1. Introduction
The last decade has witnessed a steady stream of reports from all over the world about vitamin D insufficiency in the general population, even in regions with high year-round sun exposure (the so-called “Mediterranean paradox”) [
1,
2]. The first reports on vitamin D insufficiency appeared in the 80s of the previous century but it was not until recently that the problem gained significant attention due to the growing tendency to stay indoors, avoid sun exposure, and use of sunscreen [
2,
3]. Vitamin D insufficiency in pregnant women increases the risk for a complicated pregnancy course and results in fetal insufficiency [
4,
5].
The literature offers no consensus on the recommended vitamin D concentrations or unified diagnostic criteria. Furthermore, the official nomenclature also varies (optimal, adequate, recommended, sufficient, insufficient, intermediate, suboptimal, hypovitaminosis, deficient, medium deficient, severe deficient) [
5,
6,
7,
8], which may hinder epidemiological estimates on the global scale. In light of the ongoing debate on the optimal vitamin D concentration in pregnant women, most experts are of the opinion that the 25(OH)D concentration in maternal blood should exceed 30 ng/mL [
2,
8,
9]. However, other authors claim that maternal concentration that fully normalizes vitamin D metabolism and calcium homeostasis is at least 40 ng/mL [
10], and still others that >20 ng/mL is necessary to prevent vitamin D deficiency in the newborn [
11].
As dietary sources of vitamin D have failed to meet the recommended levels for years [
12,
13], vitamin D concentration is in fact enhanced by supplementation and exposure to sunlight [
2,
5]. In consequence, season of the year, geographic location and lifestyle have become the key factors as far as sources of vitamin D are concerned. The aim of the study was to evaluate maternal and umbilical cord blood vitamin D concentrations and determine the extent to which the season of the year (summer) improves these levels in Poland.
4. Discussion
In light of current recommendations, mean maternal vitamin D concentration in our study population was deficient. The summer group had higher values, which most probably had its source in more sun exposure, as there were no differences in vitamin D intake between both groups. Nevertheless, the patients were asked to estimate only the amount of time they spent outside in the summer between 10:00 a.m. and 3:00 p.m., which does not provide direct information about the exact duration of exposure to sun. Our results are consistent with the findings of other authors, who have also confirmed vitamin D deficiency among pregnant women in Poland [
3,
5,
17,
18,
19], although reports in the Polish literature on better vitamin D saturation in the summer are less unambiguous. In that respect, our results are in agreement with the study by Skowrońska-Jóźwiak et al. and Zasimovich et al. who also confirmed season-dependent variability in vitamin D levels, with the highest values in summer and the lowest in winter [
3,
17]. However, Bartoszewicz et al. demonstrated only slight and statistically insignificant increase in their summer group [
6], while Domaracki et al., reported no differences between the two seasons [
18]. Reports from other European countries also found a tendency for higher vitamin D levels among pregnant women during summer, e.g., in Germany which is located at a similar latitude as Poland [
20], as well as countries with very different geographic location and sun exposure, such as Greece [
12], or Norway [
21]. Higher maternal vitamin D concentration during summer resulted in lower rates of mothers with vitamin D deficiency as compared to the winter group (34% vs. 66%), but the scale of vitamin D insufficiency remains significant. Overall, only 11% of the subjects reached the recommended level, which is consistent with the recent study in the Polish population (10.8%) [
18]. As far as vitamin D deficiency is concerned, the situation in Poland is comparable to that seen in neighboring countries. The recommended levels of vitamin D were found in only 5% of the pregnant women in Germany [
20], and 16.5% in Ukraine [
22]. In our study, the high number of women with vitamin D deficiency might result from avoidance of sun exposure between 10:00 a.m. and 3:00 p.m. and using sunscreen, which was reported by 46% and 26% of the women from the summer group, respectively. Baczyńska-Strzecha et al. found that adequate, according to the patients, time of sun exposure was declared by only 25% of the women from the premature labor group and 47.5% from the term group [
19]. Avoidance of sun exposure has also been observed in other countries [
23,
24]. A study conducted in Belgium found that pregnant women who avoid sun exposure are at a 2-fold higher risk for severe vitamin D deficiency [
25].
