Zinc Status in Hair Samples and Common Neurodevelopmental Disorders

Torsak Tippairote1, Dunyaporn Trachootham2*

1BBH Hospital, Bangkok, Thailand

2Institute of Nutrition, Mahidol University, Nakhon Pathom, Thailand

Zinc status is an important modifiable factor in Attention Deficit and Hyperactive Disorders (ADHD) and Autistic Spectrum Disorders (ASD), the two most common neurodevelopmental disorders. Many studies reported low serum or plasma zinc level in children with these conditions. While hair zinc level can be obtained non-invasively in young children, the reports of the hair zinc levels in ASD and ADHD children were varied. ASD children were reported to have lower than or indifferent level of hair zinc from that of healthy children. In ADHD children, hair zinc levels were reported as either lower or higher than their healthy control groups. Many factors interplayed and affected measurement of hair zinc level. Until more standardized method has been established, currently the zinc level in hair samples may be used as screening or supporting evidence. Other functional zinc markers such as serum zinc concentrations, dietary zinc intakes, and percentage of stunting rate, are still needed to assess the zinc status in susceptible children.

Prevalence of neurodevelopmental disorders, particularly ADHD and ASD, were increasing in the last two decades1-3. Many studies reported numerous factors, including non-modifiable factors, such as genetic predispositions, pre-, peri-, or postnatal events, and many modifiable factors, such as the nutritional and environmental factors that were accounted for pathogenesis and pathophysiology of these disorders4-6. Among those nutritional factors, zinc status in various tissue samples was frequently reported to be associated with ADHD and ASD children. A number of studies further supported the role of zinc supplement as the possible safe and important treatment in ASD and ADHD7,8. Analysis of zinc level in children’s hair was done via non-invasive procedure and was preferably used to assess zinc status in many settings9. We here reviewed the current knowledge on hair zinc status in ADHD and ASD children.

Undeniable role of zinc in healthy brain development are well established. The unique zinc-containing molecules, known as several classes of Zinc finger proteins (ZFP), are responsible for the DNA-binding ability of many transcription factors. Thus, zinc indirectly regulates various gene expressions. While most of central nervous system (CNS) zinc are tightly bound to proteins such as transcription factors, enzymes, metallothionein, approximately 10% of zinc is freely localized in presynaptic vesicles of glutamatergic or GABA containing neurons10-12. These free zinc help in modulating a variety of postsynaptic receptors, affecting neuronal signal transduction for moods, memory, cognition and attention11,13-16. The role of CNS zinc affects brain and neuron formation, remodeling and functions throughout the life span. During brain developmental processes, starting from the stage of neuron proliferation, axon and dendrite growth, synapse formation, pruning and function to differentiation, zinc always plays its key supporting roles in various processes such as DNA synthesis, cell division, and modulation of apoptosis, oxidative stress and synapse functions11,17. Even in adult brain, these zinc involvements still persist and significantly affect important variety of brain functions15.

Studies in cell, animal and human models supported the crucial roles of zinc in healthy neurodevelopmental process. Inadequacy of zinc during neuronal differentiation stages decreased cell survival and altered synaptogenesis and synapse pruning (selective elimination of unused synapse for development of learning and memory). These process occur through different pathways such as DNA replication, transcriptional control, mRNA translation, apoptosis and microtubule stability15,18-20. Studies using animal models demonstrated that during critical brain development period, functional zinc level was tightly regulated by zinc transporters and buffering proteins18,21.

