Dr. Rosemary Waring in the UK found that children with autism have low levels of an enzyme called phenol sulfotransferase (PST). This enzyme, which is made in the gut, is used in the brain for "housekeeping" jobs such as preparing the synapses (spaces) between neurons each time the nerve needs to "fire" its neurotransmitters (chemical messengers) across that tiny space. This happens millions of times per second.
However, this same enzyme is needed in the GI tract to handle high-phenolic foodstuffs that were eaten - most especially items such as food additive chemicals, food dyes, and salicylates. What the enzyme does, specifically, is attach a sulfate ion to the phenol part of a chemical (e.g., a neurotransmitter or Red #40) and this prepares it for processing.
When there is not enough of the PST, there is a backup system called the "glucuronide" system in the GI tract, but there is no backup system for the brain. Thus, when you're out of PST, you're out of it in your brain, creating problems of mood, activity, and cognitive function.
Interestingly, not only does the salicylate chemical need PST for itself, but salicylate also suppresses your actual production of the PST enzyme. So if you start out with a low level of the enzyme for reasons unknown ... and you further suppress it by eating lots of apples, grapes, and oranges (all otherwise healthy foods), and then you use up what little PST you have left with the Red #40 and Yellow #5 in your dessert .... well, you're in trouble.
Although Dr. Waring's studies were on children with autism, it is expected that a similar situation is going on with ADHD. This may also explain why long-term avoidance of salicylates appears to allow some "recovery" of the ability to handle accidental off-diet items.
Links: - Studies on PST, Salicylates, and Sulfation
- More information about PST and behavior