June 26, 2021
A team of U.S. endocrinologists recently published the results of a meta-analysis examining a possible association between bisphenol A(BPA) and childhood ADHD. BPA is used in a variety of consumer products, including plastic bottles for food and drink, epoxy resins used to line cans of food, dental sealants, and the thermal receipts issued by stores.
A review of the literature found 29 rodent studies, but only three with humans. The human studies were too different from each other to be suitable for meta-analysis. One found no association between prenatal exposure and ADHD. A second found prenatal BPA exposure to be associated with teacher-reported hyperactivity in 4-year-old boys, but not girls. The third found is to be associated with hyperactivity scores in 3-year-old girls.
As the authors note, “Often, there is little human data available, particularly in the environmental toxicology/health fields, due to the time and expense of conducting epidemiological studies and the ethical barriers for human-controlled trials that involve human exposure to potentially hazardous chemicals. Thus, it is important to have methods for using animal data to inform human health hazard conclusions; indeed, animal models are traditionally used to study human health.â€
Twelve of the mice and rat studies, with a total of 709 rodents, were suitable for meta-analysis.
Overall these pointed to a tiny SMD effect size of 0.09, but it was not significant, with the odds of such a result being obtained by chance being almost one in four (p = 0.237). But when results from the 356 males and353 females were looked at separately, a significant sex difference emerged. There was essentially no effect on female rodents, with an effect size of -0.07and a 95% confidence interval of -0.27 to 0.14, widely spanning the zero mark, rendering the result statistically non-significant. Among male rodents, however, there was a small but statistically significant effect size (0.24), with a 95%confidence interval from 0.04 to 0.45. The odds of obtaining this outcome by chance were only one in 50 (p = .02).
This result must be viewed with caution, as rodent physiology often differs substantially from that of humans. The authors, therefore, conclude, “early BPA exposure is associated with a presumed hazard of hyperactivity in humans. Our conclusion is based on ‘moderate’ levels of evidence for the human and ‘high’ levels of evidence for animal literature.â€
Parkinson’s disease is a chronic, progressive neurological disease, characterized by the drastic reduction of dopamine transporters and the dopaminergic neurons upon which they are expressed. The resulting symptoms include bradykinesia (slowness of initiation of voluntary movements), tremors, rigidity, and postural instability.
Taiwan’s National Health Service covers about 99 percent of its 24 million inhabitants, and maintains complete records in its National Health Insurance Research Database. The Longitudinal Health Insurance Database2000 (LHID 2000) is a nationally representative subset of the latter.
Using the LHID 2000, a Taiwanese research team identified10,726 patients with Parkinson’s disease. It paired them with an identical number of randomly selected non-Parkinson’s controls, matched by age, gender, and index date (first date of diagnosis of Parkinson’s disease).
The team then looked retroactively through the database to determine which of the 21,452 individuals had previously been diagnosed with ADHD. Fourteen of the 10,726 Parkinson’s patients had been diagnosed with ADHD, versus five of the 10,726 in the control group.
Parkinson’s patients were thus 2.8 times as likely to have had a previous diagnosis of ADHD as the controls. When adjusted for age, gender, and Carlson Comorbidity Index scores, they were 3.6 times as likely to have had a previous ADHD diagnosis.
The authors cautioned that this association between prior ADHD diagnosis and subsequent Parkinson’s diagnosis is not causal.
Only one in 766 of Parkinson’s patients (a seventh of one percent) had previously been diagnosed with ADHD. So even if there were any causal relationship, it would be extremely weak.
Sleep disorders are one of the most commonly self-reported comorbidities of adults with ADHD, affecting 50 to 70 percent of them. A team of British researchers set out to see whether this association could be further confirmed with objective sleep measures, using cognitive function tests and electroencephalography (EEG).
Measured as theta/beta ratio, EEG slowing is a widely used indicator in ADHD research. While it occurs normally in non-ADHD adults at the conclusion of a day, during the day it signals excessive sleepiness, whether from obstructive sleep apnea or neurodegenerative and neurodevelopmental disorders. Coffee reverses EEG slowing, as do ADHD stimulant medications.