Due to the fact that weather conditions, including the increasingly common heat waves which prevent exposure to sun, remain beyond human control, supplementation seems to be the best method of preventing vitamin D insufficiency. Regardless, despite vitamin/mineral supplementation in pregnancy, maternal intake of vitamin D remains below the recommended levels, mostly because 75% of the multi-component supplements for pregnant women which are available in Poland contain small amount of vitamin D (5–10 µg) [
14], as compared to the recommended dose of 37.5–50 µg [
8]. Small doses of vitamin D do not meet the requirements of the body, and remain ineffective as preventive measures [
23,
26,
27,
28]. According to the European Union law, producers of dietary supplements need not comply with the recommended doses of nutrient content. Thus, their content is established entirely by the manufacturers [
29]. The problem of low doses of vitamin D in preparations for pregnant women appears to be global. Lack of compliance with the contemporary recommendations has been voiced by experts in Belgium [
25], Turkey [
23], and Canada [
30]. Only 15% of the women from this study used single-component preparations, with higher vitamin D content (mean 25 µg) [
14], which automatically resulted in higher maternal and cord blood levels.
Consumption of sea fish in Poland continues to be low (approximately 15 g/day) [
31]. Thus, it seems impossible for it to ever become a significant source of vitamin D, especially in light of exaggerated media reports on chemical pollutants absorbed by the fish.
In all mother-infant blood sets, umbilical cord blood vitamin D concentration was higher by 7.7 ng/mL on average (approximately 40%), as compared to maternal levels, which is consistent with most studies worldwide, claiming that cord blood level was higher than maternal venous blood level by 3.9–8.0 ng/mL [
7,
12,
26,
32]. Godand et al. [
21] found a reverse significant correlation, while Halicioglu et al. [
23] reported comparable concentrations in both, maternal and umbilical cord blood. In our study, we found that maternal vitamin D concentration of >15.3 ng/mL, i.e., half the recommended dose, would be sufficient to meet the demands of the developing fetus. Streym et al. declared that maternal vitamin D concentration which is necessary for the fetus ought to be >20 ng/mL [
11].
In our study, we confirmed detrimental effects of excessive weight gain in pregnancy which results in diminished vitamin D concentration. Despite a lack of consensus in the literature [
6,
12,
23,
33], prevention of excessive weight gain is important enough to monitor and mention the problem during each check-up visit. Elevated levels of vitamin D in mothers with GDM remain a surprise. It is most probably the result of the fact that GDM women in this sample more often used single-component vitamin D preparations, or even several supplements at once, as compared to healthy women (35.8% vs. 11.5%, respectively). As vitamin D concentration was measured immediately before the delivery, we lack data on pre-pregnancy, as well as the first trimester of pregnancy, values. Thus, we were unable to evaluate the relationship between vitamin D levels and the development of GDM.
The absence of a correlation between blood vitamin D levels and its intake from the diet (no supplementation) is most probably a direct result of low vitamin D intake (2.1 µg/day), which is consistent with reports of other authors at a daily intake of 0.8 µg [
12]. In light of the current norms in Poland [
34], adequate consumption of vitamin D from dietary sources was reported by only 6% of the investigated women. It seems that the estimated vitamin D consumption is a reliable result as it is consistent with other reports, which for years have indicated a consistently low amount of vitamin D in the diet of Polish pregnant women (2.6–2.8 µg/day) [
13,
35]. The same has been found for young non-pregnant women, who consume on average 2.4 µg/day of vitamin D [
36]. In our study, vitamin D intake was not connected with education or age (data not presented). The association between vitamin D consumption and financial status was not investigated, which is one of the limitations of our study because that factor might have affected the diet of the investigated population. Although the study was conducted in the capital city of Poland, where mean net monthly income is the highest in the country, vitamin D intake was low.
Other limitations of our study include a relatively small sample size, which was the result of the number of deliveries at the Clinic and the nature of the study, i.e., the winter-summer comparison. Also, the study was not conducted on a representative sample of Polish pregnant women but only a group of patients from one city, in the central part of Poland, thus general conclusions ought to be drawn with caution. Regardless, no significant differences in sun exposure between the north and the south of the country have ever been reported, so the effect of the geographic location and climatic factors seems to be insignificant. It is consistent with the national studies, on a representative sample, which have determined the effect of the geographic latitude on serum vitamin D concentration to be of minimal importance [
37].
As for multiparity, due to small sample size we did not evaluate the relationship between parity and time elapsed between subsequent gestations, which might have affected the vitamin D status in the body. We also did not investigate pre-pregnancy vitamin D intake, which may have had some impact on its concentration in the first trimester. Also, it is vital to bear in mind that in our study the analysis of vitamin D level was conducted on the day of the delivery, which does not signify that the concentration was typical for the entire course of pregnancy or at least its significant part. Noteworthy, type and factor of the sun screen was also not specified, which might have been a confounding factor.
