NCEP), were similar with recommendations to limit trans fat intake in the diet. In 2003, FDA amended its regulations on nutrition labeling to require that trans fatty acids be declared in the nutrition label of conventional foods and dietary supplements. This rule is intended to provide information to assist consumers in maintaining healthy dietary practices. In 2015, FDA determined that partially hydrogenated oils, the major source of artificial trans fat in the food supply, are no longer “Generally Recognized as Safe,” or GRAS. This new rule will be fully in effect in 2020.

Eligible Sample

Examined participants aged 6 years and older were eligible.

Description of Laboratory Methodology

The laboratory method used in this study was described previously (Kuiper et al. 2018) and measures the total (free and esterified) content of selected trans fatty acids in plasma and provides results in concentration. The trans fatty acids measured with this method are: trans-9-hexadecenoic acid (palmitelaidic acid, C16:1n-7t), trans-9-octadecenoic acid (elaidic acid, C18:1n-9t), trans-11-octadecenoic acid (vaccenic acid, C18:1n-7t), trans-9, trans-12-octadecadienoic acid (linolelaidic acid, C18:2n-6t, 9t).

The fatty acids in plasma are converted into free fatty acids by subsequent acidic and alkaline hydrolysis. The free fatty acids are extracted from the sample solution using liquid-liquid extraction and derivatized with pentafluorobenzylbromide (PFB-Br). The derivatized fatty acids are separated by capillary gas chromatography and detected by mass spectrometry using negative chemical ionization. 

The fatty acids are identified based on their chromatographic retention time and on the specific mass to charge ratio of the ion formed in the ion source. Retention times are compared against those obtained with known standards. Quantitation is performed with standard solution using stable isotope-labeled fatty acids as internal standards.

Refer to the Laboratory Method Files section for a detailed description of the laboratory methods used.

Trans fatty acids is a new component in the 1999-2000 survey cycle.

Laboratory Method Files

Trans Fatty Acids (Updated February 2019)

Laboratory Quality Assurance and Monitoring

Plasma specimens were processed, stored and shipped to Divisions of Laboratory Sciences, National Center for Environmental Health, Centers for Disease Control, Atlanta, GA for analysis.

Detailed instructions on specimen collection and processing are discussed in the NHANES Laboratory Procedures Manual (LPM). Vials are stored under appropriate frozen (–30°C) conditions until they are shipped to National Center for Environmental Health for testing.

The NHANES quality assurance and quality control (QA/QC) protocols meet the 1988 Clinical Laboratory Improvement Act mandates. Detailed QA/QC instructions are discussed in the NHANES LPM.

Mobile Examination Centers (MECs)
Laboratory team performance is monitored using several techniques. NCHS and contract consultants use a structured competency assessment evaluation during visits to evaluate both the quality of the laboratory work and the quality-control procedures. Each laboratory staff member is observed for equipment operation, specimen collection and preparation; testing procedures and constructive feedback are given to each staff member. Formal retraining sessions are conducted annually to ensure that required skill levels were maintained.

Analytical Laboratories
NHANES uses several methods to monitor the quality of the analyses performed by the contract laboratories. In the MEC, these methods include performing blind split samples collected on “dry run” sessions. In addition, contract laboratories randomly perform repeat testing on 2% of all specimens.

NCHS developed and distributed a quality control protocol for all CDC and contract laboratories, which outlined the use of Westgard rules (Westgard et al, 1981) when running NHANES specimens. Progress reports containing any problems encountered during shipping or receipt of specimens, summary statistics for each control pool, QC graphs, instrument calibration, reagents, and any special considerations are submitted to NCHS quarterly. The reports are reviewed for trends or shifts in the data. The laboratories are required to explain any identified areas of concern.

All QC procedures recommended by the manufacturers were followed. Reported results for all assays meet the Division of Laboratory Sciences’ quality control and quality assurance performance criteria for accuracy and precision, similar to the Westgard rules (Caudill, et al, 2008).

Data Processing and Editing

The data were reviewed. Incomplete data or improbable values were sent to the performing laboratory for confirmation.

Analytic Notes

Refer to the 1999-2000 Laboratory Data Overview for general information on NHANES laboratory data.

Please refer to the NHANES Analytic Guidelines and the on-line NHANES Tutorial for details on the use of sample weights and other analytic issues.

Sample weights

Trans fatty acids were measured in a full sample of participants 6-11 years old and a fasting subsample of persons 12 years and over.

Demographics and Other Related Variables

The analysis of NHANES laboratory data must be conducted using the appropriate survey design and demographic variables. The NHANES 1999-2000 Demographics File contains demographic data, health indicators, and other related information collected during household interviews as well as the sample design variables. The recommended procedure for variance estimation requires use of stratum and PSU variables (SDMVSTRA and SDMVPSU, respectively) in the demographic data file.

The Fasting Questionnaire File includes auxiliary information, such as fasting status, length of fast, and the time of venipuncture.

This laboratory data file can be linked to the other NHANES data files using the unique survey participant identifier (i.e., SEQN).

Detection Limits

The detection limits were constant for all of the analytes in the data set. Two variables are provided for each of these analytes. The variable name ending in “LC” (ex., LBDHDTLC) indicates whether the result was below the limit of detection: the value “0” means that the result was at or above the limit of detection, “1” indicates that the result was below the limit of detection. For analytes with analytic results below the lower limit of detection (ex., LBDHDTLC=1), an imputed fill value was placed in the analyte results field. This value is the lower limit of detection divided by the square root of 2 (LLOD/sqrt[2]). The other variable prefixed URX (ex., LBXHDT) provides the analytic result for that analyte.

All results were at or above the lower detection limit. The lower limits of detection (LLOD, in µmol/L) for trans fatty acids are:

Variable Name	SAS Label	LLOD
LBXHDT	trans-9-hexadecenoic acid (µmol/L)	0.07
LBXOD9	trans-9-octadecenoic acid(µmol/L)	0.28
LBXOD1	trans-11-octadecenoic acid(µmol/L)	0.43
LBXOTT	trans-9,12-octadecadienoicacid(µmol/L)	0.02

References

• Caudill SP, Schleicher RL, Pirkle JL. Multi-rule quality control for the age-related eye disease study. Stat Med 2008;27:4094-106.
• Kuiper HC, Wei N, McGunigale SL, Vesper HW. Quantitation of trans-fatty acids in human blood via isotope dilution-gas chromatography-negative chemical ionization-mass spectrometry. J Chromatogr B 2018;1076:35-43.
• Westgard J.O., Barry P.L., Hunt M.R., Groth T. A multi-rule Shewhart chart for quality control in clinical chemistry. Clin Chem (1981) 27:493-501.

Codebook and Frequencies