There is no agreement on the part of doctors about the causes of ADHD. Because there is no objective test to confirm the diagnosis, practitioners rely on a checklist of behaviors, which can be very subjective. The 1998 NIH Consensus Development Conference on ADHD ended with the experts failing to reach a consensus, with one doctor telling the press, “Diagnosis is a mess!”
There is a common belief in scientific circles that ADHD is a genetic disease involving an imbalance in brain neurotransmitters. Genome wide surveys have shown linkage between ADHD and loci on chromosomes 7, 11, 12, 15, 16, and 17. With so many suspected genes, however, ADHD could not follow the traditional model of a "genetic disease" and is better viewed as a complex interaction among genetic and environmental factors. To date no single gene with a major contribution to ADHD has been identified.
Measurements have shown differences in brain activity and even brain size in people with ADHD. However, these differences are subtle and not sufficient for any sort of diagnostic test. Moreover, these tests were often done on children taking stimulant medication; therefore, it is hard to know what effects are caused by ADHD and which are caused by the stimulants themselves.
Measurements have also shown that people with ADHD have differences in levels of various neurotransmitters, including dopamine and serotonin. Again, this difference is not sufficient as a diagnostic test. Moreover, although levels of the various neurotransmitters - and whether they are balanced or not - are controlled in part by your genetic makeup, it is not so simple. Neurotransmitters can also be affected by what you eat (or don't eat). For example, Vitamin B6 is a cofactor required for making dopamine so a Vitamin B6 deficiency will affect dopamine levels. On the other hand, even if you have a biological or genetic problem making enough dopamine, it can sometimes be overcome by providing extra cofactor (the Vitamin B6), thus "pushing the pathway" that is weak.
To give another example: As long ago as 1974, Stokes showed that the preservatives BHA and BHT induce changes in several neurotransmitters in the brain, including serotonin, norepinephrine, and cholinesterase. In a review of the literature up to 1986, Zeisel said, "Diet clearly influences neurotransmission."
In other tests using glucose and brain scans, areas of the brain required for certain tasks may not be "lit up" (using glucose) in people with an ADHD diagnosis. However, it cannot be determined whether this lower use of glucose in those areas causes the attention deficit, or whether it is the lack of attention to the task that is causing the lower use of glucose.
For some children, the symptoms can change dramatically, even from hour to hour, suggesting that the main problem is not within the child, but may have external causes. It is likely that the ADHD child has inherited a greater sensitivity to various external things, and these items can trigger the symptoms. This would explain many puzzling aspects of ADHD, including the following:
- The incidence of ADHD symptoms has increased dramatically at the same time our food supply has changed.
- ADHD is more commonly diagnosed in developed countries where there is a greater reliance on foods with
synthetic chemical additives.
- ADHD symptoms in most children are greatly reduced when they change their diet to remove certain food additives.
- Even children with no prior history of ADHD have been found in double-blind studies to exhibit symptoms of hyperactivity and difficulty in paying attention when they consume food with additives like food dye.