Author: Ulrich Brennecke
Review: Dipl.-Psych. Waldemar Zdero
Certain physical illnesses, toxins or food intolerances seem to increase the risk of ADHD (or other mental disorders) throughout life.
The % values indicate the possible ADHD risk increase due to the respective cause. They refer to individual known, but rather selective stress factors. However, they are nevertheless helpful as an indication of the magnitude of the possible increase in risk.
4.1. Toxins¶
4.1.1. Phthalates (+ 200 % to + 900 %)¶
Higher phthalate metabolites in children’s urine correlated with increased likelihood of ADHD by 3 to 9 times.
4.1.2. Fluoridated drinking water (+ 510 % if 1 mg/L too high)¶
In Canada, a study found that a 1 mg/liter increase in fluoride levels in drinking water above acceptable limits increased the risk of ADHD by 6.1 times in 6- to 17-year-olds. In 14-year-olds living in areas where fluoride was added to drinking water, there was a 2.8-fold risk of an ADHD diagnosis compared to 14-year-olds in areas without fluoridated drinking water. Older children responded with a higher risk. Fluorine urine levels, on the other hand, did not correlate with ADHD (1,877 subjects).
A study in Mexico found a link between increased prenatal fluoride exposure and inattention and ADHD, but not hyperactivity and cognitive problems. Another study found similar results. A review summarizes the results.
In Germany, 90% of drinking water has a fluoride content of 0.3 mg/liter. Drinking water is not fluoridated in Germany.
A study found an inverse correlation between fluoride levels in the mother’s urine during pregnancy and cognitive problems in the offspring at the age of 11. The higher the pregnancy urine fluoride content, the lower the cognitive problems. This was not consistent with the results of other studies, which found an increased risk of ADHD with increased urine fluoride levels in the children themselves.
Sodium fluoride in drinking water (20 ppm to 100 ppm) led to a dose-dependent reduction of dopamine, noradrenaline and acetylcholine in the brain of rats, while the levels of adrenaline, histamine, serotonin and glutamate increased.
4.1.3. Lead (+133 %)¶
Elevated blood lead levels lead to an increased risk of ADHD. A blood lead level of ≥ 5 μg/dl was found to increase the risk of ADHD by 1.33 (OR 2.33).
Lead influenced the dopamine balance in many studies.
- Reduced dopamine signaling
- caused cognitive deficits with delayed spatial alternation, which could be corrected by L-dopa and without L-dopa only ended 8 years after the 2-year lead exposure
- in the nucleus accumbens
- Increased dopamine signaling
- in mesolimbic pathways (nucleus accumbens)
- Lead increases dopaminergic activity and has been linked to attention deficits, Alzheimer’s disease and increased drug sensitivity.
The DRD2 gene variant rs1800497r is said to promote a link between ADHD and lead. A connection to certain MAO-A gene variants is also mentioned, which causes lower serotonin degradation. A study in rats suggests interactions between lead exposure and early stress on the dopaminergic system. A long-term study found no directly increased risk of ADHD in people with previous lead exposure, but increased externalizing behaviours and increased risk of addiction
In one study, lead altered neostriatal serotonin and norepinephrine levels, increased anxiety and decreased open-field activity.
Lead exposure during pregnancy may increase the risk of ADHD. See there.
Even a lead content in drinking water below the limit values is said to be problematic.
Increased lead absorption can occur from old water pipes. In principle, lead water pipes are not very dangerous in areas with calcareous water, as lime forms a reliable protective layer in the pipes. However, if a water softening system is installed, this protective lime layer can be lost. If old lead pipes are still present, this can lead to increased lead absorption.
Lead is barely relevant as a toxin in Central Europe. In less developed countries, however, lead can be a serious problem.
In children who have been exposed to lead, succimer chelation can produce lasting cognitive benefits if chelation sufficiently reduces the lead concentration in the brain. At the same time, succimer treatment without lead exposure leads to permanent cognitive dysfunction.
4.1.4. Inorganic arsenic (+ 102 %)¶
Those children who were among the 20% with the highest urinary arsenic levels were found to have double the risk of ADHD (OR 2.02).
4.1.5. Benzene, toluene, ethylbenzene, xylene/xylene (BTEX) (+ 54 %)¶
Higher exposure to these substances in the air correlated with a 1.54-fold increase in the risk of ADHD at kindergarten age.
