Visual Abstract

Vitamin D is an endocrine regulator of calcium and bone metabolism. Yet, its effects include other systems, such as innate and adaptive immunity. Unique to pregnancy, circulating 1,25-dihydroxyvitamin D (1,25[OH]2D) increases early on to concentrations that are 2–3 times prepregnant values. At no other time during the lifecycle is the conversion of 25-hydroxyvitamin D (25[OH]D) to 1,25(OH)2D directly related and optimized at ≥100 nmol/L. Vitamin D deficiency appears to affect pregnancy outcomes, yet randomized controlled trials of vitamin D supplementation achieve mixed results depending on when supplementation is initiated during pregnancy, the dose and dosing interval, and the degree of deficiency at the onset of pregnancy. Analysis of trials on an intention-to-treat basis as opposed to the use of 25(OH)D as the intermediary biomarker of vitamin D metabolism yields differing results, with treatment effects often noted only in the most deficient women. Immediately after delivery, maternal circulating 1,25(OH)2D concentrations return to prepregnancy baseline, at a time when a breastfeeding woman has increased demands of calcium, beyond what was needed during the last trimester of pregnancy, making one question why 1,25(OH)2D increases so significantly during pregnancy. Is it to serve as an immune modulator? The vitamin D content of mother’s milk is directly related to maternal vitamin D status, and if a woman was deficient during pregnancy, her milk will be deficient unless she is taking higher doses of vitamin D. Because of this relative “deficiency,” there is a recommendation that all breastfed infants receive 400 IU vitamin D3/day starting a few days after birth. The alternative – maternal supplementation with 6,400 IU vitamin D3/day, effective in safely raising maternal circulating vitamin D, that of her breast milk, and effective in achieving sufficiency in her recipient breastfeeding infant – remains a viable option. Additional research is needed to understand vitamin D’s influence on pregnancy health and the effect of maternal supplementation on breast milk’s immune signaling.

  • The active form of vitamin D – 1,25-dihydroxyvitamin D (1,25[OH]2D) – increases during pregnancy, remains elevated throughout, and, unlike at other times during the lifecycle, is directly affected by circulating 25-hydroxyvitamin D (25[OH]D) concentration with the optimal point of conversion of 25(OH)D to 1,25(OH)2D at 100 nmol/L (40 ng/mL).

  • Lactation has increased demands on the mother regarding nutrient intake delivered through her breast milk to her recipient infant: when a mother is vitamin D deficient, her milk is deficient, which can be remedied by direct infant supplementation; however, this treats only the infant.

  • A safe alternative during lactation to infant supplementation is direct maternal vitamin D supplementation at higher doses than usual (6,400 IU/day), improving the vitamin D status of the mother, the content of the milk, and, consequently, the vitamin D status of the infant, effectively treating both the mother and the infant.

Conception Onward

From the moment of conception, there are tremendous changes that must occur for growth and shaping of a single-cell organism to billions of cells as the construct of diverse systems, which function in concert to form a living human being. It is in the context of this timing, this concert of matter and energy transfer across cells, that we can appreciate what is happening surrounding conception. Conception does not occur in a hostile or nonnurturing environment, yet the very invasion of extravillous cytotrophoblasts into the uterine wall is an invasive and inflammatory process [1-3]. Pregnancy is a state of change and flux that must balance between negen-tropy—organization of tissue—and cellular death and apoptosis – necessary for refinement of tissue and organ structure. The very event of conception is dependent upon a functional neuroendocrine system in both the mother and father, a functioning uterus with a rich lining to allow for invasion of the extravillous cytotrophoblasts into the uterine wall, and a dynamic synchrony of cell division and cell death. The initiation of human life at the moment of conception involves a myriad of ancient signaling hormones, which include vitamin D [4, 5].

Long known as an endocrine facilitator in its role as a preprohormone affecting calcium and bone metabolism and homeostasis, vitamin D is something more as well. Our understanding of vitamin D has expanded in the decades since its discovery in the early 20th century. There are provocative experimental models in animals that extend to observational and some clinical trials in humans, which suggest that vitamin D plays a role in both innate and adaptive immunity, affecting our ability to survive infectious insults as well as long-latency diseases, such as autoimmune diseases and cancers, all of which depend on a balanced and functional immune system [6].

