The best source of vitamin D is exposure to natural sunlight and 90% of vitamin D is derived from sunlight with 10% derived from food and plant sources (Paxton et al, 2013; Cannell, 2019). Approximately 1 in 5 people have low vitamin D levels (defined as serum levels below 25 nmol/L) in the UK. Recent statistics reveal that vitamin D deficiency affects the population as follows: 19% aged 4–10 years old, 37% 11–18 years old, 29% 19–64 years old, and 27% for people aged 65 years and older (National Diet and Nutrition Survey, 2019). There is sufficient research to support a correlation between certain risk factors and vitamin D deficiency including lack of exposure to sunlight, darker skin pigmentation, increased body mass index and certain ethnic groups which involves the wearing of cultural garments (Paxton et al, 2013; Fallon et al, 2020).
Vitamin D screening and supplementation during pregnancy has generated significant interest in the last decade and is currently a topic of ongoing debate. According to current guidelines by the Royal College of Obstetricians and Gynaecologists ([RCOG], 2014), antenatal screening for vitamin D levels is not advised universally and only recommended for woman at high risk of vitamin D deficiency. The guidelines further recommend against routine vitamin D supplementation; and state that supplementation should be reserved for pregnant women who are found to have vitamin D serum levels below 50 nmol (RCOG, 2014). However, there is ongoing dispute regarding vitamin D screening in pregnancy and routine vitamin D supplementation (De Laine et al, 2013; Roth et al, 2017), with an argument for universal vitamin D serum screening, as opposed to the argument that vitamin D supplementation should be recommended for all pregnant women (Davies-Tuck et al, 2015).
Discussion
Vitamin D is an essential fat-soluble vitamin which plays an important role in the absorption of calcium and phosphorus which are vital for optimal bone and muscle health (Hasanpour Dehkordi, 2018). In addition to the well-established muscle and skeletal benefits of vitamin D, there is emerging evidence to support vitamin D being effective in reducing the risk of illness such as diabetes and cardiovascular heart disease, as well as enhancing immune function and protecting against certain forms of cancers (Wimalawansa, 2018a; 2018b).
Vitamin D comprises of two active forms: vitamin D2 and D3. Vitamin D2, otherwise known as ergocalciferol, is derived from natural plant and food sources, whereas vitamin D3, known as cholecalciferol, is synthesised from exposure to ultraviolet (UV) light B radiation (De-Regil et al, 2016). Both active forms of vitamin D are metabolised by the liver to produce 25-hyrdroxyvitamin D (25(OH)D) which is then converted by the kidneys into calcitriol (De-Regil et al, 2016). The majority of vitamin D is synthesised from exposure to sunlight, with approximately 10% of vitamin D requirements derived from food sources such as oily fish, fortified dairy products, fortified breakfast cereals and eggs (Fischer, 2020). The latest evidence suggests fish oil supplements are suitable and are recommended (Chen et al, 2016; Kashroo et al, 2020), and the best sources of EPA and DHA are cold-water fish such as salmon, tuna, sardines, anchovies and herring. Many people are justifiably concerned about mercury and other toxins in fish, especially during pregnancy. For this reason, purified fish oil supplements are often the safest source of EPA and DHA (Middleton et al, 2018).
Risk factors for vitamin D deficiency
There are several risk factors which have been linked to increased incidence of vitamin D deficiency; many of the risk factors relate to reduced synthesis of vitamin D from lack of exposure to sunlight. Risk factors for vitamin D deficiency include women who have limited exposure to sunlight for behavioural reasons including those working indoors, nightshift workers and those who are predominately housebound (Aji et al, 2019). Moreover. it is well-documented that latitude and season have a significant influence on vitamin D deficiency around the world, with a higher prevalence of vitamin D deficiency in temperate zones in comparison to tropical zones and during the winter months (Griffiths, 2020).
Many cultural factors influence vitamin D levels and it is widely accepted that women from ethnic backgrounds who have darker skin have a higher prevalence of vitamin D deficiency (Richard et al, 2017). Women with darker skin have increased melanin pigment in their skin which protects against UV light exposure and inhibits synthesis of vitamin D. Therefore, women with darker skin require 3–6 times more exposure to sunlight to achieve optimal vitamin D synthesis in comparison to women with fairer skin (Mitchell et al, 2019).