4.1.6. Smoking by parents (+ 30 %)¶
Postnatal smoking by parents correlates with a 1.3-fold risk (increased by 30%) of ADHD in offspring.
This could be related to genetic factors, as people with ADHD are significantly more likely to smoke. The co-morbidity of smoking with ADHD is 40%. In contrast, around 25 % fewer of the total population smoke, namely 26.9 % of women and 32.6 % of men.
4.1.7. Polychlorinated biphenyls (PCBs) / polychlorinated biphenyl ethers (+ 26 % to + 92 %)¶
Polychlorinated biphenyls and polychlorinated biphenyl ethers are suspected of causing ADHD.
PCBs are banned in many countries, in Germany since 1989. PCBs were used in particular as lubricants and coolants in electrical appliances and as building materials. Due to their chemical stability, many areas around the world are still contaminated with PCBs. Contaminated food, especially seafood from contaminated rivers and lakes, is the most common source of contamination today.
Even low levels of PCB exposure during development impair neurobiological, cognitive and behavioral functions.
One study found a 26% to 92% increased risk of ADHD. Individual studies found contradictory or weak impairments, however, the vast majority show evidence of relevance in ADHD.
Polychlorinated biphenyls affect the dopamine system. PCBs inhibit dopamine synthesis as well as the storage of dopamine in the vesicles and its release, thereby causing low dopamine levels in the basal ganglia and PFC , as well as reduced DAT in the striatum, which overall corresponds quite closely to the picture of ADHD.
Prenatal exposure to PCBs has an adverse effect:
- Hyperactivity (in rats even at subtoxic doses)
- IQ, memory, attention
- Memory, attention
-
Impulsivity (via corpus callosum) in rats even at subtoxic doses
- Male and female offspring were trained as adults to perform asymptotically in a differential reinforcement of low rates (DRL) task. The PCB-exposed groups had a lower ratio of reinforced to non-reinforced responses than the control groups.
- no effect on sustained attention
4.1.8. Polyvinyl chloride (PVC)¶
One review describes a suspected correlation between PVC exposure and ADHD.
4.1.9. Pesticides¶
With regard to pesticides (especially organochlorine compounds, pyrethroids, organophosphates), there are indications of relevance in ADHD.
For pesticides during pregnancy and ADHD, see there.
4.1.9.1. Organochlorine compounds¶
With regard to organochlorine compounds, there are indications of relevance in ADHD.
A study of Greek schoolchildren with ADHD found no elevated blood serum levels of
- Dichlorodiphenyltrichloroethane (DDT) Metabolites
- Hexachlorocyclohexane (HCH) isomers
- Cyclodienes
- Methoxychlorine
4.1.9.2. Organophosphates¶
According to a large number of studies, organophosphate pesticides have a correlation between prenatal and postnatal exposure and ADHD or a theoretically possible increase in ADHD risk. One source suggests an increased risk of ADHD from organophosphates, particularly when coinciding with a particular MAO-A gene variant that causes lower serotonin degradation.
In contrast, two larger studies found no influence
With regard to hyperactivity, 2 studies found an association between organophosphates and hyperactivity, 4 studies found no association.
One study found no correlation of chlorpyrifos with hyperactivity in rats while another found it in females.
A study in rats was able to induce ADHD-like behaviors in Wystar and SHR rats by organophosphates and found strong circumstantial evidence that these are mediated by reductions in fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) via the cannabinoid receptor.
Blood values were measured in Egyptian adolescents, some of whom used pesticides, and the parents were asked about ADHD symptoms in the adolescents: No correlation was found with ADHD in relation to the organophosphate chlorpyrifos.
Higher vitamin D levels appear to reduce the negative effect of chlorpyrifos on the risk of ADHD.
4.1.9.3. Pyrethroids (+ 142 %)¶
Various studies indicate a correlation between pyrethroid exposure in childhood and neurodevelopmental disorders such as ADHD with a 2.42-fold risk of ADHD Other studies also found an association with ADHD , ASD or developmental delay.
Blood levels were measured in Egyptian adolescents, some of whom used pesticides, and the parents were asked about ADHD symptoms in the adolescents: A correlation to ADHD was found in relation to the pyrethroid λCH through the measured value Cis-DCCA (all persons with ADHD reported clinical ADHD symptoms).
