Low birth weight associated low Vitamin D (little sunshine in China in this case) - Dec 2019

Associations between prenatal sunshine exposure and birth outcomes in China

Science of the Total Environment https://doi.Org/10.1016/j.scitotenv.2019.136472 STOTEN 136472
Xin Zhang, Yixuan Wang, Xi Chen, Xun Zhang

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Perhaps 50% less likely to have low birth weight if there was lots of sunshine while pregnant

This paper is one of the first to examine the associations between prenatal sunshine exposure and birth outcomes, specifically the incidence of low birth weight (LBW) and small for gestational age (SGA), based on a nationally representative birth record dataset in China. During the sample period in the 1990s, migration was limited in rural China, allowing us to address the identification challenges, like residential sorting and avoidance behaviors. We found a nonlinear relationship between the length of sunlight and birth outcomes. In particular, prenatal exposure to increasing sunshine was associated with a reduction in the incidence of LBW and SGA, especially in the second trimester during pregnancy. This finding was consistent with the clinical evidence suggesting positive effects of sunshine on birth outcomes via obtaining vitamin D or relieving maternal stress.

Introduction

The effects of environmental factors on birth outcomes have received much attention in the literature. For example, humidity has been proven to be an important determinant of mortality (Barreca 2012). Air pollution can affect infant health and its cognitive condition, as well as the incidence of low birth weight and premature birth (Currie et al. 2009; Knittel et al. 2016; Bharadwaj et al. 2017). Hot weather can result in low birth weight (Deschenes et al. 2009) and preterm delivery (Basu et al. 2010; Andalon et al. 2016; Beltran et al. 2014). Extreme cold can also lead to adverse birth outcomes, such as mortality (Deschenes and Moretti 2009; Deschenes 2014) and low birth weight (Murray 2000; Beltran et al. 2014).
Recent epidemiological studies revealed that certain metal elements and organic compounds could also affect birth weight. Rodosthenous et al. (2017) suggested that prenatal lead (Pb) exposure was negatively associated with infant birth weight. Hou et al. (2019) revealed that low level serum cobalt (Co) was positively correlated with low birth weight and showed a nonlinear dose relationship. Ding et al. (2015) found that prenatal exposure to pyrethroids was adversely correlated with birth weight. Rhee et al. (2015) reported that high caffeine intake during pregnancy increased the risk of low birth weight significantly. Additionally, maternal smoking behaviors and passive smoking were also associated with low birth weight (Lindbohm et al. 2002).
Compared to other environmental stressors, evidence on the effects of sunshine exposure on health-related outcomes is limited and inconclusive. Tustin et al. (2004) found that first trimester maternal exposure to sunlight increased birth weight in human infants in New Zealand. Trudeau et al. (2016) found that sunshine exposure had a negative effect on birth weight for white infants but a positive effect for black infants, potentially through different mechanisms of vitamin D and folic acid by race. Wernerfelt et al. (2017) suggested that increasing sunlight in the second trimester during pregnancy could lower the probability of asthma through the effect of vitamin D. Slusky and Zeckhauser (2018) revealed that sunlight in late summer and early fall would strongly protect newborns against influenza.
Sunshine can affect birth outcomes through two offsetting physiological channels. First, sunshine is associated with folate depletion because of ultraviolet (UV) radiation (Fukuwatari et al. 2009). Folate and folic acid are different forms of vitamin B9. The well-studied consequence of folic acid deficiency for newborns is neural tube defects (De- Regil et al. 2010; Blencowe et al. 2010; Chen et al. 2008; De Wals et al. 2007; Ray et al. 2002; Green 2002; Scholl and Johnson 2000; Hernandez-Diaz et al. 2000; Berry et al. 1999; Milunsky et al. 1989). Folic acid deficiency can also lead to defective cellular growth (Hibbard 1964; Scholl and Johnson 2000; Sram et al. 2005) and negative birth outcomes, such as preterm delivery, fetal growth retardation, and low birth weight (Siega-Riz et al. 2004; Scholl et al. 1996; Scholl and Johnson 2000; Butterworth and Bendich 1996; Iyengar and Rajalakshmi 1975).
Second, exposure to sunshine can allow obtaining vitamin D, which plays an important part in pregnancy and birth. Vitamin D comprises a group of fat-soluble secosteroids. The main sources of vitamin D in the human body are from photosynthesis within the skin under exposure to ultraviolet B (UVB) radiation and dietary intake. The effect of vitamin D in pregnancy has been examined quite thoroughly. Maternal and infant vitamin D levels are highly correlated (Ponsonby et al. 2010). Appropriate vitamin D supplements can reduce the risk of low birth weight (Gernand et al. 2013; Scholl and Chen 2009; Mannion et al. 2006) and small for gestational age (Bodnar et al. 2010). Vitamin D deficiency, on the other hand, can cause fertility impairment (Lewis et al. 2010), maternal skeletal preservation, and infant skeletal formation impairment, which raise the risk of chronical diseases for fetuses (Lapillonne 2010).
Sunshine may also affect birth outcomes via a psychological channel. As documented in the literature, sunshine enhances happiness and mental well-being (Kämpfer and Mutz 2013; Hirshleifer and Shumway 2003; Keller et al. 2005), while prenatal exposure to stressful events (e.g., natural disasters, violence, and family loss) has detrimental effects on birth outcomes, such as low birth weight and preterm (Persson and Rossin-Slater 2018; Aizer et al. 2016; Koppensteiner and Manacorda 2016; Torche 2011). Therefore, a long-time window of sunlight exposure may promote positive birth outcomes through alleviating maternal stress during pregnancy. However, too much sunshine may also be a stressor and result in negative birth outcomes.
The existing literature details mixed conclusions on the influence of prenatal exposure to sunshine on birth outcomes, and reports that the effect differs by race. The goal of the present study was to examine how and to which extent birth outcomes, i.e., low birth weight (LBW) and small for gestational age (SGA), were related to in utero exposure to sunshine for a Chinese population using a nationally representative birth record dataset. Taking advantage of the unique aspects of rural China in the 1990s, we were able to address the identification challenges, like residential sorting and avoidance behaviors.
Early life has been well recognized as a critical period that shapes long-term health and well-being (Barker 1990). For example, the Fetal Origins Hypothesis shows that the nine months in utero could have a substantial effect on future outcomes (e.g., Almond and Currie 2011). As birth weight strongly predicts adult health and earnings (Bharadwaj et al., 2018; Black et al., 2007; Behrman and Rosenzweig 2004), this study may help effectively target newborns more vulnerable to inadequate sunshine to promote their health as well as shedding light on other labor and fertility policies in China.