In later neurodevelopmental stage, behavioral changes which were related to dopaminergic pathway genes expression, were shown to be associated with ZFP gene variants22. Abnormal behaviors and hippocampal developmental disorders were demonstrated in ZFP521 mutant mice23. Study of zinc deprivation in rhesus monkeys showed changes in attention and behaviors24. Human with zinc deficiency may show impairment in concentration and delay in cognitive development25,26. A recent study reported low hair Zn/Cu ratio in children with Phelan McDermid Syndrome (PMDS), a genetic disorder characterized by features of Autism spectrum disorders and deletion in SHANK3 (SH3 and multiple ankyrin repeat domains 3) genes. Mechanistic studies in cell culture model revealed that the observed Zn deficiency was likely due to decreased expression of Zn uptake transporters27. Besides attention and cognitive deficits, several studies using animal depression model showed that zinc-deficient diets could induce quantitative and qualitative changes in phospholipid-protein balance, and up-regulation of N-methyl-D-aspartate receptor (NMDAR) in hippocampus and prefrontal cortex28-30.

Many reports showed lower serum or plasma zinc level in ASD and ADHD than that of healthy children31-33. However, the association between zinc status in hair samples and ASD or ADHD risk were somewhat inconclusive. While most of reports showed lower hair zinc level in ASD children than healthy controls, some reports showed no differences. For examples, a roentgen-fluorescence spectrometry study from Georgia and an atomic absorption spectrophotometry (AA) study from India showed significantly lower zinc in hair of ASD children than that of their matched controls34,35. Another ICP-MS study in Kingdom of Saudi Arabia (KSA) also reported lower hair zinc level in ASD children than their control groups36. In contrast to these findings, recent inductively coupled plasma mass spectrometry (ICP-MS) study from Russia reported no difference, with average levels at 124.6 to 113.3 µg/g, of hair zinc level in ASD children, to their age-matched controls in concordance to the report of AA study from KSA in 200537,38. A Meta-analysis of 12 studies from 1978 to 2012 also reported no difference in hair zinc level between their study groups39. Based on existing information, there was no report of significantly higher hair zinc level in ASD than healthy children.

In ADHD children, a recent Egyptian ICP-MS study reported lower hair zinc level in 20 ADHD children as compared to their cut-off value at 108.9 µg/g, and to their healthy controls31. In contrast, a Polish AA study reported higher hair zinc level in ADHD than healthy children40. Our recent Thai ICP-MS study also reported higher hair Zn/Cu and Zn/P ratios in 45 newly diagnosed ADHD children than their healthy controls. The average hair zinc levels in this study were 92.9 and 85.4 µg/g in ADHD and healthy children, respectively. Interestingly, the higher hair zinc level was associated with higher number of inattention, hyperactivity and total ADHD symptoms41.

Reports of hair zinc status in these common neurodevelopmental disorders were apparently swung to both possible conflicting ends, of particularly in ADHD children. These indicated the dynamic association between hair zinc status and these common neurodevelopmental disorders. Further studies are required to elucidate factors contributing to these observations. Furthermore, zinc is environmentally ubiquitous. It is possible that some, but not all reported results of analyses of zinc in hair were obtained with inadequate clean sample preparation technique42. Zinc also has ICP-MS analytical interferences42. It is possible that some groups are less adept at exploiting instrument capabilities for the effective elimination of interferences. The fact that zinc has analytical interferences and is environmentally ubiquitous might be possible additional explanations for the conflicting results reported by different groups.

Mineral deposition into hair samples are different from other tissues. Unlike minerals in blood, hair minerals have no reported roles in any physiologic functions. Instead it may preferably be viewed as excretory or wasting minerals43. External incorporation of minerals into hair samples is possible and probably affected the reported hair mineral levels. However, proper hair sample collection procedure can minimize such possibility42. Moreover, diverse analytical methods and different laboratory standards and settings may create variations in detected levels. Currently, there are still no universal standard and references in hair mineral levels42-45. Without proper regional references or control groups, it is then premature to determine the deficiency or excess state of minerals merely from hair level.

Genetic predisposition of individual affected zinc status in hair samples. For example, a patient with ASD related mutation in SHANK3 showed re-emerging of low Zn shortly after discontinuation of zinc supplementation27. This evidence implied that in susceptible individuals regular diet may not be sufficient to maintain functional zinc level and the level of measured serum/plasma or hair zinc level may greatly vary and be rapidly changed on different state of mineral exposure.