Study participants were either on stable treatment with ADHD medication (stimulant or non-stimulant medication) or on no medication. Participants had to refrain from taking any stimulant medications for at least 48 hours prior to taking the tests. Persons with IQ below 80 or with recurrent depression or undergoing a depressive episode were excluded.
The team administered a cognitive function test, The Sustained Attention to Response Task (SART). Observers rated on-task sleepiness using videos from the cognitive testing sessions. They wired participants for EEG monitoring.
Observer-rated sleepiness was found to be moderately higher in the ADHD group than in controls. Although sleep quality was slightly lower in the sleepy group than in the ADHD group, and symptom severity slightly greater in the ADHD group than the sleepy group, neither difference was statistically significant, indicating extensive overlap.
Omission errors in the SART were strongly correlated with sleepiness level, and the strength of this correlation was independent of ADHD symptom severity. EEG slowing in all regions of the brain was more than 50 percent higher in the ADHD group than in the control group and was highest in the frontal cortex.
Treating the sleepy group as a third group, EEG slowing was highest for the ADHD group, followed closely by the sleepy group, and more distantly by the neurotypical group. The gaps between the ADHD and sleepy groups on the one hand, and the neurotypical group on the other, were both large and statistically significant, whereas the gap between the ADHD and sleepy groups was not. EEG slowing was both a significant predictor of ADHD and of ADHD symptom severity.
The authors concluded, “These findings indicate that the cognitive performance deficits routinely attributed to ADHD … are largely due to on-task sleepiness and not exclusively due to ADHD symptom severity. … we would like to propose a simple working hypothesis that daytime sleepiness plays a major role in cognitive functioning of adults with ADHD. … As adults with ADHD are more severely sleep deprived compared to neurotypical control subjects and are more vulnerable to sleep deprivation, in various neurocognitive tasks they should manifest larger sleepiness-related reductions in cognitive performance. … One clear testable prediction of the working hypothesis would be that carefully controlling for sleepiness, time of day, and/or individual circadian rhythms would result in a substantial reduction in the neurocognitive deficits in replications of classic ADHD studies.”
A growing body of studies suggests a link between inflammation and autoimmune diseases on the one hand and ADHD on the other. It has been hypothesized that excessive release of cytokines (small signaling proteins that regulate immune response and inflammation and repair) and keratinocytes (skin cells) under allergic conditions may cause structural and functional changes to the nervous system and brain, which can contribute to psychiatric disorders, including ADHD.
Noting that previous studies have focused primarily on associations between ADHD and respiratory allergies(asthma) and skin allergies, a joint Chinese and American study team set out to see what, if any, association there might be with food allergies.
To this end, they turned to the national health Interview Survey (NHIS), conducted annually by the National Center for Health Statistics at the Centers for Disease Control and Prevention. This survey relies on a very large, nationally representative sample of the U.S. population.
The study encompassed 192,573 youths aged 4 through 17 years old. Of these, 15,376 had an ADHD diagnosis, 8,603 had food allergies, 24,218 had respiratory allergies, and 18,703 had skin allergies.
After adjusting for age, sex, race/ethnicity, education level, family income to poverty ratio, and geographic region, youths with food allergies were found to be over 70% more likely to be diagnosed with ADHD than those without food allergies. After further mutual adjustment for other allergic conditions, they were still well over 40% more likely to be diagnosed with ADHD than their non-allergic peers.
How did that compare with respiratory and skin allergies? In the same study population, making identical adjustments for potential confounders, youths with respiratory allergies were 50% more likely to be diagnosed with ADHD than those without such allergies. Those with skin allergies were 65% more likely to be diagnosed with ADHD. After further mutual adjustment for other allergic conditions, those with respiratory allergies were still over a third more likely to be diagnosed with ADHD, and those with skin allergies were 50% more likely to be diagnosed with ADHD.
The authors concluded, “The current study found a significant and positive association between common allergic conditions, including food allergy, respiratory allergy, skin allergy, and ADHD in children. Although the detailed mechanisms linking food allergy and other allergic conditions to ADHD remain to be understood, physicians should be aware of the increased risk of ADHD as a comorbidity of children with allergic conditions.â€
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