Clinical characteristics of the study population
A total of 2538 participants (1360 men and 1178 women) were included in this study, and 853 (33.61%) had NAFLD. The prevalence of NAFLD was 13.10% in lean participants (BMI <24 kg/m2), and was 53.32% in obese participants (BMI ≥24 kg/m2). We compared clinical characteristics based on NAFLD status (Table 1). We found that both lean and obese NAFLD patients were older, had higher BMI, larger waist circumference, higher systolic and diastolic blood pressure, and had elevated serum levels of alanine aminotransferase, γ-glutamyl transpeptidase, triglyceride, LDL-cholesterol, uric acid and fasting glucose, but lower serum HDL-cholesterol levels than corresponding controls. Besides, vitamin D levels of obese NAFLD patients were lower than those of obese NAFLD-free controls (59.03 ± 19.46 versus 63.56 ± 22.09 nmol/L, P <0.001), but this was not observed in lean participants (Table 1).
Table 1. Clinical characteristics of the study population according to obese and NAFLD categories
Variables
|
Overall
(n =2538)
|
Lean participants
|
|
Obese participants
|
NAFLD
(n =163)
|
Without NAFLD
(n =1081)
|
P value
|
|
NAFLD
(n =690)
|
Without NAFLD
(n =604)
|
P value
|
Gender (male/female)
|
1360/1178
|
86/77
|
430/651
|
0.002
|
|
485/205
|
359/245
|
<0.001
|
Age (year)
|
54.08 (6.85)
|
54.84 (6.59)
|
53.81 (6.69)
|
0.078
|
|
54.23 (6.60)
|
54.18 (6.99)
|
0.901
|
Body mass index (kg/m2)
|
24.22 (2.96)
|
22.77 (1.14)
|
21.71 (1.54)
|
<0.001
|
|
27.09 (2.18)
|
25.83 (1.73)
|
<0.001
|
Waist circumference (cm)
|
85.13 (8.67)
|
83.34 (5.12)
|
78.66 (6.12)
|
<0.001
|
|
92.88 (6.39)
|
88.35 (6.37)
|
<0.001
|
Systolic blood pressure (mmHg)
|
127.93 (18.38)
|
127.64 (16.86)
|
122.77 (18.51)
|
0.002
|
|
134.37 (17.41)
|
129.88 (16.89)
|
<0.001
|
Diastolic blood pressure (mmHg)
|
78.37 (11.56)
|
78.08 (9.98)
|
74.77 (11.26)
|
<0.001
|
|
82.96 (10.72)
|
79.65 (11.34)
|
<0.001
|
Albumin (g/L)
|
46.13 (2.49)
|
46.58 (2.34)
|
45.98 (2.49)
|
0.004
|
|
46.51 (2.36)
|
45.84 (2.60)
|
<0.001
|
Alanine aminotransferase (U/L)
|
23.21 (19.06)
|
28.64 (45.74)
|
18.24 (12.69)
|
0.004
|
|
30.78 (16.61)
|
22.00 (15.59)
|
<0.001
|
Aspartate aminotransferase (U/L)
|
21.77 (10.64)
|
23.35 (20.40)
|
20.51 (9.70)
|
0.082
|
|
23.77 (9.15)
|
21.30 (9.60)
|
<0.001
|
γ-Glutamyl Transpeptidase (U/L)
|
29.90 (31.84)
|
35.50 (35.39)
|
23.00 (31.34)
|
<0.001
|
|
40.49 (32.64)
|
28.64 (26.98)
|
<0.001
|
Triglyceride (mmol/L)
|
1.63 (1.14)
|
2.02 (1.16)
|
1.26 (0.82)
|
<0.001
|
|
2.17 (1.43)
|
1.54 (0.95)
|
<0.001
|
Total cholesterol (mmol/L)
|
4.71 (0.89)
|
4.83 (0.92)
|
4.67 (0.86)
|
0.031
|
|
4.75 (0.89)
|
4.71 (0.92)
|
0.382
|
HDL-cholesterol (mmol/L)
|
1.22 (0.34)
|
1.12 (0.26)
|
1.36 (0.01)
|
<0.001
|
|
1.04 (0.25)
|
1.19 (0.28)
|
<0.001
|
LDL-cholesterol (mmol/L)
|
2.75 (0.72)
|
2.83 (0.79)
|
2.69 (0.68)
|
0.016
|
|
2.79 (0.75)
|
2.79 (0.74)
|
0.839
|
Serum uric acid (μmol/L)
|
325.71 (81.72)
|
332.94 (73.72)
|
293.72 (71.12)
|
<0.001
|
|
369.41 (78.22)
|
331.10 (80.78)
|
<0.001
|
Fasting blood glucose (mmol/L)
|
5.21 (1.21)
|
5.53 (1.34)
|
4.99 (1.03)
|
<0.001
|
|
5.57 (1.50)
|
5.12 (0.94)
|
<0.001
|
Smoking history (yes/no)
|
525/2013
|
32/131
|
175/906
|
0.271
|
|
195/495
|
123/481
|
0.001
|
Vitamin D (nmol/L)
|
61.21 (21.64)
|
60.95 (18.54)
|
61.32 (22.97)
|
0.844
|
|
59.03 (19.46)
|
63.56 (22.09)
|
<0.001
|
Data are expressed as mean (SD).