There are 2 forms of vitamin D: ergocalciferol (or vitamin D2, which is synthesized by plants and fungi) and cholecalciferol (or vitamin D3, which is synthesized in the skin of humans and animals). Humans can metabolize both forms of vitamin D. Pre-vitamin D3 is synthesized in the epidermal layer of the skin by keratinocytes mainly in the stratum basale and stratum spinosum when 7-dehyrocholesterol is exposed to ultraviolet B light in the wavelength of 290–320 nm [7]. Through this photolytic energy transfer, pre-vitamin D is formed, and with further thermally induced isomerization in the skin, the parent compound vitamin D3 is produced. Vitamin D3 is carried into the bloodstream bound to vitamin D-binding protein (VDBP) or, less frequently, to albumin. Once vitamin D (either form D2 or D3) enters the circulation, either through epidermal transfer or intestinal absorption, it associates with VDBP, a 58-kD globular protein that binds vitamin D and its metabolites with various affinities based on the number and position of polar functional groups and/or methyl groups [8]. The initial step in the metabolic activation of vitamin D is the enzyme-catalyzed insertion of an OH group at carbon 25; this oxidation process is primarily a hepatic microsomal function mostly by CYP2R1, a 25-hydroxylase [9], producing 25-hydroxyvitamin D (25[OH]D), the most abundant circulating form of vitamin D [10].

As shown in Figure 1 (from Hollis and Wagner [11], with permission), following formation in the liver, 25(OH)D appears in the circulation – bound primarily to VDBP. The half-life of the parent compound is 12–24 h, while that of its first metabolite 25(OH)D is 2–3 weeks. The conversion of 25(OH)D to the active hormone 1,25-dihydroxyvitamin D (1,25[OH]2D) through the CYP27B1 enzyme mainly occurs in the proximal tubules of the kidney, and then it is carried throughout the body also bound to VDBP.

Fig. 1.

Diagram of the metabolic processes providing vitamin D and its metabolites to various tissues of the body. Tissue distribution of vitamin D and 25(OH)D based on simple diffusion (red arrows) or endocytosis (green arrows). Endocytosis requires the tissue-specific megalin-cubilin system, whereas simple diffusion is primarily controlled by the dissociation constant of the vitamin D compound for VDBP. Bolder red lines indicate greater diffusion rates due to a higher dissociation constant. t1/2, half-life. (Hollis and Wagner [11], 2013, with permission.)

Fig. 1.

Diagram of the metabolic processes providing vitamin D and its metabolites to various tissues of the body. Tissue distribution of vitamin D and 25(OH)D based on simple diffusion (red arrows) or endocytosis (green arrows). Endocytosis requires the tissue-specific megalin-cubilin system, whereas simple diffusion is primarily controlled by the dissociation constant of the vitamin D compound for VDBP. Bolder red lines indicate greater diffusion rates due to a higher dissociation constant. t1/2, half-life. (Hollis and Wagner [11], 2013, with permission.)

Close modal

Unlike 25(OH)D, 1,25(OH)2D has a much shorter half-life of 4–8 h. VDBP preferentially binds 25(OH)D with higher affinity than 1,25(OH)2D or the parent compound [12]. The high affinity of VDBP for the vitamin D and its metabolites, coupled with the excessive binding capacity, keeps “free” or unbound concentrations of vitamin D and its metabolites at quite low relative concentrations [13, 14]. This is important because only the “free” concentrations of the vitamin and its metabolites have transmembrane diffusion capabilities, thus exerting their biologic function. What influences vitamin D status throughout the lifecycle is parathyroid hormone (PTH). When circulating 1,25(OH)2D concentrations decrease, PTH increases, affecting intestinal absorption of vitamin D and conversion of vitamin D from its precursor in the skin. The measurement of intact PTH (iPTH) has long been considered an indicator of vitamin D deficiency and is used as a marker [15].

