Objective: Infants who are born small for gestational age (SGA) are at risk for developmental delays, which may be related to deficiencies in zinc, an essential trace metal, or to deficiencies in their ability to elicit caregiver responsiveness (functional isolation hypothesis). The objective of this study was to evaluate at 6 and 10 months of age the impact of a 9-month supplementation trial of 5 mg of zinc on the development and behavior of infants who were born SGA and to evaluate infants' ability to elicit responsive caregiver behavior.
Methods: A randomized, controlled trial of zinc supplementation was conducted among 200 infants in a low-income, urban community in Delhi, India. Infants were recruited when they were full term (>36 weeks) and SGA (birth weight <10th percentile weight-for-gestational age). Infants were randomized to receive daily supplements of a micronutrient mix (folate, iron, calcium, phosphorus, and riboflavin) with or without 5 mg of zinc sulfate. The supplement was administered by field workers daily from 30 days to 9 months of age. At 6 and 10 months, infant development and behavior were measured in a clinical setting using the Bayley Scales of Infant Development II. Caregiver responsiveness, observed on an Indian version of the Home Observation for Measurement of the Environment scale, was measured during a home visit at 10 months. During both the clinic and home visits, caregivers reported on their infant's temperament.
Results: There were no direct effects of zinc supplementation on the infants' development or behavior at either 6 or 10 months. In a subgroup analysis among the zinc-supplemented infants, lower birth weight infants were perceived to be more temperamentally difficult than higher weight infants; in the control group, birth weight was not associated with temperament. Heavier birth weight infants had better scores on all measures of development and behavior at 6 months and on changes in mental and motor development from 6 to 10 months, compared with lighter birth weight infants. Boys had better weight gain and higher scores on mental development and emotional regulation than girls. Infants who were from families of higher socioeconomic status (indexed by parental education, house size, and home ownership) had higher scores on mental development and orientation/engagement (exploratory behavior) than infants who were from families of lower socioeconomic status. In keeping with the functional isolation hypothesis, caregiver responsiveness was associated with infant irritability, controlling for socioeconomic status, gender, birth weight, and weight gain. Responsive mothers were more likely to perceive their infants to be temperamentally easy than less responsive mothers.
Conclusion: Possible explanations for the lack of effects of zinc supplementation on infant development and behavior include 1) subtle effects of zinc supplementation that may not have been detected by the Bayley Scales, 2) interference with other nutritional deficiencies, or 3) no impact of zinc deficiency on infants' development and behavior. The link between birth weight and irritability among infants in the zinc supplementation group suggests that the response to zinc supplementation may differ by birth weight, with irritability occurring among the most vulnerable infants. Longer term follow-up studies among zinc-supplemented infants are needed to examine whether early supplementation leads to developmental or behavioral changes that have an impact on school-age performance. The relationship between infant irritability and low maternal responsiveness lends support to the functional isolation hypothesis and the importance of asking caregivers about infant temperament.