Table 2.

Simple linear regression and multiple linear regression results between covariates and SAF

Parameter	Simple linear regressiona				Multiple linear regressionb
	Unstandardized coefficient B [95% CI]	Standardized coefficient β	Explained variance (%)c	P value	Unstandardized coefficient B [95% CI]	Standardized coefficient β	Explained variance (%)d	P value
Age	0.020 [0.017, 0.023]	0.248	6.2	< 0.0001	0.017 [0.013, 0.022]	0.213	2.3	< 0.0001
Sexe	− 0.208 [− 0.246, − 0.169]	− 0.209	4.4	< 0.0001	− 0.197 [− 0.235, − 0.159]	− 0.199	3.6	< 0.0001
BMI	0.009 [0.004, 0.014]	0.072	0.5	< 0.0001	0.005 [0.001, 0.010]	0.044	0.2	0.022
Smoking status			2.7				2.2
Current smokerf	0.162 [0.113, 0.212]	0.131	–	< 0.0001	0.208 [0.155, 0.262]	0.168	–	< 0.0001
Ex-smokerf	0.022 [− 0.017, 0.062]	0.023	–	0.268	0.039 [− 0.003, 0.082]	0.040	–	0.067
25(OH)D3 (nmol/L)	− 0.002 [− 0.003, − 0.001]	− 0.105	1.1	< 0.0001	− 0.002 [− 0.003, − 0.002]	− 0.125	1.5	< 0.0001
DM statusg	0.220 [0.146, 0.294]	0.119	1.4	< 0.0001	0.112 [0.042, 0.182]	0.060	0.3	0.002
eGFR (mL/min per 1.73 m2)	− 0.004 [− 0.006, − 0.003]	− 0.123	1.5	< 0.0001	− 0.001 [− 0.003, 3.06 × 10−4]	− 0.035	0.1	0.115
RS subcohorts			4.8				1.0
RS-Ih	0.143 [0.098, 0.188]	0.126	–	< 0.0001	0.096 [0.038, 0.155]	0.085	–	0.001
RS-IIh	0.103 [0.063, 0.144]	0.102	–	< 0.0001	0.128[0.083, 0.174]	0.127	–	< 0.0001
Medication number	0.020 [0.009, 0.031]	0.074	0.5	< 0.0001	0.022 [0.012, 0.032]	0.082	0.6	< 0.0001
Coffee intake (g/d)	2.71 × 10−4 [2.01 × 10−4, 3.41 × 10−4]	0.153	2.3	< 0.0001	1.74 × 10−4 [1.04 × 10−4, 2.44 × 10−4]	0.098	0.8	< 0.0001
Cholesterol	− 0.019 [− 0.038, 0.001]	− 0.038	0.1	0.062	–	–	–	–
C-reactive protein	0.007 [0.002, 0.012]	0.054	0.3	0.009	–	–	–	–
Time interval	0.023 [0.019, 0.027]	0.220	4.9	< 0.0001	–	–	–	–

SAF skin auto-fluorescence; BMI body mass index; 25(OH)D₃ 25-hydroxyvitamin D₃; DM status diabetes mellitus status; eGFR estimated glomerular filtration rate calculated by the CKD-EPI

^aOutcome was SAF, without adjustment for covariates

^bModel 2: SAF ~ Age + sex + BMI + smoking status + 25(OH)D₃ + DM status + eGFR + coffee intake + medication numbers + RS subcohorts. N = 2388, R² = 0.190, p < 0.0001. Collinearity test satisfied, no apparent collinearity

^cPercentage of SAF variance explained by the variable in the simple linear regression model (%), equals R² of the model × 100%

^dPercentage of SAF variance explained by the variable in the multiple linear regression model (%), equals partial R² of the variable × 100%

^eReference group is male participants

^fReference group is non-smoking participants

^gReference group is non-diabetic participants

^hReference group is participants from RS-III