All vitamin D moieties are capable of binding to the vitamin D receptor (VDR). As shown in Figure 1, the conversion of vitamin D to 25(OH)D and of 25(OH)D to 1,25(OH)2D in the nuclear membrane of the cell is not limited to the liver and kidneys, respectively; keratinocytes and many cells throughout the body, including monocytes, macrophages, and prostate and breast cells, can convert vitamin D3 to 25(OH)D and then to the active form 1,25(OH)2D [16, 17]. 1,25(OH)2D’s endocrine effects include the following classic triad of action: (1) increase in intestinal calcium (as Ca2+ ions) absorption through the actions of calbindin; (2) increase in urinary calcium reabsorption; and (3) regulation of PTH in a negative feedback loop that allows calcium to be absorbed from the gastrointestinal tract, reabsorbed from urine, and metabolized from bone in order to maintain calcium homeostasis within the body. Because calcium is essential to all tissues and organs, particularly the heart, skeletal muscle, and brain, the body will claim calcium, if necessary, from the skeleton. In individuals with vitamin D deficiency, only trace amounts of vitamin D will be found in the body because whatever comes into the circulation is quickly converted to 25(OH)D and then to 1,25(OH)2D to maintain calcium homeostasis [18]. For this reason, 1,25(OH)2D is not the indicator of vitamin D status and why 25(OH)D with its longer half-life should be used.

Another important factor influencing the conversion rate of 25(OH)D to 1,25(OH)2D is the tissue transport mechanism for these secosteroids referred to as the megalin-cubilin system. The megalin-cubilin endocytic system [19] serves as an essential delivery system of 25(OH)D to the 25(OH)D-1-α-hydroxylase in the kidney, necessary in the conversion of 25(OH)D to 1,25(OH)2D [19]. This system also exists in the parathyroid glands and, therefore, plays an important role in the endocrine function of vitamin D to maintain calcium homeostasis. Interestingly, the megalin-cubilin system also functions in the placenta and likely orchestrates maternal-fetal calcium homeostasis [20]. For those tissues that lack this endocytic transport system, free circulating concentrations of vitamin D moieties reach target cells through passive diffusion. For additional information, there are excellent reviews available that detail vitamin D metabolism in the nonpregnant individual [17, 21-23].

Also of importance is that 1,25(OH)2D itself is responsible for reducing 1,25(OH)2D concentrations in cells primarily by stimulating its catabolism through the induction of 24-hydroxylase, 24CYP24A1. This enzyme hydroxylates both 25(OH)D and 1,25(OH)2D in the 24 position to form 24,25(OH)2D and 1,24,25(OH)3D [24]. As is discussed next, during pregnancy, there is increased 1,25(OH)2D concentration presumed to be due to decreased catabolism.

From early on in pregnancy, circulating 1,25(OH)2D concentrations increase without the predicted surge in PTH that causes a rise in calcitriol in nonpregnant individuals. While calcitriol is synthesized by the placenta, during pregnancy it is mainly synthesized by the kidneys [25]. There appears to be a slower rate of catabolism of 1,25(OH)2D to 24,25(OH)2D [26]. What purpose does this early and sustained rise in 1,25(OH)2D serve? There has been much speculation about this. It has been theorized for decades that this increase during pregnancy was due to increased fetal calcium requirements, most notable during the last trimester [27]. Elevated circulating 1,25(OH)2D was also thought to continue during lactation [28], but later, with more sensitive assay methodology surrounding the measurement of 1,25(OH)2D, this was shown not to be the case [29, 30]. The return to prepregnancy circulating concentrations of 1,25(OH)2D during lactation is poorly understood and suggests that the role of 1,25(OH)2D during pregnancy may be for reasons that extend beyond calcium metabolism and which surround vitamin D’s role in immune function [25]. The above occurs in the presence of a continued high calcium requirement of the breastfeeding infant of at least 200–350 mg/day for growth that is comparable to fetal requirements during the last trimester of pregnancy.

Specific to pregnancy, there are changes in states of inflammation: early in pregnancy, there is inflammation – to allow the conceptus to invade the uterine milieu and for a network of channels between maternal and embryo to develop that give rise to the placenta, following a time of relative quiescence of those inflammatory processes that facilitate fetal growth beginning in the middle of the first trimester toward the end of pregnancy, with a return to a relatively inflammatory state with the onset of labor and the expulsion of the placenta [3]. Pregnancy represents tremendous change in numerous systems with most notable increases in estrogen, progesterone, human placental growth factor, the interleukins, as well as 1,25(OH)2D. Each has its purpose, but with any system, various growth factors and cytokines do not operate in isolation, but there is much interaction.