HDL-C, high-density lipoprotein cholesterol; LDL-C, low-density lipoprotein cholesterol; NAFLD, nonalcoholic fatty liver disease.
Association of serum vitamin D levels with the prevalence of NAFLD
We classified all participants into quartiles by their serum vitamin D levels and analyzed the association of vitamin D quartiles with the prevalence of NAFLD. We found that serum vitamin D quartiles were negatively associated with the prevalence of NAFLD in obese participants (Table 2). The prevalence of NAFLD was 56.54%, 59.77%, 52.15%, and 44.51% in the first, second, third, and fourth quartiles of serum vitamin D in obese participants (P for trend <0.001; Table 2). However, serum vitamin D quartiles were not associated with the prevalence of NAFLD in lean participants (Table 2).
Table 2. Association of vitamin D quartiles with prevalence of NAFLD in lean and obese participants
VD quartiles
|
Lean participants
|
|
Obese participants
|
Total
|
NAFLD
|
PR%
|
PR
|
χ2
|
P value
|
|
Total
|
NAFLD
|
PR%
|
PR
|
χ2
|
P value
|
Quartile 1
|
327
|
37
|
11.31
|
0.95
|
4.133
|
0.247
|
|
306
|
173
|
56.54
|
1.27
|
17.118
|
<0.01
|
Quartile 2
|
292
|
38
|
13.01
|
1.09
|
|
343
|
205
|
59.77
|
1.34
|
Quartile 3
|
306
|
50
|
16.34
|
1.37
|
|
326
|
170
|
52.15
|
1.17
|
Quartile 4
|
319
|
38
|
11.91
|
1.00
|
|
319
|
142
|
44.51
|
1.00
|
VD, vitamin D; PR%, prevalence rate; PR, prevalence ratio.
Participants were classified into quartiles according to their serum vitamin D levels: quartile 1, <45.5 nmol/L; quartile 2, 45.5-59.5 nmol/L; quartile 3, 59.5-74.2 nmol/L; and quartile 4, ≥74.3 nmol/L.
We also divided all participants into three groups according to their vitamin D adequacy status and analyzed the association of vitamin D adequacy status with the prevalence of NAFLD in lean and obese participants, respectively. We found that participants with vitamin D deficiency had the highest prevalence of NAFLD (57.38%), followed by those with vitamin D insufficiency (52.06%), and vitamin D sufficiency (41.82%) in the obese group (P for trend =0.045; Table 3). However, the prevalence of NAFLD was comparable among lean participants with different vitamin D adequacy status (Table 3).
Table 3. Association of vitamin D sufficiency with prevalence of NAFLD in lean and obese participants
VD classification
|
Lean participants
|
|
Obese participants
|
Total
|
NAFLD
|
PR%
|
PR
|
χ2
|
P-value
|
|
Total
|
NAFLD
|
PR%
|
PR
|
χ2
|
P value
|
VD deficiency
|
427
|
49
|
11.48
|
1.28
|
3.114
|
0.211
|
|
413
|
237
|
57.38
|
1.37
|
6.194
|
0.045
|
VD insufficiency
|
750
|
108
|
14.40
|
1.61
|
|
826
|
430
|
52.06
|
1.24
|
VD sufficiency
|
67
|
6
|
8.96
|
1.00
|
|
55
|
23
|
41.82
|
1.00
|
VD, vitamine D; PR%, prevalence rate; PR, prevalence ratio.
