Table 2

The characteristic of all Northern Finland Birth Cohort (NFBC) 1966 participants (N=4758) in the present study at 31 years by serum 25-hydroxyvitamin D tertiles* (I= the lowest tertile; III= the highest tertile)

	I		II		III
Tertile of serum 25(OH)D† N	1592		1589		1577
Tertile of serum 25(OH)D† N	n or mean	% or 95% CI	n or mean	% or 95% CI	n or mean	% or 95% CI	p Value
Sex n %
Males	782	32.9	800	33.7	792	33.4	0.75
Females	810	33.9	789	33.2	785	32.9
Environmental factors
Season of blood drawn‡ n %
High sunlight	566	19.2	1012	34.3	1375	46.5	<0.0001
Low sunlight	1026	56.9	577	31.9	202	11.2
Latitude§ n %
65°N	210	23.6	305	34.2	376	42.2	0.0006
>65°N	923	29.7	1042	33.6	1140	36.7
Anthropometry
Body mass index (kg/m²) mean 95% CI	24.8	24.6 to 25.0	24.8	24.6 to 24.9	24.4	24.2 to 24.6	0.017
Waist circumference (cm) mean 95% CI	84.6	83.9 to 85.2	84.0	83.4 to 84.6	82.9	82.3 to 83.4	0.0003
Socioeconomic position n %
I+II (Professional)	421	37.2	374	32.9	339	29.9	0.0046
III (Skilled worker)	501	33.8	503	33.9	479	32.3
IV (Unskilled worker)	386	31.4	427	34.8	415	33.8
V (Farmer)	60	36.4	49	29.7	56	33.9
VI (Other)	224	29.9	236	31.6	288	38.5
Lifestyle factors
Smoking n %
Non-smoker	742	34.9	686	32.2	700	32.9	0.055
Former/occasional smoker	366	30.2	438	36.1	410	33.7
Active smoker	484	34.2	465	32.9	467	32.9
Alcohol consumption (g/day) n %
Abstainer	165	38.7	146	34.3	115	27.0	0.053
Low risk drinker	1335	32.9	1349	33.3	1369	33.8
At-risk drinker	92	32.9	94	33.7	93	33.4
Leisure time computer use n %
Never	537	31.4	599	35.1	572	33.5	0.0012
No more than once per week	208	30.1	234	33.9	249	36.0
On 2 to 5 days per week	487	34.3	447	31.5	485	34.2
On more than 5 days per week	360	38.3	309	32.9	271	28.8
Quartile of physical activity (MET hours per week) n %
QI: 0.0–3.79	444	36.6	394	32.5	376	30.9	<0.0001
QII: 3.80–11.29	403	33.9	421	35.4	365	30.7
QIII: 11.30–21.99	415	34.5	397	33.0	391	32.5
QIV: >22.0	330	28.7	377	32.7	445	38.6
Diet score n %
0–1	478	32.7	477	32.7	506	34.6	0.26
2–3	912	33.3	920	33.6	907	33.1
4–5	202	36.2	192	34.4	164	29.4
Females only
Contraception n %
No contraception	441	38.2	401	34.8	312	27.0	<0.001
Other kinds of contraception	216	36.6	187	31.6	188	31.8
Oral contraceptive pills	140	23.1	190	31.3	277	45.6

The values are expressed as mean and 95% CIs; numbers and %.
*Differences between males and females were tested with ANOVA for normally distributed variables and Pearson's χ² test for categorical variables.
†Mean (95% CI) of 25-hydroxyvitamin D tertiles for all were 41.50 (41.11 to 41.89), 63.87 (63.55 to 64.19) and 100.01 (98.81 to 101.22). Serum total 25(OH)D may differ slightly from the actual sum of D2 and D3 because of amendment of undetectable D2 values (see methods).
‡The season of blood sampling were categorised as high sunlight (summer (1 June–30 August), autumn (1 September–31 October)) and low sunlight (winter (1 November–31 March) and spring (1 April–31 May)).
§Data included only on samples taken during all seasons from Oulu city and other provinces of Oulu and Lapland. Data not included on N=343 in men and N=419 in women with samples taken during winter months from Helsinki region.
MET, metabolic equivalent of task of physical activity; 25(OH)D, 25-hydroxyvitamin D;