There is evidence that maternal vitamin D deficiency – however this is defined – affects maternal and fetal outcomes. Although scientific inquiry on the topic with published observational and clinical vitamin D supplementation trials did not consistently appear in the literature until the late 1970s/early 1980s [31], there is historical information as early as the 1940s with halibut liver oil – rich in both vitamins A and D and other vitamins – given as a supplement to pregnant women that showed benefit [32]. Specifically, a study conducted by the People’s League of Health in 1938–1939 involving over 5,000 pregnant women who were randomized to receive a cocktail of vitamins and halibut liver oil (a source of both vitamins A and D) or placebo was rediscovered by Olsen and Secher [32] and the results published in 1990. This nutritional supplement was superior compared to control in achieving reductions in preterm birth and preeclampsia. Since that time, studies that have focused on one nutrient instead of a combined nutritional supplement, with the exception of higher-dose vitamin D studies in the most deficient women, and more recently in systematic reviews and meta-analyses, have failed to demonstrate this effect. Much research has occurred with far more studies published each year on the topic. With those trials, there have been mixed results, with some studies showing a positive effect and others showing a minimal or no effect. There are, however, indisputable findings surrounding gene expression on the basis of maternal vitamin D status.

Focusing on vitamin D, the metabolism of this important preprohormone during pregnancy is vastly different when compared to the nonpregnant state. As noted earlier, 1,25(OH)2D increases 2- to 3-fold within days of conception, while 25(OH)D remains relatively stable within a certain range [33-35]. It is 25(OH)D which crosses the placenta to the fetus and, thus, is the main pool of vitamin D in the fetus, not 1,25(OH)2D; the fetus must synthesize 1,25(OH)2D from that pool. While the main source of the increased 1,25(OH)2D during pregnancy comes from the kidney, its other source is the placenta, with VDR and regulatory metabolic enzymes synthesized in the placenta and decidua. This is considered a potential critical point in the immunomodulation at the maternal-fetal interface and raises the question if maternal hypovitaminosis D during pregnancy leads to pregnancy-related disorders [36, 37].

There is an increasing number of genetic studies to evaluate vitamin D’s effect on gene expression. One of the first was a study by Al-Garawi et al. [38] who, in their post hoc analysis of a randomized clinical trial of maternal vitamin D supplementation in women who themselves or of whom a first-degree relative had allergy or asthma, sought to explore vitamin D’s effect on genomic changes during pregnancy, which is one of the first reports of its kind. Women were randomized at 10–18 weeks of gestation to 400 and 4,400 IU vitamin D3/day [39] with the primary outcome wheezing in the offspring at 3 and 6 years. An analysis of a subset of blood samples for RNA gene expression changes between the first and third trimesters was conducted. Using significance of analysis of microarrays (SAM) and clustered weighted gene co-expression network analysis (WGCNA) to identify major biological transcriptional profiles between those time points, 5,839 significantly differentially expressed genes were studied. Transcripts from these genes clustered into 14 co-expression modules, of which 2 (associated with immune defense pathways, extracellular matrix reorganization, and Notch signaling and transcription factor networks) showed significant correlation with maternal 25(OH)D concentrations. The findings show that maternal gene expression changes during pregnancy are affected by maternal vitamin D status, which, in turn, is a direct reflection of maternal vitamin D supplementation.

Additional evidence of vitamin D’s effect on gene expression comes from Baca et al. [40] and another from Barchitta et al. [41] in their focus on vitamin D-related genes. Baca et al. [40] conducted a meta-analysis of 2 large cohorts – the Epidemiology of Vitamin D Study (EVITA) and the Collaborative Perinatal Project (CPP) – where the combined analysis of more than 4,000 randomly selected samples showed that the maternal genotypes of 7 SNPs in VDR, 3 SNPs in GC (VDBP), and 1 SNP in the flanking regions of Cyp27B1 were associated with maternal vitamin D status as expressed as the log25(OH)D concentration. Adjusting for multiple comparisons, 1 SNP in VDR and 2 SNPs in GC remained significant. The investigators theorized that SNPs in VDR may influence circulating 25(OH)D by changing the rate at which 25(OH)D is hydroxylated either directly or indirectly through a negative feedback loop. The 2 SNPs in GC are likely related to the response of an individual to vitamin D supplementation, with certain GC polymorphisms associated with an attenuated or refractory response to supplementation compared to other genotypes, such as 1S or 2 [42].

