[A single-center prospective study of vitamin D levels and its supplementary effect in the first trimester].

Objective: To investigate vitamin D levels and the effect of exogenous vitamin D supplementation in the first trimester among pregnant women in our center. Methods: This was a prospective cohort study. A total of 98 women in the first trimester who were followed-up regularly in Peking Union Medical College Hospital from December 1^st 2020 to December 1^st 2021 were enrolled. Subjects who had medical conditions that affect vitamin D absorption or metabolism were excluded. Questionnaires were administered, and 25-hydroxyvitamin D [25(OH)D] levels were detected using liquid chromatography tandem mass spectrometry (LC/MS/MS) method. According to the basic 25(OH)D level, different dosages of exogenous vitamin D were supplemented. After 4 weeks, 25(OH)D levels were detected again to evaluate the effect of supplementation. T test, analysis of variance, χ² test, and multiple linear regression analysis were used for analysis. Results: The mean age of enrolled subjects was (33.5±4.0) years. The baseline 25(OH)D level was (41.2±20.0) nmol/L. Briefly, 70.4% (69/98) subjects were deficient in vitamin D, and 42.9% (42/98) patients were using vitamin D supplementation at the time of 25(OH)D testing. Single-factor analysis showed that vitamin D supplementation (t=-4.21, P<0.001), season (t=2.59, P=0.011), and nut-eating frequency (t=2.67, P=0.009) were related to 25(OH)D levels. Multiple linear regression analysis showed that only vitamin D supplementation had a relationship with 25(OH)D level (B=13.84, P=0.006). According to the baseline 25(OH)D level, 400-5 000 U/d vitamin D3 was supplemented regularly for (4.1±2.5) weeks, and 25(OH)D levels significantly increased after supplementation [(64.1±18.1) vs (37.3±16.6) nmol/L, t=-9.36, P<0.001]. The ascending range was negatively associated with basic 25(OH)D level (B=-0.66, P<0.001) and positively associated with supplementary dosage (B=0.51, P<0.001). 25(OH)D levels increased by 0.51 nmol/L on average per 1 μg (40 U) of vitamin D supplementation daily. Conclusions: The proportion of vitamin D deficiency was high in the first trimester among pregnant women in our center. Exogenous vitamin D supplementation could significantly increase 25(OH)D levels, and the effect was negatively associated with basic 25(OH)D level but positively associated with supplementary dosage.