Participants
This was a cross-sectional study. Two health examination datasets were analyzed in this study. The US dataset was retrieved from the Third American National Health and Nutrition Examination Survey (NHANES III), a nationally representative, cross-sectional study conducted by the National Center for Health Statistics of the United States from 1984 to 1994. The dataset of this study and further information are available at https://www.cdc.gov/nchs/nhanes/about_nhanes.htm. The Chinese health examination dataset was retrieved from Wenzhou Medical Center of Wenzhou People’s Hospital, China, from January 2010 to December 2010.
Exclusion criteria included the presence of any of the following: without ultrasonography results, with missing data, with hepatitis B or C, and participants with excessive alcohol intake. In Chinese datasets, it was defined as >140g/week for men and>70g/week for women[11]. In US datasets, it was defined as two drinks a day. A drink means a 12-oz beer, a 4-oz glass of wine or an ounce of liquor, approximately 35g alcohol.
According to the presence of fatty liver in ultrasonography, participants were divided into the NAFLD group and non-NAFLD group.
Anthropometric and biochemical measurements
The diagnosis of hepatosteatosis was based on ultrasonography. Serum cholesterol, triglyceride, serum creatinine, and uric acid were obtained from the original datasets. Body mass index (BMI) was calculated as weight (in kilograms) divided by the square of the height (in meters). Mean arterial pressure (MAP) was calculated as MAP= (systolic pressure + 2 x diastolic pressure) /3.
Estimated glomerular filtration rate (eGFR) was calculated according to the 2009 CKD-EPI eGFR formula[12]: eGFR = 141 x min(SCr/κ, 1)α x max(SCr /κ, 1) -1.209 x 0.993Age x [1.018 if female] x [1.159 if Black] ;κ = 0.7 (females) or 0.9 (males); α = -0.329 (females) or -0.411 (males), where Scr is serum creatinine concentration (in mg/dL) and age refers to age in years.
CKD was defined as either decreased eGFR (<60ml/min/1.732) and/or abnormal albuminuria and/or overt proteinuria, in accordance with the Kidney Disease: Improving Global Outcome (KIDGO) 2012 Practice guideline for CKD[13].
CKD was classified into five stages based on the eGFR categories. Decline in GFR category represented CKD stage G1 to G5 (≥90 [G1], 60-89 [G2], 59-30 [G3], 15-29 [G4], <15 [G5] ml/min/ 1.73 m2). The early stages of CKD were defined as CKD stage G1-G2, while the late stages of CKD were defined as stages 3-5.
Statistical analysis
Continuous variables were represented as mean ± standard deviation and compared using the Student’s t-test. Categorical variables were expressed as counts (percentages) and compared using the Chi-squared test or the Fisher’s exact test when the samples were limited in number. Binary logistic regressions including two models (Model 1 adjusted for age, sex, BMI. Model 1 adjusted for age, sex, BMI, history of diabetes and history of hypertension) were employed to find the relationship between renal function decline and the presence of NAFLD after correcting for different confounding factors of renal function. All tests were two-sided, and a p-value <0.05 were considered statistically significant. All analysis was conducted by SPSS version 23.0.