Barchitta et al. [41] conducted a study to examine the association of VDR polymorphisms and preterm birth and neonatal anthropometric measures. Utilizing the Italian “Mamma and Bambino” cohort (n = 187), they studied the most common polymorphisms – BsmI, ApaI, FokI and TaqI. The investigators found that for the FokI polymorphism, gestational duration (age) and birth weight (that are clearly linked) were statistically significantly lower with increasing number of the A allele. In addition, when compared to mothers with the GG or GA genotype, those mothers who carried the AA genotype had a higher risk of preterm birth (OR 12.049, 95% CI 2.606–55.709, p = 0.001). Further, the BsmI polymorphism appeared to be protective against preterm birth, both allelic (A vs. G: OR 0.74, 95% CI 0.59–0.93) and recessive (AA vs. GG + AG: OR 0.62, 95% CI 0.43–0.89, p = 0.0001). Mothers with the AA genotype exhibited a 12-fold increased risk of preterm birth that was independent of sociodemographic characteristics, lifestyle, vitamin D intake/use of supplements, type of delivery, and parity. The results of this study were combined with earlier reported studies, which strengthened the robustness of these findings.

These genetic studies collectively suggest that genotyping of common allelic variants and polymorphisms may play an important role in vitamin D metabolism during pregnancy. The findings further suggest that certain functional genetic variants may contribute to vulnerability or risk of vitamin D deficiency. The findings suggest that there may be subgroups of women based on their genotype profile for relevant vitamin D-related genes who would benefit from certain dosing regimens while others would not. The changes in gene expression from the first trimester compared to the third may also suggest that the prescription of one vitamin D supplement dose throughout pregnancy does not meet the physiological needs of the pregnant woman and might be based more on convenience than what is needed for optimal vitamin D status.

The issue with nutrient studies is that they often are designed more like a drug study, where the starting concentration of the “drug” is zero, compared with a nutrient study, such as vitamin D, where there is some vitamin D concentration in everyone, and, thus, baseline 25(OH)D concentration is variable and not zero. Heaney [43] described the qualities that should define a nutrient study:

1basal nutrient status must be measured, used as an inclusion criterion for entry into the study, and recorded in the report of the trial;

2the intervention must be large enough to change nutrient status and must be quantified by suitable analysis;

3the change in nutrient status produced in those enrolled in the report of the trial must be measured and reported;

4the hypothesis to be tested must be that a change in nutrient status produces the sought-after effect; and

5the status of other nutrients must be optimized to guarantee that the nutrient being studied is the only nutrition-related limiting factor in the response.

One might add another stipulation to the list – that the nutrient being investigated has to follow an appropriate dosing schedule to match what happens naturally. In the case of vitamin D, there is a plethora of data that show substantial physiological differences between daily, weekly, and monthly vitamin D dosing regimens [11].

In reviewing past clinical trials of vitamin D, most lack all 5 of the Heaney criteria and often the 6th dosing criterion. While systematic reviews and meta-analyses provide larger combined sample sizes, the analyses of several limited studies only compound the problem. More recently, the rigor of clinical trials with increased sample sizes has improved the consistency of pooled/aggregate data, and some compelling evidence from these clinical trials suggests that vitamin D sufficiency during pregnancy enhances maternal and fetal health. The “translation” of the laboratory data to the clinic and bedside supports this emerging concept that vitamin D plays a role not only in calcium homeostasis and bone function but also in immune function. Beyond the scope of this review for an exhaustive summary, below are some of the highlights of those systematic reviews and meta-analyses to date, with an emphasis on the salient findings, and their strengths and weaknesses.