4.1.9.4. Carbamate (-)¶
One review found no associations between carbamates and ADHD.
4.1.9.5. Neonicotinoids (- ?)¶
One review found no links between neonicotinoids and ADHD, although there were few studies on this topic.
4.1.10. Mercury / Amalgam (Mercury)¶
There is weak evidence (= not proven) of relevance in ADHD.
A large study with n = 2073 participants was unable to establish a connection between amalgam and ADHD.
Mercury is also suspected of being a possible contributory cause of Parkinson’s disease. This would be a clear indication of a damaging effect on the dopamine system.
4.1.11. Manganese¶
There is weak evidence of relevance in ADHD, although elevated manganese levels were only found in the hair, but not in blood levels, of people with ADHD.
An animal model with developmental manganese exposure showed that manganese can cause permanent attention and sensorimotor deficits resembling ADHD-I. Oral methylphenidate was able to fully compensate for the deficits caused by early manganese exposure.
A doubling of the manganese content in teeth from the prenatal and postnatal period increased the risk of attention problems and ADHD symptoms in the school years by 5%. Manganese from the childhood period showed no influence
A study reports benefits of choline supplementation during pregnancy in rats to prevent manganese-induced developmental disorders in the offspring
4.1.12. Bisphenol A¶
Bisphenol A is suspected of increasing the risk of ADHD. A connection with certain MAO-A gene variants that cause lower serotonin degradation and an influence on the thyroid balance is being discussed.
A meta-analysis found a clear link between bisphenol exposure and ADD(H)S.
4.1.13. Perfluoroalkyl compounds¶
Elevated levels of perfluoroalkyl compounds have been observed in ADHD.
4.1.14. Triclosan¶
Prolonged exposure to the environmental pollutant triclosan induced ADHD symptoms in rats. Triclosan appears to cause a reduction in dopamine levels in the PFC.
A 60-day continuous exposure of rats to triclosan caused ADHD-like behavior in the offspring. It activated microglia in the PFC, which led to the release of inflammatory factors. In vitro, triclosan increased the levels of inflammatory cytokines, including IL-1β, IL-6 and TNF-α, in HMC3 cells. In addition, triclosan upregulated PKM2 via hnRNPA1, which affects the STAT3 signaling pathway and thus continuously activates microglia, promoting the release of inflammatory cytokines.
4.1.15. Synergy effects of neurotoxins¶
The synergistic effects of neurotoxins must be taken into account:
- Formaldehyde increases the toxicity of mercury.
- Amalgam increases the toxicity of PCBs and formaldehyde.
- Mercury and PCBs potentiate each other’s effects.
4.2. Interrupted breathing during sleep¶
Breathing interruptions in children’s sleep can trigger cognitive stress, causing symptoms that resemble ADHD.
It remains to be seen whether breathing interruptions during sleep can cause such stress that they can contribute to ADHD through epigenetic changes, or whether they merely cause symptoms that are similar to those of ADHD. In the latter case, people who did not previously have ADHD and who have developed ADHD (similar) symptoms as a result of breathing interruptions during sleep should see these symptoms disappear completely once the breathing interruptions during sleep have been eliminated. We are not yet aware of any studies on this.
4.3. Food intolerances, allergies¶
It is certain that ADHD is not caused by individual, specific foods, phosphates or additives.
However, individual food intolerances or allergies are just as much stressors as illnesses, toxins or psychological stress and can therefore worsen the stress situation of people with ADHD to such an extent that symptoms develop. This is not a finding specific to ADHD. For example, in a group of children with schizophrenia problems, dietary treatment of an existing gluten intolerance was able to eliminate the schizophrenia symptoms in the children affected by this. The same was found in people with ADHD and non-affective psychosis.
Food additives (here: Sun yellow, carmoisine, tartrazine, ponceau 4R; quinoline yellow, allura red, sodium benzoate) can cause histamine release from circulating basophils. This is not allergic, i.e. not dependent on immunoglobulin E. The increased release of histamine can - in carriers of certain gene variants of the genes that encode histamine-degrading enzymes - increase ADHD symptoms
To identify rare food intolerances (which, unlike allergies, cannot be detected by blood tests), an elimination diet can be helpful. However, such a diet is very difficult to implement and maintain and is barely adhered to, especially in younger children. In particular, any benefits must be weighed against the sometimes serious social consequences.
