NHvars DATA DICTIONARY
NHANES 2021–2023 (Continuous Release)

------------------------------------------------------------
DATASET OVERVIEW
------------------------------------------------------------
The NHvars dataset is derived from the National Health and 
Nutrition Examination Survey (NHANES) 2021–2023 cycle.

NHANES combines:
- demographic interviews
- physical examinations
- laboratory testing

Each row represents one individual participant.
All datasets are linked using the unique identifier SEQN.

Only participants aged 18 years and older are included.

------------------------------------------------------------
KEY IDENTIFIER
------------------------------------------------------------
SEQN
  Unique participant identifier used across all NHANES files


------------------------------------------------------------
DEMOGRAPHIC VARIABLES (DEMO_L)
------------------------------------------------------------

RIAGENDR
  Sex of participant
  Values: Male, Female

RIDAGEYR
  Age in years at time of survey
  Units: Years

AgeBand (derived)
  Age grouped into categories:
    0–19
    20–39
    40–59
    60–79
    80+


------------------------------------------------------------
BODY MEASUREMENTS (BMX_L)
------------------------------------------------------------

BMXWT
  Body weight
  Units: kilograms (kg)

BMXHT
  Standing height
  Units: centimetres (cm)

BMXBMI
  Body Mass Index
  Units: kg/m²

BMXWAIST
  Waist circumference
  Units: centimetres (cm)


------------------------------------------------------------
BLOOD PRESSURE (BPXO_L)
------------------------------------------------------------

BPXOSY1
  Systolic blood pressure (first reading)
  Units: mmHg

BPXODI1
  Diastolic blood pressure (first reading)
  Units: mmHg

Note: Values are measured using an automated oscillometric device.


------------------------------------------------------------
LIPID PROFILE
------------------------------------------------------------

From TCHOL_L:
LBXTC
  Total cholesterol
  Units: mg/dL

From HDL_L:
LBDHDD
  HDL cholesterol (high-density lipoprotein)
  Units: mg/dL

From TRIGLY_L:
LBXTR
  Triglycerides
  Units: mg/dL

LBDLDLM
  LDL cholesterol (derived)
  Units: mg/dL


------------------------------------------------------------
METABOLIC MARKERS
------------------------------------------------------------

From GLU_L:
LBXGLU
  Blood glucose
  Units: mg/dL

From INS_L:
LBXIN
  Serum insulin
  Units: µU/mL

Note: Insulin may contain missing values depending on sampling.


------------------------------------------------------------
HORMONES (TST_L)
------------------------------------------------------------

LBXTST
  Total testosterone
  Units: ng/dL

LBXEST
  Estradiol
  Units: pg/mL


------------------------------------------------------------
INFLAMMATION MARKER (HSCRP_L)
------------------------------------------------------------

LBXHSCRP
  High-sensitivity C-reactive protein (CRP)
  Units: mg/L


------------------------------------------------------------
VITAMIN MEASURE (VID_L)
------------------------------------------------------------

LBXVD3MS
  Vitamin D (25-hydroxyvitamin D)
  Units: nmol/L


------------------------------------------------------------
PHYSICAL ACTIVITY (DERIVED)
------------------------------------------------------------

PhysActive
  Physical activity status

  Values:
    Yes = participant reports moderate or vigorous activity
    No  = no reported activity

  Derived from PAQ_L dataset using:
    PAD800 > 0 OR PAD820 > 0 → Yes
    otherwise → No

  Interpretation:
    PAD800 = moderate activity
    PAD820 = vigorous activity


------------------------------------------------------------
IMPORTANT NOTES
------------------------------------------------------------

1. Missing values:
   Not all variables are measured for every participant.
   Missing values appear as NA.

2. Derived variables:
   AgeBand and PhysActive were created from raw NHANES variables.

3. Measurement context:
   - Anthropometric and BP measurements were taken in
     mobile examination centres (MEC).
   - Laboratory variables are based on blood samples.

4. Population:
   This dataset includes adults aged 18 years and older only.

------------------------------------------------------------
END OF DATA DICTIONARY
------------------------------------------------------------