Essential nutrients, including micronutrients, amino acids and fatty acids, are necessary for proper functioning of the central nervous system (CNS) and play a role in the maintenance of normal neuronal plasticity. Micronutrient deficiencies are a critical health issue among children throughout the world. Approximately 25% of the world’s children under age 3 years have iron-deficiency anaemia, with higher rates in developing countries 1.
Dietary deficiencies of iron, zinc, iodine, selenium, copper, manganese, fluoride, chromium and molybdenum are suspected to be associated with mild to significant changes in neuronal function, which can lead to poor health and adverse effects on behaviour and learning 2.
Zinc deficiency related to poor dietary intake may be a factor in the development of several conditions that, ultimately would, make learning processes more difficult for children.
Zinc deficiency also decreases thyroxine production 3 and develops over time with the consumption of high fructose corn syrup 4 and tartrazine, which are used as colouring substances in foods 5. Poor nutrition leading to dietary deficiencies of zinc is considered the main aetiology. Zinc deficiency throughout childhood is estimated to be high, primarily related to the low consumption of foods high in bioavailable zinc 6.
Impairment of brain function and neuronal plasticity by exacerbating heavy metal neurotoxicity is suspected in case of zinc deficiency 7,8. In a preliminary investigation of lead, zinc and iron levels with respect to child cognition and behaviour in a small sample of children aged 3–5 years, zinc and ferritin were linked to verbal abilities 9.
In another study, including children aged 6–8 years, iron and zinc were of limited usefulness for improving cognition; however, a role for the supplements was suggested in case of their deficiency 10.
Copper is critical for the normal development and function of the CNS. Precise regulation of brain copper is essential to ensure appropriate levels and distribution for the maintenance of brain function. Copper acts as a cofactor for numerous critical enzymes that are involved in vital CNS processes, including neurotransmitter synthesis, activation of neuropeptides and protection from oxidative damage 11.
Iron is suspected to be linked to cognitive performance, although various iron parameters are being studied at various age groups. Ferritin showed relationship with cognition in children suffering from other symptoms – for example, attention deficit hyperactivity disorder (ADHD) 12.
This study aims to assess the blood level of zinc, copper and iron parameters such as ferritin and haemoglobin (HgB)% in children with decreased cognitive abilities of various degrees in comparison with control children.
Patients and methods
This study included 68 children with cognitive impairment aged 7.4±2.6 years (46 boys and 22 girls). They were recruited from clinic of learning disabilities, centre of medical excellence, National Research Centre (NRC), Cairo. Consent of parents was obtained and study was explained. Assessment of cognitive functioning was performed using Primary Scale of Intelligence-Revised to determine the intelligence quotient (IQ). The measured IQ was 80.6±10. In addition, children with profound mental retardation (IQ<60) were excluded. Exclusion of children with ADHD was performed using the DSM-V criteria.
The control group included 48 healthy age-matched control children aged 9±1.6 years (30 boys and 18 girls). Any children suffering from chronic disorders or perinatal insults – for example, perinatal hypoxic ischaemic insults – were excluded from the study. Each family of a patient with cognitive impairment was asked to bring another family from the same area with normal child (IQ was checked, clinical examination and exclusion of any chronic health problem). All control children had IQ more than 105.
A blood sample of 2 ml was withdrawn by venipuncture after wrapping the skin by alcohol 70% and placed into a pyrogen-free tube for complete blood count. Another 2 ml of blood was withdrawn and centrifuged for serum separation and stored at −70° for other bioelements assay. A colourimetric test without deproteinization (Spinreact, Spain) was used for zinc and copper assay. For ferritin assay, a Direct ELISA Kit was used (Abcam, Cambridge, USA). The parents or guardians of all children had to give informed consent to participate in the study.
HgB concentration was determined by an automated cell counter from a tube of well-mixed EDTA-anticoagulated blood. Values are given as mean±SD. The Student t-test was used for group comparisons of normally distributed variables, and the Mann–Whitney U-test was used for comparison of variables with skewed distribution.
All procedures in the study are in compliance with the Helsinki Declaration.
No significant difference in age between children with cognitive impairment and control children was found (P=0.2), and no significant difference in sex distribution was found (P=0.6), but the difference in IQ was significant (P=0.005).
In plasma of cognitively impaired children, zinc level was 90±32 μg/dl, whereas in controls it was 98.5±3 μg/dl. Zinc level decreased in affected children compared with controls, with a highly significant difference (P<0.0001; Table 1).
Serum/plasma copper in children with cognitive impairment showed a mean of 118.6±39 μg/dl, whereas in controls it was 129±10.6 μg/dl, with highly significantly decreased copper level in the affected group (P<0.0001; Table 1). Correlation between zinc and copper levels in the affected children group does not show significant value (P=0.1; Fig. 1), and correlation between individualized IQ scores and zinc and copper levels does not show significant value (P=0.9 and 0.3, respectively; Fig. 2).
Ferritin level was 39±42 μg/dl in the affected group and 72±21 μg/dl in controls. Ferritin was significantly lower in the affected group (P<0.001; Table 1 and Fig. 3), whereas HgB concentration in children with cognitive impairment was 11.9±1 and in controls it was 12.4±0.9, with no significant difference between both groups (P=0.5; Table 1). Correlation between HgB% and ferritin did not show significant value (P=0.6).
