Table 2.

Results of unadjusted and multivariate-adjusted linear regression analyses with log-transformed 25-hydroxyvitamin D concentrations as the dependent variable (R ² = 0·274): adolescents aged 12–18 years, Korea National Health and Nutrition Examination Survey (KNHANES) 2008–2009

	Unadjusted (n 1510)				Multivariate-adjusted (n 1056)
	β	se	95 % CI	P value	β	se	95 % CI	P value
Constant	–	–	–	–	2·656	0·057	2·543, 2·769	<0·001
Age*	−0·041	0·005	−0·052, −0·030	<0·001	–	–	–	–
Sex
Males (v. females)	0·058	0·021	0·017, 0·098	0·006	0·035	0·020	−0·004, 0·075	0·080
Season (v. winter)				<0·001				<0·001
Spring	−0·009	0·033	−0·074, 0·055	0·774	0·010	0·034	−0·056, 0·076	0·757
Summer	0·335	0·031	0·274, 0·397	<0·001	0·314	0·036	0·243, 0·384	<0·001
Autumn	0·277	0·030	0·218, 0·336	<0·001	0·257	0·032	0·194, 0·320	<0·001
Degree of obesity (v. obese)				0·043				0·100
Lean	0·072	0·036	0, 0·143	0·048	0·063	0·033	−0·003, 0·128	0·060
Overweight	0·098	0·042	0·015, 0·182	0·021	0·075	0·038	−0·002, 0·151	0·050
Education level reached (v. elementary-school student)				<0·001				<0·001
College student	−0·228	0·060	−0·346, −0·111	<0·001	−0·256	0·065	−0·384, −0·127	<0·001
High-school student	−0·228	0·038	−0·302, −0·153	<0·001	−0·202	0·040	−0·280, −0·124	<0·001
Middle-school student	−0·111	0·038	−0·185, −0·037	0·003	−0·098	0·039	−0·175, −0·020	0·014
Regular physical activity† (v. yes)
No	−0·067	0·021	−0·109, −0·025	0·002	−0·037	0·020	−0·076, 0·002	0·060
Vitamin D supplement use (v. no)				0·006				0·012
≥5 μg/d	0·146	0·049	0·049, 0·243	0·003	0·259	0·047	0·166, 0·352	<0·001
0–<5 μg/d	0·118	0·058	0·004, 0·233	0·043	0·249	0·087	0·077, 0·421	0·005
Milk intake (v. ≥400 ml/d)				0·058				0·591
0–<200 ml/d	−0·105	0·048	−0·200, −0·011	0·029	−0·035	0·034	−0·101, 0·031	0·295
200–<400 ml/d	−0·309	0·051	−0·138, 0·060	0·441	−0·001	0·036	−0·070, 0·071	0·978
Interaction between season and vitamin D supplements (season × supplements)‡	–	–	–	–	–	–	–	<0·001
Ca intake (mg/d)§ (v. <DRI)
≥DRI	0·006	0·031	−0·066, 0·055	0·856	–	–	–	–

DRI, Dietary Recommended Intake.

*Age was excluded in multivariate-adjusted linear regression analysis because of collinearity with the education level reached.

†Regular physical activity: participants who performed moderate or vigorous physical activity for at least 60 min/d on 3 d/week, and/or walking activity for at least 60 min/d on 5 d/week, and/or muscular strength activity on more than 3 d/week were assigned to the ‘regular physical activity’ group. Those participants who performed moderate or vigorous physical activity for at least 180 min/d on 3 d/week were also assigned to the ‘regular physical activity’ group.

‡The modifying effect of season on the association between vitamin D supplement use and vitamin D status was significant in unadjusted and multivariate-adjusted linear regression analyses.

§The recommended DRI is 1000 mg/d for boys aged 12–14 years, 900 mg/d for girls aged 12–14 years, 900 mg/d for boys aged 15–18 years and 800 mg/d for girls aged 15–18 years.