Participants were classified into three groups according to their serum vitamin D levels: VD sufficiency, ≥100 nmol/L; VD insufficiency, 50-100 nmol/L; and VD deficiency, <50 nmol/L.
Association of serum vitamin D levels with risk of NAFLD
Next, multiple logistic regression analyses were conducted to explore the risk factors of NAFLD in lean and obese participants. We found that male gender, high BMI and waist circumference, high serum levels of albumin, alanine aminotransferase, aspartate aminotransferase, uric acid, and fasting blood glucose, and low serum levels of HDL-cholesterol were correlated with increased risks of NAFLD in both lean and obese participants (Table 4). We also found that serum vitamin D concentrations were another factor associated with the risk of NAFLD in obese participants, with an adjusted OR (95% CI) of 0.986 (0.979–0.992). However, serum vitamin D concentrations were not associated with the risk of NAFLD in lean participants (Table 4).
Table 4. Multivariable analysis for factors associated with risk of NAFLD in lean and obese participants
Variables
|
Lean participants
|
|
Obese participants
|
Wald χ2
|
OR (95% CI)
|
P value
|
Wald χ2
|
OR (95% CI)
|
P value
|
|
Male gender
|
16.793
|
2.809 (1.714–4.604)
|
<0.001
|
|
10.171
|
1.782 (1.250–2.542)
|
0.001
|
|
Age (years)
|
3.363
|
1.028 (0.998–1.059)
|
0.067
|
|
3.601
|
1.020 (0.999–1.042)
|
0.058
|
|
Body mass index (kg/m2)
|
12.505
|
1.432 (1.174–1.747)
|
<0.001
|
|
15.075
|
1.205 (1.097–1.323)
|
<0.001
|
|
Waist circumference (cm)
|
8.647
|
1.071 (1.023–1.122)
|
0.003
|
|
8.709
|
1.046 (1.015–1.079)
|
0.003
|
|
Albumin (g/L)
|
8.411
|
1.129 (1.040–1.226)
|
0.004
|
|
13.816
|
1.109 (1.050–1.172)
|
<0.001
|
|
Alanine aminotransferase (U/L)
|
21.812
|
1.070 (1.040–1.101)
|
<0.001
|
|
36.361
|
1.055 (1.037–1.074)
|
<0.001
|
|
Aspartate aminotransferase (U/L)
|
10.249
|
0.922 (0.877–0.969)
|
0.001
|
|
14.961
|
0.945 (0.918–0.972)
|
<0.001
|
|
Total cholesterol (mmol/L)
|
9.794
|
1.407 (1.136–1.742)
|
0.002
|
|
4.790
|
1.181 (1.018–1.372)
|
0.029
|
|
HDL-cholesterol (mmol/L)
|
25.698
|
0.146 (0.069–0.307)
|
<0.001
|
|
45.129
|
0.154 (0.089–0.266)
|
<0.001
|
|
Serum uric acid (μmol/L)
|
10.789
|
1.005 (1.002–1.008)
|
0.001
|
|
16.539
|
1.004 (1.002–1.006)
|
<0.001
|
|
Fasting blood glucose (mmol/L)
|
6.690
|
1.196 (1.044–1.370)
|
0.010
|
|
11.248
|
1.230 (1.090–1.387)
|
0.001
|
|
Vitamin D (nmol/L)
|
-
|
-
|
-
|
|
19.253
|
0.986 (0.979–0.992)
|
<0.001
|
|
Backward stepwise regression was used in multivariate logistic regression analyses (probability to enter = 0.05 and probability to remove = 0.10).
OR, odds ratio; CI, confidence interval; HDL-C, high-density lipoprotein cholesterol.
We further analyzed the correlation between vitamin D quartiles and the risk of NAFLD (Table 5). Among obese participants, compared with participants in the fourth quartile, participants with serum vitamin D levels in the first, second, and third quartiles all showed increased risks of NAFLD, with the adjusted OR (95% CI) of 1.942 (1.329–2.838), 1.853 (1.286–2.669) and 1.453 (1.009–2.093), respectively. However, the risk of NAFLD was comparable among lean participants with different serum vitamin D quartiles (Table 5). Similarly, we analyzed the correlation between vitamin D adequacy status and risk of NAFLD (Table 6). Among obese participants, vitamin D deficiency showed an increased risk of NAFLD compared with those with vitamin D sufficiency, with an adjusted OR (95% CI) of 1.906 (1.005–3.614). However, vitamin D deficiency was not associated with an increased risk of NAFLD in lean participants (Table 6). These results showed that decreased serum vitamin D concentrations were associated with an increased risk of NAFLD in obese but not lean participants.
