Table 1. Baseline characteristics by working hours per week among 44,627 participants without nonalcoholic fatty liver disease
Characteristics
|
Overall
|
Working hours per week
|
Men
|
p for trend
|
Women
|
p for trend
|
35–40 hours
|
41–52 hours
|
≥53 hours
|
35–40 hours
|
41–52 hours
|
≥53 hours
|
Number
|
44,627
|
2,949
|
12,028
|
6,402
|
|
7,461
|
11,585
|
4,202
|
|
Age (years)
|
35.1 (±6.2)
|
39.3 (±8.0)
|
36.2 (±6.2)
|
36.0 (±6.0)
|
<0.01
|
34.9 (±6.0)
|
33.3 (±5.3)
|
33.3 (±5.6)
|
<0.01
|
Married (%)
|
71.4
|
79.4
|
71.7
|
71.7
|
<0.01
|
82.9
|
66.2
|
59.0
|
<0.01
|
Income (≥400*, %)
|
68.1
|
64.1
|
61.6
|
64.1
|
0.014
|
75.4
|
71.3
|
71.2
|
<0.01
|
Education (≥college, %)
|
91.7
|
87.4
|
93.3
|
94.1
|
<0.01
|
87.0
|
92.6
|
92.7
|
<0.01
|
Center (Seoul, %)
|
63.2
|
63.1
|
57.5
|
58.4
|
<0.01
|
62.4
|
68.1
|
75.1
|
<0.01
|
BMI (kg/m2)
|
20.6 (±1.5)
|
21.3 (±1.2)
|
21.3 (±1.3)
|
21.3 (±1.3)
|
0.887
|
20.1 (±1.5)
|
20.0 (±1.5)
|
19.9 (±1.5)
|
<0.01
|
Hypertension (%)
|
3.4
|
8.3
|
5.5
|
5.4
|
<0.01
|
1.7
|
1.0
|
1.2
|
0.004
|
Diabetes mellitus (%)
|
2.0
|
2.0
|
1.9
|
1.7
|
0.138
|
2.3
|
2.1
|
1.8
|
0.073
|
Stress (score)
|
17.2 (±6.5)
|
14.8 (±5.5)
|
16.1 (±5.7)
|
18.3 (±6.8)
|
<0.01
|
16.3 (±6.2)
|
17.9 (±6.7)
|
19.8 (±7.5)
|
<0.01
|
Depression (%)
|
10.9
|
5.9
|
6.2
|
9.6
|
<0.01
|
10.9
|
14.2
|
20.9
|
<0.01
|
Current smoker (%)
|
17.5
|
37.1
|
31.9
|
35.9
|
0.414
|
1.6
|
1.8
|
2.9
|
<0.01
|
Alcohol drinking (≥10 g, %)
|
16.7
|
32.2
|
26.9
|
28.7
|
0.056
|
5.4
|
5.8
|
8.3
|
<0.01
|
HEPA (%)
|
11.4
|
17.5
|
14.6
|
13.5
|
<0.01
|
9.1
|
8.0
|
7.8
|
0.007
|
Sleep duration (<8 hours, %)
|
85.9
|
85.8
|
90.3
|
93.5
|
<0.01
|
75.1
|
83.2
|
88.9
|
<0.01
|
Sitting time (≥8 hours, %)
|
73.4
|
55.3
|
72.7
|
79.0
|
<0.01
|
61.9
|
78.9
|
83.1
|
<0.01
|
* 1,000 KRW; Abbreviations: BMI, body mass index; HEPA, health-enhancing physical activity
Table 2. Development of lean NAFLD according to the working hour category in men and women
Categories of
working hours
|
PY
|
Incident cases
|
Incidence rate
(Cases per 1000 PY)
|
Age-adjusted
HR (95% CI)
|
Multivariable-adjusted HR (95% CI) a
|
HR (95% CI)b in a model using time-dependent variables
|
Total
|
|
|
|
|
|
|
35–40 hours
|
39,205.4
|
958
|
24.4 (22.9-26.0)
|
1.00 (reference)
|
1.00 (reference)
|
1.00 (reference)
|
41–52 hours
|
91,972.7
|
3,158
|
34.3 (33.1-35.5)
|
1.07 (0.99-1.15)
|
1.14 (1.01-1.29)
|
1.14 (1.05-1.23)
|
≥53 hours
|
42,135.5
|
1,622
|
38.4 (36.6-40.4)
|
1.07 (0.98-1.16)
|
1.10 (0.96-1.26)
|
1.25 (1.13-1.37)
|
p for trend
|
|
|
|
0.078
|
0.146
|
<0.01
|
Men
|
|
|
|
35–40 hours
|
11,230.9
|
572
|
50.9 (46.9-55.2)
|
1.00 (reference)
|
1.00 (reference)
|
1.00 (reference)
|
41–52 hours
|
48,180.1
|
2,563
|
53.2 (51.1-55.3)
|
1.06 (0.96-1.16)
|
1.20 (1.03-1.40)
|
1.16 (1.06-1.28)
|
≥53 hours
|
26,412.9
|
1,395
|
52.8 (50.1-55.6)
|
1.04 (0.94-1.15)
|
1.16 (0.98-1.37)
|
1.25 (1.12-1.39)
|
p for trend
|
|
|
|
0.293
|
0.156
|
<0.01
|
Women
|
|
|
|
35–40 hours
|
27,974.4
|
386
|
13.8 (12.4-15.2)
|
1.00 (reference)
|
1.00 (reference)
|
1.00 (reference)
|
41–52 hours
|
43,792.6
|
595
|
13.5 (12.5-14.7)
|
1.10 (0.96-1.25)
|
1.06 (0.86-1.31)
|
1.10 (0.94-1.29)
|
≥53 hours
|
15,722.6
|
227
|
14.4 (12.6-16.4)
|
1.15 (0.98-1.36)
|
1.02 (0.77-1.33)
|
1.12 (0.89-1.41)
|
P for trend
|
|
|
|
0.118
|
0.666
|
0.039
|
Abbreviations: NAFLD, non-alcoholic fatty liver disease; PY, person-years; HR, hazard ratio; CI, confidence interval
p <0.001 for the overall interaction between sex and working hour category for incident NAFLD (time-dependent model).