As mentioned in the attributes of a well-designed nutrient study as outlined by Heaney [43], part of the issue is that these supplementation trials have varied by the onset of supplementation during pregnancy, the dosing and timing of that dose, the degree of vitamin D deficiency at the onset of the trial, and different methodology used in measuring 25(OH)D. There have been numerous systematic reviews and meta-analyses on the topic, also with mixed findings. Restricting systematic reviews and meta-analyses to the 2 recent years 2018–2019, there are over 30 publications on the topic of vitamin D and pregnancy outcomes. The analyses cover such topics as gestational age, infant birth weight, gestational diabetes and insulin resistance, small-for-gestational age, preeclampsia, and maternal and neonatal vitamin D status at delivery. Compared to analyses performed in earlier years when there were few published randomized controlled trials that were often plagued with small sample sizes, the more recent reviews consistently have shown benefit of maternal vitamin D supplementation during pregnancy. The highlights of some of the larger systemic reviews and meta-analyses published in the past 2 years (2018–2019) are summarized in Table 1. With each review, there is evidence that there are still limitations to the clinical studies and there is a need for continued research, especially with genetic and epigenetic considerations in place and design of nutrient studies that take into account the Hea-ney criteria [43].

Table 1.

Systematic Reviews and Meta-Analyses of Various Clinical Studies Reporting the Effect of Maternal Vitamin D Status on Pregnancy Health Outcomes, 2018–2019

Systematic Reviews and Meta-Analyses of Various Clinical Studies Reporting the Effect of Maternal Vitamin D Status on Pregnancy Health Outcomes, 2018–2019
Systematic Reviews and Meta-Analyses of Various Clinical Studies Reporting the Effect of Maternal Vitamin D Status on Pregnancy Health Outcomes, 2018–2019

Taken together, there is evidence to suggest that vitamin D deficiency increases the risk of adverse pregnancy outcomes in both the mother and her developing fetus. The question at the heart of the matter is what 25(OH)D concentration should be the target for pregnant women? What this target might be is suggested by this kinetic reaction saturation graph (Fig. 2) of 25(OH)D and 1,25(OH)2D, which shows that 25(OH)D has direct influence on 1,25(OH)2D concentrations throughout pregnancy, an event which does not occur during any other time during the human lifecycle. As is noted in our study reporting these results, as lower concentrations of 1,25(OH)2D increase, first-order kinetics becomes zero-order kinetics, with a plateauing of the graph and an inflection point at 40 ng/mL (100 nmol/L) 25(OH)D – the level required to optimize 1,25(OH)2D production during pregnancy [34].

Fig. 2.

The relationship of circulating 25(OH)D to control the production of 1,25(OH)2D during pregnancy. All data points for all subjects in all groups were included in this analysis. (Hollis et al. [34], 2011, with permission.)

Fig. 2.

The relationship of circulating 25(OH)D to control the production of 1,25(OH)2D during pregnancy. All data points for all subjects in all groups were included in this analysis. (Hollis et al. [34], 2011, with permission.)

Close modal

If this inflection point of 40 ng/mL (100 nmol/L) represents where there is optimal conversion of 25(OH)D to 1,25(OH2D, one would predict that attaining this level during pregnancy would be critical for both maternal and fetal well-being. In our post hoc analysis using multivariable log-binomial regression of maternal 25(OH)D status during pregnancy, McDonnell et al. [44] reported that in women who attained a maternal 25(OH)D concentration ≥40 ng/mL (100 nmol/L) compared to those who remained with a concentration ≤20 ng/mL (50 nmol/L), adjusted for covariates, their risk of preterm birth was reduced by 59%. Based on cellular, animal studies and genetic analyses, it appears that early vitamin D status may have greater bearing on pregnancy outcomes than later status [45, 46], but dissecting the factors that influence these early processes has been a challenge [47, 48]. At the very least, a woman who is considering becoming pregnant or who becomes pregnant should be vitamin D sufficient as defined by the kinetics curve of Figure 2, attaining at least a circulating 25(OH)D concentration of 40 ng/mL (100 nmol/L) as early as possible during pregnancy.