In other cases, such a diet can help to alleviate the symptoms of existing intolerances.
When assessing the effectiveness of diets (and other “desirable” therapies), parents’ assessments are often far higher than what tests or teacher evaluations can confirm.
More information at ⇒ Nutrition and diet for ADHD.
4.4. Gut-brain axis, gut bacteria, gut flora¶
See under Gut-brain axis
4.5. Polycystic ovary syndrome (PCOS)¶
Women with polycystic ovary syndrome (PCOS) appear to have an increased risk of mental disorders, primarily anxiety disorders and depression, but also ADHD.
4.6. (Untreated) type 1 diabetes¶
A study of persons with ADHD with and without insulin pump treatment found a 2.45-fold increased risk of ADHD in untreated people with type 1 diabetes, with ADHD considered a risk factor for inconsistent diabetes treatment.
4.7. Phenylketonuria (PKU)¶
Phenylketonuria (Følling disease, phenylpyruvic acid oligophrenia) is a genetically caused metabolic disorder in which the amino acid phenylalanine cannot be broken down into tyrosine due to the lack of the enzyme phenylalanine hydroxylase (PAH). Tyrosine, in turn, is required for the synthesis of dopamine, so dopamine deficiency is a consequence of PKU. PKU has a prevalence of 1 in 8000 people.
One study found an ADHD rate of 38% in phenylketonuria despite adequate treatment.
ADHD is also associated with dopamine deficiency.
4.8. Anabolic androgenic steroids (AAS)¶
Strength athletes who take anabolic androgenic steroids are significantly more likely to have ADHD than strength athletes who do not take them.
4.9. Infections¶
4.9.1. Susceptibility to infection and infection burden¶
A higher burden of infection may have a cumulative association with psychiatric disorders beyond what has been described for individual infections. Susceptibility to infections is reflected in the infection burden (the number of specific infection types or sites). An increased burden of infection correlates with an increased risk of
-
ADHD
-
ASS
- bipolar disorders
- Depression
- Schizophrenia
- psychiatric diagnoses overall.
A modest but significant heritability was found for the burden of infection (h2 = 0.0221) and a high degree of genetic correlation between it and the overall psychiatric diagnosis (rg = 0.4298). There was also evidence of genetic causality of the overall infection for the overall psychiatric diagnosis.
4.9.2. Viral infections¶
4.9.2.1. Enteroviruses in general¶
(Non-polio) enteroviruses cause a good half of all cases of aseptic meningitis and are therefore among the most important known causes. In addition to encephaltitis, (non-polio) enteroviruses also frequently cause febrile illnesses, hand-foot-and-mouth disease, herpangina, aseptic meningitis and encephalitis, as well as sometimes serious and threatening infections such as myocarditis or neonatal sepsis.
A previous study found an increased risk of ADHD from mild enterovirus infections (16%) and severe enterovirus infections (182%).((Chou IC, Lin CC, Kao CH (2015): Enterovirus Encephalitis Increases the Risk of Attention Deficit Hyperactivity Disorder: A Taiwanese Population-based Case-control Study. Medicine (Baltimore). 2015 Apr;94(16):e707. doi: 10.1097/MD.0000000000000707. PMID: 25906098; PMCID: PMC4602682.))
4.9.2.2. Enterovirus A71 (EV-A71) (+ 200 %)¶
A longitudinal study of 43 adolescents who had a central nervous system infection with enterovirus A71 (EV-A71) between the ages of 6 and 18 found that 34.9% had ADHD. This more than tripled the risk of ADHD. There was also an increase in autistic symptoms. Other psychiatric diagnoses were not elevated. Another study found ADHD to be particularly common when the A71 infection was accompanied by cardiopulmonary failure.
EV-A71 often shows weakness, atrophy of the limbs, seizures, hand-foot-and-mouth disease, encephalitis and reduced intelligence.
4.9.2.3. HIV¶
A study of children and adolescents with HIV in stable health found ADHD symptoms in 20%.
4.9.2.4. Zoster encephalitis¶
In one isolated case, ADHD was mentioned in association with zoster encephalitis.