Women with darker skin including those from South East Asia, Middle East and Africa are associated with higher prevalence of vitamin D deficiency of up to 80% (Davies-Tuck et al, 2015). Furthermore, women who wear covering garments and veils as part of cultural or religious practices are also at significant increased risk of vitamin D deficiency during pregnancy (Al-Yatama et al, 2019). Significant evidence also supports that vitamin D deficiency is higher amongst women from indigenous backgrounds around the world including Native Americans; Native Canadians, Maori, Pacific Islanders and Australian Aboriginals (O'Brien et al, 2016; Naqvi et al, 2017; Malacova et al, 2019; Porter et al, 2019).
There are many physical factors and health conditions that are associated with increased risk of vitamin D deficiency. There is significant evidence to support a correlation between obesity and vitamin D deficiency; synthesised vitamin D being stored in adipose tissue and reducing its bioavailability (Carrelli et al, 2017). In addition, certain conditions such as kidney disease, liver failure and intestinal malabsorption and medications (anti-retroviral and anti-epileptic) affect the synthesis and absorption of vitamin D, and are associated with increased risk of vitamin D deficiency (Weinert and Silveiro, 2015; Magro and Borg, 2017). There is also evidence to suggest a link between lifestyle behaviours such as smoking, alcohol and inactivity to vitamin D deficiency (Skaaby et al, 2016). Therefore, it is important that midwives have sound knowledge of the various risk factors associated with vitamin D deficiency and ensure thorough screening of women during the antenatal period.
Current research and guidelines regarding vitamin D in pregnancy
There has been a significant increase in interest regarding vitamin D in pregnancy, with a current lack of consensus around the world regrading vitamin D screening and supplementation during pregnancy (Davies-Tuck et al, 2015). There is also much debate regarding optimal ranges for vitamin D and what qualifies as vitamin D deficiency. Current guidelines in the UK recommend a daily vitamin D supplementation of 10 μg for all pregnant women which is considered safe with minimal risks of causing harm or toxicity; universal screening of vitamin D levels is not currently recommended, with universal supplementation of vitamin D is considered more cost-effective (Hynes et al, 2017). In contrast, current guidelines in Australia and New Zealand advise that universal vitamin D supplementation during pregnancy is not recommended; supplementation is only advised for women at risk of deficiency or those with levels below 50 mmol (Department of Health, 2018).
While the consensus against universal screening for vitamin D during pregnancy cannot be disputed in terms of substantial economic savings on the healthcare system, there is evidence to suggest that the lack of screening results in a poor identification rate of those with or at risk of moderate to severe vitamin D deficiency (González-Chica and Stocks, 2019). Therefore, the healthcare costs related to treatment of moderate to severe vitamin D deficiency, the potential impact on the child and the related costs should be considered. There are conflicting views and evidence regarding the recommendation of routine vitamin D supplementation during pregnancy. There is evidence from randomised controlled trials which argues that vitamin D supplementation during pregnancy is safe and effective in achieving optimal vitamin D levels and preventing against maternal and fetal adverse health effects (Mir et al, 2016; Divakar et al, 2018). There is evidence from a Cochrane review which suggests that vitamin D supplementation during pregnancy is associated with decreased risk of pre-eclampsia, gestational diabetes, low birthweight and severe postpartum haemorrhage (Palacios et al, 2019)
In addition, the review reported that a combination therapy of vitamin D and calcium supplements during pregnancy can potentially reduce the risk of pre-eclampsia but may increase the risk of preterm birth; thus warranting the need for further research (Palacios et al, 2019). However, the current available guidelines by the World Health Organization (2016) state that there is limited evidence to support routine supplementation of vitamin D during pregnancy and in the prevention of maternal and fetal adverse effects, recommending exposure to sunlight and adequate nutritional intake to optimise vitamin D levels during pregnancy. The lack of consensus regarding vitamin D supplementation during pregnancy underlines the need for further research and the provision of consistent guidelines for midwives and health practitioners.
