Table 2.

Characteristics of study participants according to categories of serum 25(OH)D concentration; adolescents aged 10–19 years, Korea National Health and Nutrition Examination Survey (KNHANES) 2009–2010

	<37·5 nmol/l		37·5 to <50 nmol/l		≥50 nmol*
Variable	Mean	se	Mean	se	Mean	se	P value
Total participants (n, %)	553	37·7	543	37·1	370	25·2
Age (years)	15·6||	0·14	14·5	0·16	14·0	0·22	<0·001
Sex (n (males/female))	265/288		285/258				219/151						0·062**
Season (n (spring/summer/autumn/winter))	112/92/111/238		74/162/194/113				20/181/133/36						<0·001**
Height Z-score	0·50¶	0·060	0·56	0·071	0·69	0·062	0·033
Weight Z-score	0·29	0·066	0·32	0·061	0·33	0·056	0·582
BMI Z-score	0·07	0·069	0·09	0·055	0·06	0·063	0·917
FM (g) Z-score	0·15	0·074	0·15	0·067	0·06	0·079	0·414
FM (%) Z-score	0·07	0·068	0·10	0·067	−0·03	0·091	0·453
FMI Z-score	0·12	0·074	0·12	0·068	0·00	0·086	0·320
Degree of obesity (n (lean/overweight/obese))	435/68/50		427/69/47				296/52/22						0·440**
WC ≥90th percentile (n (no/yes))	500/53		497/46				343/27						0·609**
Regular PA (n (no/yes))†	242/311		188/355				85/285						<0·001**
IFG (n (no/yes))‡	523/30		522/21				360/10						0·460**
Fasting blood glucose (mmol/l)	4·94	0·02	4·89	0·02	4·89	0·02	0·039††
Fasting insulin (pmol/l)§	96·5||	2·5	92·1||	1·9	81·0	2·0	<0·001††
HOMA-IR§	3·116||	0·086	2·933||	0·064	2·575	0·068	<0·001††
HOMA-β§	198·39||	4·732	199·2||	4·795	173·17	4·894	0·001††
QUICKI§	0·329||	0·001	0·330||	0·001	0·336	0·001	<0·001††

25(OH)D, 25-hydroxyvitamin D; FM, fat mass; WC, waist circumference; PA, physical activity; IFG, impaired fasting glucose; HOMA-IR, homeostatic model assessment–insulin resistance index; HOMA-β, homeostatic model assessment–β-cell function; QUICKI, quantitative insulin-sensitivity check index.

All continuous variables are given as means with their standard errors.

*Reference category.

†Regular PA (n (yes)) means the number of 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, who were assigned to the ‘regular physical activity’ group. Those participants who performed moderate or vigorous physical activity for at least 180 min/day on 3 d/week were also assigned to the ‘regular physical activity’ group.

‡IFG (n (no)) means the number of participants with fasting glucose <5·6 mmol/l.

§The analysis was performed using natural log-transformed values.

||The value was significantly different from that of the reference category (P < 0·001).

¶The value was significantly different from that of the reference category (P=0·027).

**The Rao–Scott χ ² test was used to analyse the associations between the categorical variables and the three categories of serum 25(OH)D.

††Tests for linear trend (P for linear trend) across the three categories of serum 25(OH)D levels were performed. We calculated the age- and sex-adjusted means for fasting glucose, fasting insulin, HOMA-IR, HOMA-β and QUICKI according to the categories of serum 25(OH)D. Subgroup analyses were then performed to analyse the mean differences in the dependent variables between the reference category (25(OH)D ≥50 nmol/l) and the other two categories.