1. Introduction
Pro-vitamin A carotenoids, including α-carotene, β-carotene, and β-cryptoxanthin, are anti-inflammatory nutrients found in vegetables, fruits, and fish which can be converted into vitamin A in the body [
1,
2,
3]. Vitamin A plays an important role in pregnancy and fetal development by regulating normal organogenesis, tissue differentiation, and development of the immune system and the inner ear [
4]. Furthermore, numerous studies have shown that the antioxidant and anti-inflammatory properties of carotenoids can ameliorate pregnancy-associated morbidities such as pre-eclampsia, intrauterine growth restriction, gestational diabetes, and pregnancy induced hypertension [
5,
6,
7,
8]. However, vitamin A deficiency is still prevalent and is considered a worldwide public health problem, affecting an estimated 19 million pregnancies every year [
9]. This deficiency is associated with long-term health consequences for the infant, including hearing loss [
10].
Vitamin A is indispensable in inner ear development. During in utero development, essential enzymes in the inner ear convert vitamin A (retinol) into a biologically active form, retinoic acid. Previous studies reported that retinoic acid regulates fibroblast growth factors (FGF) such as FGF3 and FGF10, which may in turn modulate several downstream target molecules that are necessary for normal inner ear development [
11,
12]. Surface ectoderm gives rise to the otic placode during the first trimester of gestation, which in turn forms the mature inner ear. Studies have shown that vitamin A induces development of the otic placode, which further corroborates the role of vitamin A in inner ear development. In fact, experiments in rats, zebrafish, and chicks demonstrate that deficiency of vitamin A leads to aberrant otic placode development [
12,
13].
Developmental defects of the inner ear, which is composed of the cochlear and vestibular systems, are a major cause of congenital sensorineural hearing loss [
14,
15] and approximately one-third of children with congenital sensorineural hearing loss have significant hearing impairment [
16]. The American Academy of Pediatrics recommends that a newborn hearing screen (NHS) should be done within the first month of life to detect congenital sensorineural hearing loss [
17]. There are several known causes of congenital hearing loss including genetic factors and congenital infections (such as congenital cytomegalovirus, toxoplasmosis, and rubella infection). Early detection of and intervention for congenital sensorineural hearing loss is important, as previous studies have shown that children diagnosed with congenital sensorineural hearing loss earlier have better language and socioemotional outcomes, likely resulting from early interventions [
18]. The protocols for NHS vary in different countries; in the United States, NHS may include otoacoustic emissions (OAE) or automated auditory brainstem response (AABR) tests [
19]. OAE is helpful in assessing the inner ear (cochlear function) whereas automated AABR assesses the auditory pathway in addition to assessing the function of the external and inner ear [
20,
21]. This comprehensive screening of newborns reduces the chance of missing auditory neuropathy spectrum disorders [
22]. However, although previous studies have demonstrated that vitamin A is required for normal ear development, there is a gap in our knowledge regarding how maternal and infant nutritional status of pro-vitamin A carotenoids are associated with the results of these newborn hearing screens.
To address this gap, we conducted a study to assess the relationship between NHS results and retinol and pro-vitamin A carotenoid maternal intake, maternal plasma levels, and umbilical cord plasma levels. As pro-vitamin A carotenoids can be converted into vitamin A and vitamin A plays a key role in inner ear development, we hypothesized that higher concentrations of retinol, α-carotene, β-carotene, and β-cryptoxanthin in maternal and umbilical cord plasma would correlate with better NHS outcomes.
4. Discussion
Contrary to our original hypothesis, we detected that higher infant plasma concentrations of retinol, as well as higher maternal and infant plasma concentrations of β-carotene, were associated with increased odds of abnormal NHS. Other studies investigating the association between retinol and carotenoid nutritional status and hearing loss have focused on intake in adult populations, rather than plasma concentrations in infants. Although not statistically significant, median maternal intake of β-carotene was also higher for dyads in this study with an abnormal NHS result (5570.9 vs. 4881.8 μg/day,
p = 0.15). These findings are in stark contrast to studies in adult populations that have demonstrated an opposite effect, with increased β-carotene intake being associated with a decreased likelihood of acquired hearing loss [
33,
34,
35]. Differences in population age and mechanism of hearing loss (congenital vs. acquired) between our study cohort and previous studies in adult populations could explain our divergent findings.
Studies examining the relationship between acquired hearing loss and retinol in children, rather than adults, have reported mixed results. One randomized placebo-controlled trial by Schmitz et al. reported that supplementation with vitamin A during preschool was associated with a decreased risk of hearing impairment during adolescence [
36]. In contrast, a randomized placebo-controlled trial by Ambalavanan et al. reported that there was no difference in the risk of infant hearing impairment at 18 to 22 months adjusted age between extremely low birth weight infants who receive vitamin A supplementation compared to those who received placebo [
37]. Similar to Ambalavanan et al., we observed no association between maternal retinol intake and odds of infant hearing impairment as measured by NHS results. However, infant plasma concentrations of both retinol and β-carotene were associated with increased odds of abnormal NHS results in our cohort.
