Iodine nutrition in mothers and their infants during breastfeeding and complementary feeding

Abstract

Background : Iodine is important for normal growth and psychomotor development. Infants and lactating women are susceptible to iodine deficiency. The iodine requirements per kg of body weight of babies are higher than any other age group, while lactating women lose iodine through breast milk. Infants younger than six months of age receive iodine from breast milk or fortified infant formula, however, the introduction of complementary feeding poses a risk for deteriorating iodine status at a later age. Iodine fortification of complementary foods is therefore recommended, whilst continued breastfeeding is encouraged to ensure sufficient intake in these infants. This is the first research to investigate iodine nutrition in South African infants and lactating women. Methods : In a cross-sectional study, urinary iodine concentrations (UIC), thyroid function and breast milk iodine concentrations (BMIC) of 100 lactating women and their two to four-month-old breastfed infants from a South African township were assessed. Potential predictors of UIC, thyroid function and BMIC, including household salt iodine concentrations (SIC) and maternal sodium excretion were further explored. In a randomized controlled trial, baseline characteristics of infants aged six months were assessed to determine associations of iodine status with feeding practices and psychomotor milestone development. Iodine concentrations were measured in infant (n=386) and maternal (n=371) urine, as well as breast milk (n=257) and household salt (n=143). Feeding practices and psychomotor milestone development were assessed in all infants. Within the same trial, 750 infants aged six months were randomly selected to receive the following: a daily fortified small-quantity lipid-based nutrient supplement (SQ-LNS) with essential fatty acids providing 45 μg of iodine (SQ-LNS A); a daily fortified SQ-LNS with docosahexaenoic acid, arachidonic acid providing 45 μg of iodine (SQ-LNS B); or a control group receiving no SQ-LNS. Urinary iodine concentrations (UIC) were measured at baseline (n=386) and at 12 months (n=262). Results : In breastfed infants aged two to four months and their mothers, the median (25th-75th percentile) UIC was 373 (202-627) μg/L and 118 (67-179) μg/L, respectively. Half (53%) of the infants had a UIC >300 μg/L. Median household SIC was 44 (27-63) ppm. Household SIC and maternal urinary sodium excretion predicted UIC of lactating mothers. Median BMIC was 179 (126-269) μg/L; the age of infants, SIC and maternal UIC predicted BMIC. In turn, infant age and BMIC predicted UIC of infants. Infant thyroid stimulating hormone (TSH), total thyroxine (TT4) and thyroglobulin (Tg) concentrations were 1.3 (0.8-1.9) mU/L, 128±33 mmol/L and 77.1 (56.3- 105.7) μg/L, respectively, and did not correlate with infant UIC or BMIC. In infants aged six months, baseline median UIC was 345 (213-596) μg/L, and was significantly lower in stunted (302 [195-504] μg/L) than non-stunted infants (366 [225-641] μg/L). Only 6.7% of infants had a UIC <100 μg/L. Infant UIC correlated with maternal UIC (128 [81-216] μg/L) (rs=0.218, p<0.001) and BMIC (170 [110-270] μg/kg) (rs=0.447, p<0.0001). Most infants (72%) were still breastfed and tended to have higher UIC than non-breastfed infants (p=0.074). Almost all infants (95%) consumed semi-solid/solid foods, with commercial infant cereals (60%) and jarred infant foods (20%) being the most common foods that were first introduced. Infants who reported to frequently consume commercial infant cereals had significantly higher UIC (372 [225-637] μg/L) than those reported to seldom/never (308 [200-517] μg/L) (p=0.023) consume commercial infant cereals. There were no associations between infant UIC and psychomotor developmental scores. Results from the intervention study showed that the geometric mean (95% CI) UIC at baseline was 333.8 (310.5, 358.9) μg/L and decreased significantly to 214.9 (189.2, 242.6) μg/L at 12 months. Non-breastfed infants had significantly lower UIC (159.6 [65.9, 397.5]) μg/L and higher odds (OR=4.9 [2.5, 9.3]) for being iodine deficient (UIC <100 μg/L; [38%]) than infants who continued to be breastfed (373.2 [202.6, 522.9] μg/L) at 12 months. Infants receiving SQ-LNS (combined group) had higher UIC (P=0.025) and lower odds for having a UIC <100 μg/L (OR=0.289 [0.11, 0.75]) at 12 months than infants in the control group, adjusting for maternal baseline UIC, age, sex and continued breastfeeding. In sub-group analysis, the effect of SQLNS for higher UIC at 12 months was only apparent in the infants who no longer received breast milk at 12 months (P=0.039). This effect was insignificant after adjusting for infant baseline UIC, which resulted in a smaller sample size (n=124). Conclusion : The results of this research suggest adequate iodine intakes in lactating mothers and infants residing in peri-urban areas in South Africa. However, better monitoring of salt iodine content of the mandatory salt iodization programme in the country is required. Iodine in breast milk contributed to the adequate iodine status in infants. Commercial infant cereals potentially contributed to the sufficient iodine intakes in infants at ages six and 12 months, however, the iodine content in frequently consumed commercial infant cereals in South Africa needs to be investigated. The provision of 45 μg of iodine per day as SQ-LNS can improve UIC in nonbreastfed weaning infants, but the dose is not efficacious in counteracting an overall decline in iodine status. Therefore, it may be necessary to increase the iodine content in home fortification products such as SQ-LNS to the recommended iodine fortification level of 90 μg.