If you’ve never heard or seen the term phenylketonuria (PKU) before, you are not alone. However, here’s a quick experiment. Go look at the back of a 2-Liter bottle of diet soda. Near the bottom of the back label, you will probably see a small warning label that says something along the line of “Phenylketonurics: contains phenylalanine” (individuals with phenylketonuria are often referred to as phenylketonurics).
The reason that this warning is on the back of only diet sodas and not regular ones is because the artificial sweetener Aspartame (Nutrasweet) contains the amino acid phenylalanine as one of its two primary components. When phenylketonurics, take in large amounts of this artificial sweetener, they get a large buildup of this amino acid in their bloodstream which they have trouble clearing. As a result, they often suffer a number of physiological problems, in, but not limited to, the nervous system.
The conversion process of phenylalanine to dopamine and how it relates to ADHD:
Phenylketonurics are those individuals who, for typically genetically predetermined reasons, are unable to break down and process the amino acid phenylalanine. This process actually has several implications that can relate to ADHD. We have spoken extensively about the neurochemical dopamine in various other posts. In general, chemical imbalances of this important neurotransmitter are frequently at the helm of ADHD and related disorders (typically shortages of dopamine are found in the "gaps" between neuronal cells, and most stimulant medications for ADHD work by resetting dopamine levels within these gaps). As we can see below, the body can actually manufacture this important brain chemical from various sources or starting materials, including phenylalanine (providing that the individual is capable of manufacturing all of the necessary enzymes in the conversion process. For PKU patients, this conversion process is hindered, and typically leads to shortages of dopamine). A rough sketch of the conversion process is listed below:
So what’s the point?
I have highlighted the chemical changes above, using different colors to represent the enzymes used and the chemical changes that these enzymes are responsible for (note the red and blue colors). As we can see above, the first step of the metabolism of phenylalanine to dopamine is done by adding a hydroxyl ("OH") group to phenylalanine, converting it to another amino acid, tyrosine. The chemical change is highlighted in red. (As an interesting side note, tyrosine is sometimes used as an ADHD supplement or auxiliary to medication treatment, even though the effectiveness of tyrosine for treating ADHD is questionable. Note that if one with PKU were to start with tyrosine, they would bypass the step of the chemical process of converting phenylalanine to tyrosine, which would help with the deficient enzyme phenylalanine hydroxylase. This enzyme will be addressed further down in the post).
Further modifications carry it to the product dopamine, which require two other enzymes (as a side note, the conversion of tyrosine to dopamine, in addition to the two enzymes listed above, also requires an adequate supply of iron. This is one reason why maintaining ample iron stores is necessary in combating ADHD and related disorders, and why an iron deficiency can elicit some of the negative behaviors characteristic of ADHD patients). As an aside, we have previously investigated how iron deficiency can affect both ADHD and sleep disorders and how iron supplementation can potentially offset the toxic effects of lead in ADHD patients.
You'll notice that the first step of the conversion process is blocked for individuals with PKU. This is due to a mutation in the gene that codes for this enzyme, the phenylalanine hydroxylase gene. For reference sake, the phenylalanine hydroxylase gene is located on 12th human chromosome. Remember that it is the mutated form(s) of the gene that can lead to PKU, the vast majority of the human population carries the regular form.
Fortunately, phenylketonuria is a rare genetic disorder, affecting less than one percent of the population. This is due, in part, to the fact that it must be present in both parents to be passed on to a child. It is almost always detected in most newborn screenings. However, it is possible to be missed, especially if a milder form is present. While there are several key differences, some of its symptoms mimic problems that correlate with attention deficit disorders. These include:
- Hyperactivity
- Erratic Arm and Leg Movements (can be similar to tics or Tourette's-like behavior, which often accompanies ADHD individuals as a comorbid disorder)
- Social immaturity and impairment of mental skills
- Learning disabilities
As we can see, these four traits are classic behaviors seen in many children diagnosed with ADHD. The first two are more characteristic of the hyperactive/impulsive or combined subtypes of ADHD, the fourth is more tied to the inattentive form of the disorder, and the third can fall into any of the categories. Interestingly, both ADHD and PKU disorders share a common brain region of deficit, the prefrontal cortex.
Key Differences Between ADHD and PKU:
- IQ: Most individuals with ADHD typically fall within the normal range on most IQ tests (however, cases of abnormally high or low IQ scores certainly exist). For individuals with PKU, however, a depressed IQ is almost always seen (PKU is a relatively common cause of mental retardation). For example, in a British study, it was found that IQ scores for children with PKU hovered around 90. The IQ scores were closely correlated to the ability of the individual treatments to keep the phenylalanine levels below a specific benchmark in the blood. In addition, differences in subscores, indicate a possible deficit in spatial processing. Interestingly, visual-spatial deficits are often present in ADHD and its comorbid disorders as well.
- Head size: Abnormally small head sizes are often seen in PKU. While smaller relative volumes in specific brain regions are often seen in ADHD, the overall head size differences are typically not as pronounced as for PKU. Interestingly, the effects of smaller head sizes and brain regions for both PKU and ADHD, respectively found the differences to be much more pronounced in boys than in girls. Note that we have previously discussed several other gender differences in ADHD.
- Onset of symptoms: In general, PKU symptoms are fully manifested in the first two years of life. These include both mental and physical impairments. While symptoms of ADHD can often begin to appear early on, they often do not fully appear until much later in life. As a result, difficulties in pinning down age-specific aspects of ADHD persist.
As we can see, there are a number of features and methods in place such that the possibility of misdiagnosing ADHD as PKU and PKU as ADHD by a skilled professional is relatively small. However, in addition to PKU, there are genetic deficiencies which result in compromised activity of the phenylaline hydroxylase enzyme by around 5 to 10%. While these deficiencies are milder than in full-fledged phenylketonuria, it does bring up a critical point that intermediate states do exist between being diagnosed with PKU and not having PKU. It is possible that individuals in this potentially vulnerable intermediate state of enzyme deficiency may be more susceptible to disorders such as ADHD. Of course, this is just a personal hypothesis.
Nevertheless, the main goal of this post was to highlight some of the key genetic, physiological and behavioral overlaps of the two disorders. It is my personal belief that looking for common underlying trends between even the most disparate disorders can offer a wealth of information into some of the underlying causes of the individual disorders that we would otherwise miss. In other words, I think we often sell our selves short by not digging "deep" enough in our investigations of the fundamental causes of diseases and disorders such as ADHD and phenylketonuria.