There are several potential explanations for the observed association between increased retinol and β-carotene plasma concentrations and increased odds of abnormal NHS. Excessive maternal vitamin A intake during gestation is associated with abnormal fetal inner ear development in both humans and animal models [
12,
38]. The Institute of Medicine Food and Nutrition Board recommends 770 μg RAE/day of vitamin A, with a tolerable upper limit of 3000 μg RAE/day for preformed vitamin A [
30]; however, intake of up to 9000 μg RAE/day of preformed vitamin A, almost twelve times the recommended daily dose and three times the tolerable upper limit, has been shown to be safe during pregnancy [
39]. Additionally, although β-carotene can be metabolized into vitamin A, excessive intake of β-carotene is not thought to result in vitamin A toxicity during pregnancy [
39]. In this study, median maternal intake of retinol for the mothers of infants with abnormal NHS results over the past year was 1,755.9 μg RAE/day, well within recommended limits [
30]. Among mothers of infants with abnormal NHS results, only three (7.7%) consumed less than 770 μg RAE/day and only two (5.1%) consumed more than 3000 μg RAE/day. These intakes are comparable to mothers of infants with normal NHS results, where 9.9% consumed less than 770 μg RAE/day and 6.2% consumed more than 3000 μg RAE/day. Likewise, although most infants in this study had insufficient or deficient retinol plasma concentrations of retinol, the median retinol plasma concentration was significantly higher among infants with an abnormal NHS.
Alternatively, it is possible that impaired vitamin A metabolism or transport during inner ear development may be associated with both congenital hearing loss and increased plasma concentrations of retinol and β-carotene. During normal inner ear development, β-carotene is converted to retinol [
1,
2,
3], and retinol is converted to the biologically active retinoic acid [
11,
12]. Retinoic acid is then utilized in signaling cascades to promote inner ear development [
11,
12]. Impaired metabolism of retinol into retinoic acid could result in accumulation of plasma retinol. A high concentration of retinol could, in turn, impair metabolism of β-carotene into retinol and result in accumulation of plasma β-carotene. Similarly, an imbalance in retinol-binding protein could result in impaired transport of retinol to target tissues, leading to accumulation of retinol, and ultimately β-carotene, in plasma. However, vitamin A plays an important role in organogenesis, tissue differentiation, and immune system development, in addition to inner ear development [
4]. It is unlikely that impaired metabolism of vitamin A would result in congenital hearing loss without significantly affecting the development of other organ systems. In this study, 87% of infants with an abnormal NHS result were otherwise healthy infants who were not admitted to the NICU at delivery, though long-term health of these infants has not been evaluated.
Another potential explanation for the observed association between retinol and β-carotene and abnormal NHS is that infants with higher retinol and β-carotene plasma concentrations may have been exposed to higher levels of ototoxic environmental contaminants in vitamin A and carotenoid-rich foods, such as pesticides applied to fruits and vegetables. We observed a trend towards higher median maternal β-carotene intake in mothers of infants with an abnormal NHS result, which may support this theory, although the relationship was not significant and there were similarly no significant differences in maternal intake of other evaluated carotenoids. Multiple studies have reported associations between pesticide or other environmental contaminant exposure and hearing loss [
40,
41,
42,
43,
44,
45]. In a rat model, prenatal exposure to the environmental contaminants 2,3,7,8-tetrachorodibenzo-p-dioxin (TCDD) [
40] or polychlorinated biphenyls (PCB) [
41,
42] were associated with hearing deficits. In humans, higher prenatal exposure to multiple organochlorine pesticides, as measured in cord plasma concentrations, were associated with significantly worse cochlear function, as measured by distortion product otoacoustic emission (DPOAE) at 45 months of age [
43].
Alternative rationales may exist for these findings, including variables unknown or not evaluated. For example, metabolic and endocrine disorders such as hypothyroidism [
46,
47,
48,
49], diabetes [
50,
51], and polycystic ovarian syndrome (PCOS) [
52,
53] have been associated with both hearing loss and alterations in vitamin A nutritional status. Subclinical hypothyroidism is particularly common during pregnancy, affecting an estimated 15.5% of pregnant women in the United States [
54]. Interestingly, hypothyroidism is associated with increased concentrations of β-carotene [
46,
47]. Maternal hypothyroidism and congenital hypothyroidism are also linked to neurosensory hearing loss [
48,
49], although less is known about how subclinical hypothyroidism may affect infant hearing. One study by Radetti et al. failed to detected any significant association between maternal subclinical hypothyroidism and infant hearing outcomes [
55], while a study by G et al. observed that infants born to mothers with subclinical hypothyroidism had minor alterations in hearing which self-corrected within 6–8 months [
56]. As the American Thyroid Association does not currently recommend asymptomatic screening for subclinical hypothyroidism during pregnancy [
57], thyroid function was not evaluated in this study. However, it is possible that maternal–infant dyads with subclinical hypothyroidism may jointly exhibit higher β-carotene concentrations and newborn hearing screen fails, though these results would not indicate a causal relationship between nutritional status of β-carotene and infant NHS results. Future research is therefore needed to evaluate the relationship between these variables.
Limitations
This study was conducted at a single academic medical center in the Midwest United States (University of Nebraska Medical Center/Nebraska Medicine) with a majority non-Hispanic White cohort of maternal–infant dyads, which may limit generalizability of our results. We were unable to collect information on some factors that have been previously associated with hearing loss in infants, such as hyperbilirubinemia, craniofacial abnormalities, administration of loop diuretics, or various environmental exposures (e.g., viral infections) [
32]. However, our analysis did account for several other variables potentially associated with neonatal hearing loss, including gestational age of the infant, maternal age, maternal smoking status, and income-to-poverty ratio. Additionally, this analysis focused on associations between first NHS result and nutritional status at time of delivery. Neonates may have abnormal NHS results which resolve on repeat testing and nutritional status at time of delivery may differ from nutritional status during the critical period of inner ear development in the first trimester of pregnancy. Future studies should assess associations between diagnosed congenital hearing loss and retinol and β-carotene nutritional status across multiple timepoints in pregnancy.