4.9.2.5. Human endogenous retroviruses (HERV)¶
The topic Human endogenous retroviruses (HERV) and ADHD Is presented due to its high heritability in the chapter Development in the article Genetic and epigenetic causes of ADHD - Introduction
4.9.3. Bacterial infections¶
Periodontal disease is a bacterial inflammation of the gums caused by the bacterium P. gingivalis, which secretes toxins. Periodontal disease and is described as a risk factor for ADHD.
4.9.4. Parasitic infections¶
A study of 100 children with ADHD and 100 healthy children found a correlation of ADHD with:
- Toxoplasma
- Toxocara
- Cryptosporidium parvum
- Giardia lamblia
- Entamoeba histolytica
No difference was found with regard to Schistosoma (coccidia parasites).
4.10. Glucose-6-phosphate dehydrogenase deficiency (G6PD)¶
Glucose-6-phosphate dehydrogenase (G6PD) deficiency increased the risk of ADHD by 16%
G6PD deficiency is an X-linked genetic disorder and affects around 4.9% of all people.
The enzyme glucose-6-phosphate dehydrogenase (G6PD) facilitates the synthesis of nicotinamide adenine dinucleotide phosphate (NADPH) and glutathione (GSH), which are involved in oxidation-reduction equilibrium regulation. G6PD deficiency causes reduced GSH levels and thus increased oxidative stress.
G6PD deficiency is mostly food-related (favism; hemolytic reaction to the consumption of fava beans) and sometimes genetic (more common in the Mediterranean region and the Middle East, partly in Asia and Africa).
G6PD deficiency can trigger (especially in children):
- severe hemolysis
- Hyperbilirubinemia
- Jaundice
- Hearing disorders
- Behavioral disorders
- lead to long-lasting neurological damage
- increased production of reactive oxygen species (ROS)
- resulting in activation of astrocytes and microglia, increased proinflammatory chemokines and cytokines, neuroinflammation, impaired brain development
- Imbalance in the antioxidant system
- this leads to impairment of astrocytes, neuronal death and DNA damage
- oxidative cell death of leukocytes, myocytes and other immunological players.
4.11. Kawasaki syndrome¶
One study found an increased prevalence of ADHD in people with ADHD, other studies found only a tendency or no association.
4.12. Lipodystrophy (lack of fatty tissue)¶
One study found evidence of a greatly increased prevalence of ADHD in lipodystrophy.
4.13. Dystrophinopathy (muscular dystrophy, muscle weakness)¶
One study found evidence of a greatly increased ADHD prevalence of 18.4% for dystrophinopathy and 12.73% for ASD.
There are also links between ADHD gene candidates and genes associated with dystrophies. See there.
4.14. Hyperthyroidism / hypothyroidism¶
In addition to other cognitive deficits, hyperthyroidism can also cause inattention and hyperarousal. Depending on the degree of hypothyroidism, the cognitive effects can range from mild impairment of memory and attention to complete dementia.
The THRA gene encodes the thyroid receptor alpha, TRα1, TRHB the thyroid receptor isoforms TRβ1 and TRβ2.
The pituitary hormone TSH (thyroid-stimulating hormone) stimulates the thyroid gland to produce thyroxine (T4; prohormone) and then triiodothyronine (T3). The thyroid hormones (T3 and T4) in the blood in turn regulate the pituitary release of TSH within the hypothalamic-pituitary-thyroid axis, which is mediated by the receptor isoform TRβ2.
In the case of a (rarely occurring genetic) resistance to thyroid hormone β, this negative feedback loop, which stabilizes the TH level in the blood, is disrupted. This leads to increased TH and unsuppressed, i.e. normal TSH levels
4.15. Gender diversity¶
A multinational study found evidence that the frequency and severity of ADHD symptoms was particularly high in gender-diverse individuals.
4.16. Factors without relevant contribution¶
4.16.1. High blood pressure¶
One study found no statistical significance for a genetic link between high blood pressure and ADHD.
4.16.2. COVID-19 gene therapy¶
Gene predisposition, which makes people more susceptible to COVID-19, showed no signs of an increased risk of ADHD. Conversely, however, ADHD and Tourette’s are associated with an increased risk of COVID-19 and a more severe course of COVID-19.