Maternal factors such as cigarette smoking may contribute to zinc status in human offspring. A study of maternal and neonatal scalp hair revealed that cigarette smoking by mothers was associated with lower zinc but higher cadmium and lead in hair of their neonates46. A study in 1,967 infants (0-3 years old) with autism spectrum disorder (ASD) showed 43.5 and 52.5 % Zn deficiency in male and female infants, respectively47. Since development of nervous system starts from pregnancy and continues after birth, further studies of maternal factors affecting zinc status in ASD and ADHD children are worthwhile.

Regional and individual dietary pattern may affect the zinc status in hair samples (36, 41). In fact, a study in ASD children participating in the Autism Treatment Network, USA utilized prospective 3-day food records found that ASD children aged 4 to 8 years consumed significantly less mineral Zn than that of the recommended amount48. Furthermore, another study found significant correlation between calcium intake and hair zinc status in Polish infants49. More studies should be performed to investigate the relationship between dietary intake and hair Zn status in ASD and ADHD children. In addition, interaction with other minerals may have also interfered with hair zinc status. In fact, a multi-element study in hair samples reported that zinc level was positively associated with calcium, phosphorus, and magnesium, while negatively associated with arsenic, aluminum, iron and molybdenum41. Zn concentration in blood stream is controlled by metallothionein – the metal-binding protein. Displacement of zinc from metallothionein by cadmium may lead to release of zinc into hair samples50. In fact, elevated blood level of cadmium concomitantly with lower blood level of zinc and elevated urine level of zinc were shown in many patients with tobacco-related diseases51. Moreover, diverse analytical methods and different laboratory standards and settings may create variations in the detected level in hair samples42,45.

Cumulative evidences support a crucial role of zinc on neurogenesis in both developmental and adult stages15. However, there was limited evidence of zinc status in adults suffering from neurodevelopmental disorders. Repletion of zinc was shown to ameliorate depressive behaviors in mouse models28. Degree of symptoms in patients with major unipolar depression were decreased when being treated with combination of zinc supplementation and anti-depressant drugs in a double-blind, placebo controlled study52. Even if the treatments are started in adulthood, reversing of maladaptive biochemical state can result in significant functional improvement in neurodevelopmental disorders despite irreversible neuroanatomical pathologies53. When zinc deficiency was confirmed, zinc repletion may be beneficial to adults with these neurodevelopmental disorders.

Disturbance of zinc homeostasis may contribute to clinical symptoms of ASD and ADHD as shown by hair zinc level changes in ASD and ADHD children from different mentioned studies. However, the correlation of hair zinc status and neurodevelopmental disorders seems to be more complicated than just either high or low measured level. Many internal and external factors seem to play roles in this dynamic interaction and warranted further studies.

In the meantime, low hair zinc level may be used as supporting evidences of zinc status in ASD and ADHD children. However, normal or high hair zinc level, particularly in ADHD children, do not entirely exclude the possibility of functional zinc inadequacy. Other functional zinc deficiency markers, such as low serum zinc concentrations, low dietary zinc intakes, and percentage of stunting rate in children, were needed to confirm the possibility of zinc deficiency state54. Zinc supplementation will be the beneficial option if there are convincing evidences of deficiency state7,8,55.

The author thanks Mr.Eugene Kilayco, a professional English editor for proof-reading and editing the manuscript. Parts of the work described in this paper was supported by The National Research Council of Thailand (FY2016; Thesis Grant for Master Degree Student).

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Article Info

Article Notes

  • Published on: December 18, 2017


  • Zinc

  • ASD
  • ADHD
  • Hair
  • Neurodevelopmental disorders
  • Children


Dr. Dunyaporn Trachootham
Institute of Nutrition, Mahidol University
Salaya, Phutthamonthon, Nakhon Pathom, 73170, Thailand
Telephone: 66-2-800-2380 ext. 326
Email: dunyaporn.tra@mahidol.ac.th