The relationship between micronutrient deficiency and early cognitive development has attracted many researchers’ attention and interest because micronutrients are related to specific neurophysiological processes.
Our study shows decreased zinc, copper and ferritin levels with respect to decreased cognitive abilities in children. Micronutrient deficiency is most probably related to insufficient nutritional intake. Another observational study from Egypt demonstrated a link between maternal micronutrient intake and infants’ developmental skills 13.
In our study, we excluded children with ADHD to exclude the effects of this disorder on cognitive ability and to elucidate the effect of studied parameters on learning ability as determined by IQ.
Siekmann et al. 14 reported that the widespread prevalence in developing countries of multiple micronutrient deficiencies is associated with a low intake of animal source foods (ASFs). Low zinc intake represents a major public health problem. Our results showed significantly decreased zinc and copper levels in children with decreased IQ as compared with controls; however, correlation between both elements and IQ was not significant, which can be accounted for increased variance in zinc levels in the affected group due to other factors – for example, BMI or socioeconomic status. A recent study on Egyptian children showed an inverse relationship between body zinc and BMI 15, and maternal education and attitude of feeding children would also increase the difference in zinc levels 16.
Zinc deficiency could stem from as early as prenatal low maternal micronutrient intake, which would further be compromised during infancy and early childhood, in developing countries, by low intake of ASF. Furthermore, phytate derived from excess consumption of maize and beans would hinder zinc absorption from gastrointestinal tract 14.
Similarly, copper deficiency is most probably due to low consumption of ASF and seafood. Because of poor intake of micronutrients (including zinc and iron), 20% of US population suffer copper deficiency 17.
Our results support previous research highlighting the role of zinc in the nervous system; in some neurons, zinc is secreted with glutamate into the synapse, where glutamate initiates action potential and zinc being an inhibitor factor 18. With inadequate zinc, glutamate-induced induction of action potential persists and neuronal damage results from absence of proper inhibition.
Another study conducted on mice revealed that zinc deficiency impairs hippocampal-dependent cognition, with disruption of the calmodulin/phosphorylated cAMP-responsive element binding protein signalling pathway 19. Short-term (15-week) moderate zinc deprivation in prepubertal monkeys resulted in reduced motor activity and less accurate performance on measures of attention and short-term memory 6.
Disturbances of the zinc/copper ratio was studied in other neurodevelopmental disorders – foe example, autistic spectrum disorder, with mercury accumulation. Metallothionein dysfunction in children diagnosed with autistic spectrum disorder was suspected as a cause of zinc deficiency, as metallothionein is a protein linked to metal metabolism and function 20.
In animals, zinc treatment for traumatic brain injury was performed for symptoms of anxiety, depression and learning ability in rats. It showed significant improvement in cognitive ability, with no effect on other functions 21. Zinc supplementation in children was enrolled in many studies; a review study summarized the results of about 10 studies that discussed zinc supplement at various ages, including foetuses, toddlers and school children. Majority of school children showed improvement in cognitive function in response to zinc supplementation with various responses on other functions studied – for example, motor functions 1.
Copper results among our patients showed significantly decreased copper in children with decline in cognitive functioning than in controls, which is in agreement with previous studies in Alzheimer disease brains 22,23. In addition, in children, higher serum copper and ferritin levels were linked to better visuomotor integration, and hence IQ 24.
Copper was linked to impaired long-term potentiation in the hippocampus, and thus neuronal placidity, and it is essential for formation of dopamine hydroxylase – an essential enzyme in the nervous system 25.
Correlation between zinc and copper was not significant, whereas some experimental animal studies demonstrated a significant interaction between both elements in affecting memory 26. Both zinc and copper are also antagonists of the N-methyl-D-aspartate glutamate receptor in the CNS, which mediate mood, sleep and cognition 27. We, therefore, suggest that maintenance of optimum zinc/copper ratio as well as zinc or copper serum levels would play a significant role in proper cognitive function.
Ferritin was significantly decreased in children with learning disabilities than in control children, whereas HgB% did not show differences in both groups. This is in agreement with the fact that ferritin is the best biomarker of iron status 28–31. Fewer studies found that iron status (measured by HgB% and ferritin levels) was not related to cognitive function 32. Ferritin was linked to ADHD children, whereas no relationship with cognitive function was found 4. This could be explained by different basis for the cognitive decline with ADHD affecting the ability of child to perceive and communicate.
A study carried out on more than 400 children showed that cognitive function increased with increased HgB% in children with decreased ferritin but did not change with HgB% in presence of normal serum ferritin level 32. Our results are in agreement with these results, as most of our patients had decreased ferritin level, which is considered the main indication of iron stores.
Zinc, copper and ferritin levels showed significant decrease in our sample of school-aged children with defective cognitive functioning, which would favour examining the level of these elements in children with decreased IQ scores or cognitive performance as early as possible; however, other extended studies are warranted with randomized controlled trials to prove the role of these elements as interventional factors.
Conflicts of interest
There are no conflicts of interest.
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