a Estimated from parametric proportional hazard models with age, year of visit, center, sex, marital status, income level, education level, stress level, hypertension, diabetes mellitus, depression, smoking status, alcohol consumption, physical activity level, sitting time, and sleep duration.
b Estimated from parametric proportional hazard models with center, sex, marital status, and education level as time-fixed variables and working hours, stress level, hypertension, diabetes mellitus, depression, smoking status, alcohol consumption, physical activity level, sitting time, and sleep duration as a time-dependent variable
Table 3. Development of NAFLD plus intermediate/high probability of advanced fibrosis based on NFS according to the working hour category in men and women
Categories of
working hours
|
PY
|
Incident cases
|
Incidence rate
(Cases per 1000 PY)
|
Age-adjusted
HR (95% CI)
|
Multivariable-adjusted HR (95% CI) a
|
HR (95% CI)b in a model using time-dependent variables
|
Total
|
|
|
|
|
|
|
35–40 hours
|
40723.5
|
50
|
1.23 (0.93-1.62)
|
1.00 (reference)
|
1.00 (reference)
|
1.00 (reference)
|
41–52 hours
|
97784.2
|
127
|
1.30 (1.09-1.55)
|
1.15 (0.81-1.61)
|
1.37 (0.67-2.78)
|
1.72 (1.17-2.52)
|
≥53 hours
|
45247.5
|
72
|
1.59 (1.26-2.00)
|
1.28 (0.88-1.87)
|
1.68 (0.74-3.80)
|
1.66 (1.01-2.74)
|
p for trend
|
|
|
|
0.343
|
0.914
|
0.075
|
Men
|
|
|
35–40 hours
|
12175.1
|
37
|
3.04 (2.20-4.19)
|
1.00 (reference)
|
1.00 (reference)
|
1.00 (reference)
|
41–52 hours
|
53029.7
|
114
|
2.15 (1.79-2.58)
|
1.21 (0.83-1.79)
|
1.69 (0.73-3.87)
|
1.87 (1.22-2.88)
|
≥53 hours
|
29134.6
|
66
|
2.27 (1.78-2.88)
|
1.35 (0.89-2.05)
|
2.39 (0.95-6.00)
|
1.87 (1.09-3.22)
|
p for trend
|
|
|
|
0.408
|
0.441
|
0.035
|
Women
|
|
|
35–40 hours
|
28548.3
|
13
|
0.46 (0.26-0.78)
|
1.00 (reference)
|
1.00 (reference)
|
1.00 (reference)
|
41–52 hours
|
44754.4
|
13
|
0.29 (0.17-0.50)
|
0.89 (0.40-1.95)
|
0.91 (0.21-3.94)
|
-
|
≥53 hours
|
16112.8
|
6
|
0.37 (0.17-0.83)
|
1.01 (0.38-2.67)
|
-
|
-
|
p for trend
|
|
|
|
0.727
|
-
|
-
|
Abbreviations: NAFLD, non-alcoholic fatty liver disease; NFS, nonalcoholic fatty liver disease fibrosis score; PY, person-years; HR, hazard ratio; CI, confidence interval
p =0.051 for the overall interaction between sex and working hour category for incident NAFLD plus fibrosis based on NFS (time-dependent model)
a Estimated from parametric proportional hazard models with age, year of visit, center, sex, marital status, income level, education level, stress level, hypertension, depression, smoking status, alcohol consumption, physical activity level, sitting time, and sleep duration.
b Estimated from parametric proportional hazard models with center, sex, marital status, and education level as time-fixed variables and working hours, stress level, hypertension, depression, smoking status, alcohol consumption, physical activity level, sitting time, and sleep duration as a time-dependent variable