Impact of vitamin D deficiency on maternal and fetal health
In recent years, there has been a significant increase in research regarding vitamin D in pregnancy and the associated maternal adverse effects. Vitamin D is well-known for its effects in the body in regulation of bone and muscle health however, there is also evidence to support that vitamin D plays a role in increasing insulin sensitivity and responsiveness; thus improving the metabolism and transport of glucose (Tang et al, 2018). Therefore, there is mounting evidence to support that vitamin D deficiency in pregnancy is associated with an increased risk of gestational diabetes (Shao et al, 2019).
Growing evidence suggests that low levels of maternal serum vitamin D are associated with increased onset of anxiety and depressive disorders (Shah and Gurbani, 2019). A recent study (Aghajafari et al, 2018) identified that low vitamin D levels during the first 18 weeks of gestation was associated with increased risk of onset of mood disturbance and depressive symptoms in the antenatal and postnatal periods. For the last century, it has been well-established that maternal vitamin D deficiency has adverse effects on the growth and development of the fetus (Wagner and Hollis, 2018). One of the most well-known adverse effects associated with low maternal vitamin D during pregnancy is the condition vitamin D-deficient rickets, which involves abnormal bone growth and growth plate mineralisation. There have also been some significant studies linking small for gestational age neonates with maternal vitamin D deficiency (Chen et al, 2017; Wang et al, 2018).
Vitamin D supplementation
With regard to vitamin D supplementation, there is evidence to suggest that problems may arise due to patient compliance and adherence with supplementation therapy (Patil et al, 2018) and this implies that noncompliance to supplementation therapy could be the cause of inadequate levels. It is suggested that many pregnant women may be incompliant with vitamin D supplements due to concerns of side effects, optimal doses and the risk of overdose, and women have also cited factors such as frequency, taste and preparation of vitamin D supplements may also contribute to nonadherence (Bonevski et al, 2013). In addition to noncompliance, there are concerns regarding overdosing of vitamin D supplements in pregnancy, leading to vitamin D toxicity. Vitamin D toxicity is associated with symptoms such as weight loss, polyuria, arrhythmias and anorexia; and a worse-case scenario being multi-organ failure (Sanders and Lamb, 2015).
There is a wide discrepancy regarding recommended dosage for vitamin D supplementation where indicated according to national policy. There is recent evidence from a Cochrane review by Palacios et al (2019) to suggest that a higher dose supplement of greater than 600 IU is associated with a reduced the risk of gestational diabetes in pregnant women. Moreover, the review concluded that a higher dose vitamin D supplement (greater than 600 IU) during pregnancy appeared to be safe with few reported cases of adverse effects (Palacios et al, 2019). Therefore, highlighting the need for further research and development of consistent practice guidelines. It is vital that midwives are aware of the potential issues with compliance of vitamin D supplementation during pregnancy. Providing pregnant women with practical solutions and advice to enhance compliance to supplementation is essential, such as smartphone apps, daily alarms/reminders and combining supplementation with routine activities ie before or after brushing teeth. In addition, healthcare professionals should ensure that women are provided with adequate education on the dosage of vitamin D supplements as per national guidelines to reduce the likelihood of vitamin D overdose and toxicity.
Conclusion
Vitamin D deficiency is a significant public health concern worldwide. Vitamin D is of particular importance for women during pregnancy for there is ample evidence to support vitamin D deficiency being associated with many significant maternal and fetal adverse effects. Risk factors associated with vitamin D deficiency include limited sun exposure, obesity, ethnicity and various medical conditions that disrupt synthesis of vitamin D. Some of the maternal adverse effects include an increased risk of gestational diabetes, low birthweight, preterm birth and postnatal depression. Moreover, there is evidence that maternal vitamin D deficiency is associated with adverse effects in the development of children which include rickets, asthma, diabetes, eczema and impaired brain development. It is vital that women maintain adequate levels of vitamin D during pregnancy to ensure optimal maternal and f etal well-being.