Data and methods

Study design and population
The birth record data were obtained from the China’s National Disease Surveillance Points (DSP) system, which contained data from 145 counties in 31 provinces (autonomous regions and municipalities), using multistage cluster probability sampling to cover a 1% nationally representative sample of the Chinese population. The system was established by the Chinese Academy of Preventive Medicine, with the “Disease Prevention Unit” of the township hospitals in each DSP site responsible for the vital registration within the DSP system; see Yang et al. (2005) for a detailed introduction. The data included detailed information on each birth, including birth weight, gender, birth order, date and county of birth, gestational week, and parents’ age at birth and education years. We obtained permission from the Chinese Center for Disease Control and Prevention to use data from the Disease Surveillance Point System. Data were de-identified, and informed consent was not required.
The gestational week information was recorded according to the exact date of the mother’s last menstrual period. As displayed in Figure A1, the gestational age ranged from 28 to 45 weeks, with 93.76% of the gestational age concentrated between 37 and 41 weeks. Infants born before 37 weeks were considered preterm and infants born after 42 weeks were considered postterm, which accounted for 2.97% and 3.28% of our sample observations, respectively. The distribution of gestational age in our sample was similar to that in Dai et al. (2004), which confirmed the accuracy of the gestational age measurement. We interpolated the missing gestational age by 39 weeks in the analysis.
We used two binary indicators to represent the incidences of low birth weight (LBW) and small for gestational age (SGA), respectively. LBW refers to infants whose birth weights were below 2,500 grams, regardless of gestational age. SGA refers to babies whose birth weights were below the 10th percentile for each gestational age by gender using data from the National Population-based Birth Defects Surveillance System; see Table 2 in Dai et al. (2014) for the gestational age-specific birth weight percentiles for Chinese babies. Based on the information on gestational age and birth date, we calculated the conception date and identified the three trimesters of the pregnancy accordingly. In particular, we assigned weeks 1-13 after conception to the first trimester, weeks 14-26 to the second trimester, and weeks 27-39 to the third trimester.

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