study population
NHANES is a long-term, large-scale health survey program initiated and conducted every two years by the National Center for Health Statistics (NCHS), a division of the Centers for Disease Control and Prevention (CDC). Through a complex, multistage sampling design, NHANES collects health and nutrition data representative of the United States population, including personal interviews, physical examinations, and laboratory test results. NHANES data are widely used to study public health trends, assess the burden of disease and nutritional status, and are freely available to researchers worldwide.
This study analyzed pre-epidemic data from the 2017–2020 NHANES survey, which initially included 15,560 participants. From this cohort, 8,317 individuals aged 18 years and older with vibration-controlled transient elastography (VCTE) results were selected. We excluded 219 participants with unreliable VCTE measurements (liver stiffness quartile/median ratio ≥ 30%), 283 with hepatitis B or C, 821 with excessive alcohol intake (defined as more than two standard drinks per day for women and more than three for men), and those with missing data on neutrophil or high-density lipoprotein cholesterol (HDL-C). Consequently, 6526 participants were included in the final analysis.
Measurement of hepatic steatosis and hepatic fibrosis
NHANES staff use the FibroScan 502 Touch device to assess liver stiffness and fat content. The device measures liver elasticity and stiffness through vibration-controlled transient elastography (VCTE) technology to help determine the extent of liver fibrosis. At the same time, the device measures hepatic steatosis by ultrasound attenuation and records the Controlled Attenuation Parameter (CAP) as an indicator of hepatic fat content. Previous studies define a CAP value of ≥ 274 dB/m as a diagnostic criterion for NAFLD. A CAP value of ≥ 302 dB/m indicates severe hepatic steatosis(17, 18). In addition, liver stiffness measurements (LSM) of ≥ 8.2 kPa, ≥ 9.7 kPa, and ≥ 13.7 kPa represented the F2, F3, and F4 stages of liver fibrosis, respectively(19).
Variable
Demographic and clinical data were extracted from the NHANES database. Age, sex, race, educational level, body mass index (BMI), diabetes, hypertension, history of cardiovascular disease, smoking status, and laboratory variables were included. Diabetes mellitus was defined as HbA1c ≥ 6.5% or fasting glucose ≥ 126 mg/dL; in addition, participants had diabetes if they answered, “yes” to any of the following questions: “Do you use insulin?” or “Has your doctor told you that you have diabetes?” or “Do you take glucose-lowering medication?”, Hypertension was defined as a mean systolic blood pressure ≥ 140 mmHg or a mean diastolic blood pressure ≥ 90 mmHg on three consecutive measurements, and participants who responded to the questions “Have you been told you have high blood pressure on two or more occasions” or “Do you have to take prescription medication for high blood pressure?” A “yes” response was also defined as hypertension. A history of cardiovascular disease was described as a response confirming a physician's diagnosis of myocardial infarction, angina pectoris, coronary heart disease, congestive heart failure, or stroke. Smoking status was categorized as a smoker or never smoker based on having smoked fewer than 100 cigarettes in their lifetime.
Laboratory tests included measurements of alanine aminotransferase (ALT), aspartate aminotransferase (AST), total cholesterol (TC), triglycerides (TG), uric acid, albumin (Alb), glycosylated hemoglobin (HbA1c), γ-glutamyltranspeptidase (GGT) and high-density lipoprotein cholesterol (HDL-C). The neutrophil-to-high-density lipoprotein cholesterol ratio (NHR) was calculated by dividing the neutrophil count by the HDL-C level.
Statistical Analyses
Considering NHANES's complicated multistage sampling design, sample weights were applied in all analyses to ensure that the results were representative of the US population. Participants were divided into four groups according to NHR quartiles. Continuous variables are presented as weighted means with standard errors, while categorical variables are presented as unweighted counts and weighted percentages. One-way ANOVA for continuous variables and weighted chi-squared tests for categorical variables were used to compare differences between NHR quartiles.
NHR was analyzed as a continuous and categorical variable, with exposure variables grouped by quartiles (the first quartile served as the reference group). Outcome variables included liver steatosis parameters (CAP), NAFLD, liver stiffness measurements (LSM), and liver fibrosis. We used weighted linear regression and weighted logistic regression models for the analyses. In addition, we assessed potential non-linear associations between NHR and the prevalence of NAFLD and liver fibrosis using restricted cubic spline (RCS) analysis. The RCS model was adjusted for several confounders, including age, sex, ethnicity, smoking history, diabetes, hypertension, cardiovascular disease (CVD), body mass index (BMI), total cholesterol (TC), alanine aminotransferase (ALT), and uric acid. Subgroup analyses were conducted by stratifying participants according to age, gender, BMI, presence of hypertension, diabetes, and history of cardiovascular disease (CVD). All data analyses were done using R software (version 4.4.0), and the statistical significance level was set at P < 0.05.