NHANES

What is NHANES?

The National Health and Nutrition Examination Survey (NHANES) is a nationally representative U.S. health surveillance programme conducted by the National Center for Health Statistics (NCHS), part of the Centers for Disease Control and Prevention (CDC). Unlike many health surveys that rely primarily on self-reported information, NHANES combines health interviews, physical examinations, anthropometric assessments, laboratory testing, medical imaging, and body composition measurements within the same participants, providing a uniquely comprehensive picture of population health. Since becoming a continuous survey in 1999, NHANES has generated some of the most influential datasets in epidemiology, nutrition, anthropometry, obesity research, and body composition science.

Key Facts

NHANES has evolved from a series of periodic health examination surveys into a continuous national surveillance programme. Data are generally released in approximately two-year cycles and are collected using a complex multistage probability sampling design that enables findings to be generalized to the wider U.S. civilian, non-institutionalised population. To improve precision for certain analyses, selected demographic groups are routinely oversampled. Importantly, NHANES relies on direct measurement rather than self-report, making it one of the most robust population health resources available. Data collection takes place in specially designed Mobile Examination Centers (MECs), which provide standardised environments for measurements and testing. Anthropometry has been included since NHANES I (1971-1975), making the programme one of the longest-running sources of population anthropometric data worldwide.

Anthropometry and Body Composition in NHANES

Anthropometry sits at the core of NHANES and provides valuable information on growth, nutritional status, body size, adiposity, and disease risk. Core measurements include stature (height), body mass (weight), and recumbent length in infants and young children. Depending on the survey cycle, additional measurements have included waist, arm, hip, and thigh circumferences; sitting height; knee height; upper leg length; skeletal breadths; and skinfold thicknesses. Skinfold measurements, collected in selected survey cycles, were used to estimate subcutaneous adiposity and contributed to early body composition research.

A major advancement within NHANES was the introduction of Dual-Energy X-ray Absorptiometry (DXA), allowing large-scale assessment of fat mass, lean soft tissue mass, bone mineral content, and bone mineral density. Few population studies have incorporated body composition imaging at this scale, making NHANES a highly valuable resource for obesity, ageing, bone health, and sarcopenia research.

Derived Variables

Beyond direct measurements, NHANES provides a range of widely used derived variables, including:

• • Body Mass Index (BMI)
  • BMI-for-age percentiles
  • Waist-to-height ratio
  • Fat Mass Index (FMI)
  • Lean Mass Index (LMI)

These variables have informed national obesity estimates, growth references, clinical guidelines, and epidemiological research investigating relationships between body size, body composition, and health outcomes.

Accessing the Data

One of NHANES’ greatest strengths is its commitment to scientific transparency and accessibility. Most datasets are available free of charge and can be accessed without application through the NHANES data portal. Researchers can download anthropometry, DXA, nutrition, laboratory, questionnaire, and health datasets alongside detailed codebooks, procedures manuals, and analytic guidance documents. Restricted-use variables, which are not publicly released due to confidentiality concerns, can be accessed through application to the NCHS Research Data Center (RDC) with an approved research proposal.

Analysis Essentials

Researchers using NHANES should recognise that it employs a complex survey design. Appropriate analysis requires the use of sampling weights, strata variables, and primary sampling units (PSUs). When combining multiple survey cycles, adjusted combined weights must be calculated according to NCHS guidance. Researchers should also review cycle-specific documentation carefully because measurement protocols and variable availability can change over time.

Historical Milestones

The NHANES programme has developed over more than six decades:

• • NHES: 1959-1962
  • NHANES I: 1971-1975 (anthropometry formally incorporated)
  • NHANES II: 1976-1980
  • NHANES III: 1988-1994
  • Continuous NHANES: 1999-present

These repeated surveys allow researchers to monitor secular trends in growth, obesity, body composition, and health at the population level.

NHANES has contributed substantially to:

• • Obesity and growth surveillance
  • Body composition and sarcopenia research
  • Nutritional epidemiology
  • Cardiometabolic health research
  • Bone health and osteoporosis research
  • Development of CDC growth references and dietary guidance

Its integration of anthropometry, body composition, health outcomes, biomarkers, and lifestyle data within the same participants has made NHANES one of the most heavily used resources in public health and epidemiological research.

Strengths and Limitations

NHANES offers several important strengths. It provides nationally representative, objectively measured data, includes large sample sizes, offers open access to most datasets, maintains extensive documentation and methodological transparency, and contains one of the richest publicly available DXA body composition datasets available.

However, limitations should be acknowledged. Most NHANES analyses are based on repeated cross-sectional surveys, which limits causal inference. Reference values are derived from U.S. populations and may not generalise fully to other countries. Measurement protocols have evolved across cycles, requiring careful harmonisation for trend analyses. In addition, DXA participation differs across survey cycles and some body composition datasets contain cycle-dependent missing data that must be considered during analysis.

Despite these limitations, NHANES remains one of the most valuable resources available for anthropometry and body composition research. Its combination of rigorous methodology, nationally representative sampling, detailed body composition data, and unparalleled transparency continues to support advances in research, policy, and practice across nutrition, health, epidemiology, and human measurement science.