Table 3.

Related factors of vitamin D and vitamin A with multivariable linear regression models and restricted cubic spline.

Variables	Vitamin D			Vitamin A
	β	95%CI	p	β	95%CI	p
Age (overall)a	0.16	0.07–0.26	<0.001	0.004	0.004–0.005	<0.001
Age (non–linear)b			0.033			<0.001
Season
Spring	1.99	1.49–2.50	<0.001	0.03	0.02–0.03	<0.001
Summer	9.73	9.21–10.25	<0.001	−0.01	−0.02 to −0.01	<0.001
Autumn	5.78	5.24–6.31	<0.001	−0.02	−0.03 to −0.02	<0.001
Winter	ref			ref
Gestational trimester
1st trimester	ref			ref
2nd trimester	3.35	2.93–3.77	<0.001	−0.0003	−0.004 to 0.003	0.864
3rd trimester	5.53	5.08–5.99	<0.001	−0.051	−0.054 to −0.048	<0.001
Region
Shaanxi	10.7	10.1–11.35	<0.001	0.001	−0.005 to 0.006	0.843
Ningxia	–			0.03	0.02–0.04	<0.001
Qinghai	ref			0.02	0.01–0.02	<0.001
Shanxi	–			ref

The four seasons are defined by date: spring, from March to May; summer, from June to August; autumn, from September to November; winter, from December to February.

^aoverall association;

^bnon-linear association.