Maternal iodine status during lactation and infant weight and length in Henan Province, China

Infants are very sensitive to iodine deficiency because of their high demands and low storages of iodine [1, 2]. The first 2 years of age is a crucial time for neurological development and growth. Even mild iodine deficiency may lead to irreversible damage during this period, such as low intelligence, neonatal hypothyroidism, short stature, skeleton disorders, and other growth retardation [2, 3]. It is well known that iodine is an essential material in synthesis of thyroid hormones (TH) [1, 3]. TH can regulate protein production, fat metabolism, and glucose utilization [4, 5]. TH also influences bone development by TH receptors in bone cells and the hypothalamic-pituitary-thyroid axis [6, 7]. Thereby, TH is closely involved in infant growth.

Infant iodine status is influenced by many factors, among which feeding type is the most important one. The difference in iodine status between breastfed and formula-fed infants was reported in numerous studies, but these results were conflicting. Remer T et al. observed that breastfed infants were more inclined to suffer from iodine insufficiency [2]. Gordon J et al. demonstrated that there was no difference in iodine status between breastfed and formula-fed infants in Boston [8]. Our previous study found that the median urinary iodine concentration (UIC) in breastfed infants was significantly higher than that in formula-fed infants, which was probably due to universal salt iodization in China [9].

Breastfed infants depend on maternal iodine intake. During lactation, mothers transfer iodine to their infants by mammary glands, which concentrate iodine in breast milk. Infants approximately require 7 μg I/kg of body weight to build iodine pool in their thyroid gland [10]. In order to ensure that infants get enough iodine, lactating mothers are recommended to consume iodine 250 μg/day [11].

Iodine deficiency has long been known as a risk factor for retard growth during childhood [12]. Evidence suggested that maternal iodine deficiency and hypothyroidism during pregnancy adversely affected neurodevelopment and intellectual capacity of their offspring [13‐15]. Many studies also assessed the effects of maternal iodine deficiency and hypothyroidism on pregnancy outcomes, such as intrauterine growth restriction, fetal death, preterm, and low birth weight [16‐18]. Previous studies of infant-mother pairs were limited by small sample size, not adjusting potential confounding factors. For example, an Argentina study was restricted by a small sample size (n = 77) [19]. Furthermore, few studies focused on the iodine status of lactating mother and infant size. The objective of the present study was to assess the relationship between maternal iodine status and infant anthropometric index.

In our study, the median UICs of mothers and their infants were 176.8 μg/L and 261.1 μg/L, respectively, and they were regarded as “iodine sufficient” according to WHO criterion [21]. The results can be attributed to the universal salt iodization in China in 1995. Based on the national survey in 2014, the household coverage rates with iodized salt was 93.3% in Henan Province. Some studies found that the median UIC in lactating women might be less than that in pregnant women, which was partly due to the iodine loss into breast milk and the normalization of thyroid hormone production [22‐26]. Our research indicated that infant UIC was higher than maternal UIC. The possible reason is that thyroid and renal functions in infants are not immature and therefore excess iodine can not be retained.

Breastfed infants mainly obtain iodine from breast milk. The mammary glands have better ability in concentrating of iodine by the activities of sodium/iodine symporter and deiodinase [27]. In order to meet the needs of infant growth and development, the concentration of breast milk should be maintained between 100 and 200 μg/L [28, 29]. Evidence showed that breast-milk iodine concentration (BMIC) was correlated with UIC [30, 31]. Due to the limitation of experimental condition and easily accessibility, maternal UIC were used as a proxy of BMIC in our study. A few studies indicated that infant UIC was affected by their mothers [32, 33]. Our finding showed that maternal UIC had a positive but weak correlation with infant UIC (r = 0.203, P < 0.001).

WHO-recommend daily intakes for lactating women and infants are 250 and 90 μg I/d, respectively [11]. Although iodine supplements could supply adequate iodine intake during lactation and infancy, WHO and European pediatric societies do not recommend iodine supplements for infants or their mothers in iodine-sufficient regions [34, 35]. In contrast, some reports from the United States [36], the Switzerland [24], and the New Zealand [37], recommend that lactating women should receive 150 μg I/d and iodine fortification of infant food should be encouraged. In our research, iodine-supplemented mothers had higher UIC than unsupplemented mothers (191.7 vs 173.8 μg/L, P < 0.01). Morse [38] proved that maternal supplementation of vitamin D, DHA, folic acid and iodine during pregnancy and lactation can be beneficial for brain development of their offspring.

Adequate iodine intake is important for the growth and development of fetus and infant [39]. A review reported that there was a connection between the use of iodized salt and the increased weight and arm circumference of infant, especially in the second year of life [40]. Alvarez-pedrerol et al. [41] stated that the UIC of pregnant women in the third trimester was positively correlated with birth weight of their offspring. An Argentina research [19] demonstrated that mothers with lower UIC had lower placental weight and head perimeter in their newborns. For exclusively breastfed infant, breast milk is the main source of iodine. Sufficient iodine status of lactating women is essential for their infant growth and maturation. In the present study, the lowest mean values of HAZ and WAZ were found in infants whose mothers had UIC below 50 μg/L. After adjusting for maternal age, parental weight and height, and other socioeconomic factors, we observed that maternal UIC had positive effect on infant weight. A similar model demonstrated that the mean weight and length of boys increased by 70 g and 0.41 cm for every 0.5 mg/L increase in maternal UIC [42]. Fisher [43] reported that a moderate iodine deficiency in lactating women resulted in a 30% reduction of thyroxine in their infants, which played an important role in bone development. For another view, a study from Korea [44] found that excessive iodine intake of lactating women may be linked with hypothyroidism in their preterm infants. Infant with hypothyroidism may experience intellectual and growth retardation. These facts may be explained why maternal iodine affects infant anthropometric indices.

In our study, UIC was divided into five groups instead of a continuous variable, which could decrease the misclassified possibility due to high variation of UIC [39]. Although UIC only represents recent iodine intake in an individual, low iodine status could exist a long time in an individual [45]. In addition, we used HAZ and WAZ to replace the original values of height and weight in order to avoid the differences of infant gender and age. The major limitation was that UIC was not adjusted for the creatinine clearance. The ratio of urinary iodine and creatinine can reflect the real iodine intake of individuals by excluding the influence of urinary capacity.

In conclusion, our research indicates that the iodine status of breastfeeding mothers may be related to their infant weigh. Appropriate iodine intake of lactating women is beneficial for their infants.