Table 5. Association of serum Vitamin D quartiles with risk of NAFLD in lean and obese participants
Lean
|
Models
|
Odds ratios (95% confidence interval)
|
χ2 value
|
P value
|
Q1 (n=327)
|
Q2 (n=292)
|
Q3 (n=306)
|
Q4 (n=319)
|
Model 1
|
0.943 (0.583–1.527)
|
1.106 (0.684–1.789)
|
1.444 (0.917–2.275)
|
1
|
4.007
|
0.261
|
Model 2
|
1.174 (0.702–1.964)
|
1.272 (0.766–2.111)
|
1.527 (0.948–2.459)
|
1
|
3.190
|
0.363
|
Model 3
|
1.191 (0.673–2.106)
|
1.122 (0.637–1.795)
|
1.498 (0.882–2.545)
|
1
|
2.461
|
0.482
|
Obese
|
Models
|
Odds ratios (95% confidence interval)
|
χ2 value
|
P value
|
Q1 (n=306)
|
Q2 (n=343)
|
Q3 (n=326)
|
Q4 (n=319)
|
Model 1
|
1.621 (1.182–2.224)
|
1.852 (1.360–2.521)
|
1.358 (0.996–1.852)
|
1
|
17.145
|
0.001
|
Model 2
|
1.888 (1.337–2.666)
|
1.980 (1.420–2.758)
|
1.514 (1.088–2.106)
|
1
|
19.473
|
< 0.001
|
Model 3
|
1.942 (1.329–2.838)
|
1.853 (1.286–2.669)
|
1.453 (1.009–2.093)
|
1
|
15.141
|
0.002
|
Model 1 was unadjusted.
Model 2 was adjusted for age, gender, and body mass index.
Model 3 was further adjusted for waist circumference, systolic and diastolic blood pressure, albumin, alanine aminotransferase, aspartate aminotransferase, γ-glutamyl transpeptidase, triglyceride, total cholesterol, HDL-cholesterol, LDL-cholesterol, fasting blood glucose, serum uric acid and smoking history.
Participants were classified into quartiles according to their serum vitamin D levels: quartile 1, <45.5 nmol/L; quartile 2, 45.5-59.5 nmol/L; quartile 3, 59.5-74.2 nmol/L; and quartile 4, ≥74.3 nmol/L.
Table 6. Association of vitamin D sufficiency with risk of NAFLD in lean and obese participants
Lean
|
Models
|
Odds ratios (95% confidence interval)
|
χ2 value
|
P value
|
VD deficiency (n=427)
|
VD insufficiency (n=750)
|
VD sufficiency (n=67)
|
Model 1
|
1.318 (0.0.541–3.209)
|
1.710 (0.722–4.054)
|
1
|
3.085
|
0.214
|
Model 2
|
1.728 (0.680–4.392)
|
1.948 (0.797–4.761)
|
1
|
2.353
|
0.308
|
Model 3
|
1.571 (0.557–4.429)
|
1.827 (0.674–4.955)
|
1
|
1.724
|
0.422
|
Obese
|
Models
|
Odds ratios (95% confidence interval)
|
χ2 value
|
P value
|
VD deficiency (n=413)
|
VD insufficiency (n=826)
|
VD sufficiency (n=55)
|
Model 1
|
1.874 (1.059–3.313)
|
1.511 (0.869–2.626)
|
1
|
6.136
|
0.047
|
Model 2
|
2.334 (1.264–4.310)
|
1.778 (0.986–3.206)
|
1
|
9.108
|
0.011
|
Model 3
|
1.906 (1.005–3.614)
|
1.335 (0.722–2.467)
|
1
|
8.074
|
0.018
|
|
|
|
|
|
|
|
|
Model 1 was unadjusted.
Model 2 was adjusted for age, gender, and body mass index.
Model 3 was further adjusted for waist circumference, systolic blood pressure, albumin, alanine aminotransferase, aspartate aminotransferase, total cholesterol, LDL-cholesterol, fasting blood glucose, serum uric acid and smoking history.
Participants were classified into three groups according to their serum vitamin D levels: VD sufficiency, ≥100 nmol/L; VD insufficiency, 50-100 nmol/L; and VD deficiency, <50 nmol/L.