Effect of iodine and iron supplementation on physical, psychomotor and mental development in primary school children in Malawi

Primary school children (n = 424) from the Ntcheu District, Malawi, aged 6 - 8 years, were selected for a double-blind placebo-controlled study to evaluate the effect of iodine and iron supplementation on physical, psychomotor and mental development. After the baseline measurements were carried out, children were given a single dose of iodized oil (1 mL Lipiodol®; 490 mg I) or placebo and daily doses of iron (ferrous sulphate, 60 mg of Fe) or placebo on weekdays (February-July and October- November 1991). The anthropometric measurements included height, weight, mid-upper arm cicumference (MUAC), and four skinfolds while indicators of physical stamina included lung function, hand grip and sitting and standing ability. The psychomotor development tests were pegboard, ball throwing, tapping and reaction time tasks. The mental development tests included fluency and exclusion as a measure of fluid intelligence; quantity and verbal meaning to evaluate crystallized intelligence and visual memory and closure tests to measure perceptual skills. Baseline data were collected from October 1990 to January 1991 and the final tests and measurements were carried out in October-November 1991. The initial iodine and iron status was established by measuring the concentration of iodine in urine and of hemoglobin in blood. About 86% of children had urinary iodine concentrations below 0.4 μmol/L indicating moderate iodine deficiency while 18% of children had hemoglobin levels below 110 g/L which is the cut-off point for indicating anemia. Measurement of iodine in urine three months after supplementation showed a normal level of urinary iodine. After one year, the level had dropped to the baseline value. The change in haemoglobin could not be measured due to objections from the parents in obtaining further blood samples. About 27% of children were stunted while 2% were wasted when measured at both time points. One year after supplementation, the four treatment groups did not grow differentially indicating no effect of iodine or iron supplementation on physical growth. The iodinetreated group scored higher in tests of physical stamina as measured by sitting-standing and hand grip. The iodine treated group also significantly improved scores on the ball throwing exercise indicating an improvement in eye-hand coordination. No changes were noticed on reaction time, movement time and tapping indicating no effect of iodine on speed, dexterity and manual-motor coordination. The iron-treated group showed improvement only in eyehand coordination tests. No improvements were seen in other psychomotor tests. Subjects in all four groups improved their scores on all mental development tests. The analysis of the placebo group indicated a considerable learning effect especially in fluency. Detailed analysis were conducted based upon the differential improvement when compared with the placebo group during the final test. The iodine-treated groups showed a large improvement in fluid intelligence measured by the fluency and exclusion tests. This component of intelligence measures reasoning, classification and fluency. Perceptual skills, as measured by visual memory and closure, also improved considerably in the iodine-treated group. Crystallized intelligence, as measured by quantity and verbal meaning, was also improved in the iodine-treated group but to a lesser extent than fluid intelligence. Crystallized intelligence is associated with word meaning, factual knowledge, short termmemory and decision making. These skills are closely associated with the classroom environment. In the iron-treated group, a significant improvement was noted only in fluid intelligence and in the quantity test, a component of crystallized intelligence. No significant interaction of iodine and iron was noted. However, combined supplementation with iodine and iron sometimes resulted in an additive effect. It has been shown in several studies in which iodine has been administered to mothers prior to pregnancy or during gestation that iodine supplementation reduces spontaneous abortion and stillbirths and improves the birth weight, and the mental and psychomotor performance of children. Previous studies in which children have been supplemented with iodine have not been able to demonstrate unequivocally that such supplementation affects mental or psychomotor development. In the present study we were able to demonstrate that iodine supplementation to children as old as 6-8 yr could improve mental and psychomotor development. The present study also demonstrates that iodine supplementation was more effective than iron supplementation in the population studied but the low prevalence of iron-deficiency anemia in children could have been responsible for such an outcome. Concerning supplementation with iron, several previous studies have indicated that correction of iron deficiency early in infancy tends to improve psychomotor and mental development more than when the correction takes place in later life. In the present study, supplementation with iron was found to improve eye-hand coordination and fluid intelligence which justify prevention and control of iron-deficiency anemia in infants and children as soon as such a problem is detected. An important finding from the perspective of planners is that 490 mg oral iodine may be inadequate to provide sufficient iodine for one year as indicated by the measurement of urinary iodine excretion. In iodine-deficient areas where provision of iodized salt is not available, the provision of iodized oil capsules could be an attractive alternative provided that the dosing schedule is adequate.