In numerous articles published during the last 3 decades, it is stated that breast milk has a relatively low vitamin D concentration and, as a result, all babies who are breastfed should receive a vitamin D supplement of 400 IU/day to prevent vitamin D deficiency that can lead to osteopenia and rickets in the exclusively breastfed infant [49-51]. This recommendation is based on the observations since the 1930s and beyond that infants and children who received one teaspoon of cod liver oil (which contains about 400 IU/teaspoon) had minimal risk of developing rickets. It does not address how we evolved as a species with such low concentrations of vitamin D. Most young infants today in technologically dependent societies are not exposed to direct sunlight until well after 6 months, and so their ability to use ultraviolet light to synthesize vitamin D endogenously is thwarted. If we look at groups throughout the world who live in sun-rich environments, we see a pattern that differs from those who live at higher latitudes – maternal vitamin D status is better if the mother is exposed to the sun [52], and, therefore, her milk anti-rachitic activity – the total amount of vitamin D moieties in human milk – is better. It is critical to understand that human milk is deficient in vitamin D only when the mother herself is deficient [53]. We know that during pregnancy, maternal vitamin D status is closely linked with fetal and neonatal vitamin D status. That connection and relationship continues during lactation.

It was hypothesized by Hollis and Wagner [54] more than 2 decades ago that if a mother had an improved vitamin D status, then her milk anti-rachitic activity would be improved and that of her recipient infant, obviating the need for infant vitamin D supplementation. Maternal vitamin D supplementation would effectively treat both the mother and her breastfeeding infant. This was studied in 2 pilot studies by our group [54, 55] and then in a larger trial sponsored by the National Institutes of Health [56] that has since been repeated by Dawodu et al. [57] in another region of the world – the Middle East – where there is profound vitamin D deficiency. In the various trials, mothers at 1 month postpartum were randomized to receive 1 of 3 treatments: 400, 2,400, or 6,400 IU vitamin D/day. Infants of mothers in the 400-IU group received the standard of care of 400 IU/day, while infants of mothers in the 2,400- and 6,400-IU group received 0 IU/day (placebo). Maternal supplementation with 2,400 IU vitamin D/day with infants on placebo resulted in higher rates of infant insufficiency and that arm of the study was stopped early-on in the study. Mothers in the 6,400-IU group had improved vitamin D status, milk anti-rachitic activity, and their infants had circulating 25(OH)D concentrations that were comparable to infants receiving 400 IU/day direct supplementation [55, 56]. There were no safety issues noted in these studies except with the 2,400 IU arm and the higher rates of infant deficiency, but no issues with toxicity from vitamin D. Similar results were reported by Dawodu et al. [57].

Oberhelman et al. [58] studied 40 exclusively breastfeeding mothers and infants who were randomized to receive either daily maternal vitamin D supplementation of 5,000 IU/day versus a single large bolus of 150,000 IU once as a higher bolus vitamin D, with the primary outcome at 28 days being maternal and infant vitamin D status. The daily versus single bolus were comparable at 28 days; however, the mother and infant pair who received the single bolus had a large increase in their circulating 25(OH)D that rapidly declined but was still improved compared to baseline.

A systematic review and meta-analysis by O’Callaghan et al. [59], reviewing relevant studies on the topic of alternatives to daily infant vitamin D supplementation through September 2018, identified 28 relevant papers of which 5 were randomized clinical trials that met inclusion criteria for the analysis. The meta-analysis suggests that the results are promising, with the need for larger studies in diverse groups of women necessary to be carried out before policy changes can be made [59].

While application of alternatives to infant supplementation are being discussed, a major issue complicating recommendations is that compliance by parents to give their breastfeeding infants daily vitamin D drops is low in many regions of the world [60, 61]. In the USA, reports of compliance with the recommendation of infant vitamin D supplementation range from 9 to 20%, leaving most breastfeeding infants in the USA deficient, dependent on their mothers who are often themselves deficient [62-64]. These are less than satisfying statistics. At the end of the day, where maternal compliance with taking a vitamin D supplement is much greater than that of parental adherence with infant supplementation, maternal vitamin D supplementation alone remains as a viable alternative to infant vitamin D supplementation that safely and effectively treats both the mother and her breastfeeding infant.

The writing of this article was supported by Nestlé Nutrition Institute, and the author declares no other conflicts of interest.

Funded in part by NIH UL1 TR000062 and Stone Foundation, Children’s Hospital Fund, Medical University of South Carolina.

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