Friday 10 February 2012

Autism, aromatic amino acids and gut bacteria: a hypothesis

Mass-ive @ Paul Whiteley
The publication of an interesting article by T. Andrew Clayton* in FEBS Letters (full-text) with a speculative hypothesis regarding the amino acid phenylalanine, gut bacteria and autism has my full attention. A quick search for the author suggests quite an accomplished research career with a scientific publication track-record to boot particularly in the area of metabolomics and with some very distinguished company. That and mention of one Prof. Glenn Gibson in the acknowledgements leads me to believe that this chap knows what he is talking about.

This paper was always going to attract my attention because it made reference to the compound trans-indolylacryloylglycine or IAG and autism. What on earth is IAG I hear you ask? Well it's a compound, a peculiar metabolite of the amino acid tryptophan, that has surrounded the research I have been involved in for many years both from a detection and possible biological marker point of view. As it happens, the initial speculation on IAG potentially being a more general biomarker for autism has subsided as per this article and this article but that's not to say that it may not be relevant to some groups on the autism spectrum, particularly those who also present with comorbid gastrointestinal issues. Indeed there still remains the possibility that there may be some 'relationship' between IAG, gluten and the so-called leaky gut (more on this later).

The crux of the current paper is that another amino acid, phenylalanine, which shares more than a passing relationship with other aromatic amino acids particularly tryptophan and tyrosine in terms of the 'give-and-take' ability of the respective hydroxylase enzymes and the shared use of cofactors such as BH4, is influenced by certain types of gastrointestinal bacteria. This in turn might also explain some of the origins of IAG, derived from tryptophan.

I will hopefully not get too technical here but there are some important points raised in the paper which are perfectly testable in an autism research and other context.


  • The proposed model is based on two different rat urinary phenotypes distinguished by a favourite metabolomics method, NMR spectroscopy - one of the gold standards. One rat phenotype - the 'HIP' phenotype - produces rather a lot of urinary hippurate, whilst the other phenotype - the chlorogenic acid phenotype - is characterised by low levels of urinary hippurate and elevated levels of 3-(3-hydroxyphenyl)propionic acid (3-HPPA). The differences between the biochemistry of the two phenotypes is gut bacterial composition not readily attributed to factors like different diets.
  • Proposals for the bacterial agents and biochemical pathways pertinent to these rat phenotypes is given with specific focus on the origin of the benzoic acid that consequently forms the hippurate levels seen in the HIP phenotype. Words like cinnamic acid and PAL are also included.
  • Transposing the various biological reactions noted in phenylalanine and tyrosine on to tryptophan, Clayton moves through the production of IAG from tryptophan via a most interesting intermediary compound called indole-3-acrylic acid (IAA). IAA is thought to be quite a reactive molecule which subsequently conjugates with glycine to form IAG, also able to potentially do various things because of its flat planar chemical geometry. There is for example, some prior speculation that IAA may have some 'membrane-busting' potential in relation to autism as a result of its dose-dependent effect on prostaglandin (E) production, but this observation still remains preliminary. 

Acknowledging that this is a hypothesis paper and so still subject to some degree of testing, I am very curious about some of the concepts detailed. Elevated levels of 3-(3-hydroxyphenyl)-3-hydroxypropionic acid (HPHPA) have been reported in cases of autism, a compound which fits rather nicely into the whole phenylalanine-bacteria connection with particular focus on an old bacterial friend, Clostridia. This accompanied by various other related findings in cases of autism such as elevated levels of urinary 4-hydroxyhippuric acid detailed in this paper adds to the curiosity.

Speculation on some role for IAG and precursors in relation to autism and particularly some reported [non-significant] differences to levels according to use of a gluten-free diet, has again been something of interest to me for a while. Allowing for interfering variables based on urine sample concentration noted in some cases of autism, the question of why a gluten-free diet in particular, might affect urinary IAG levels has always been a puzzle. Does a gluten-free diet affect gut bacterial composition? Likely, according to this paper. Does the gluten-free diet affect gut permeability? Yes it probably does to some extent and the de Magistris results seem to back that up in cases of autism. Could gut bacteria contribute to gut permeability either directly or peripherally through the formation of IAG or related mechanisms? Mmm... I dunno.

There is a lot to take in with this recent paper and even after a few reads I still find myself not fully understanding all of the chemistry. Nevertheless an important connection has been posited between our gut bacterial masters and their potential effect on important aspects of our biochemistry. I would like to think that I will post further about some of the implications of this hypothesis as more research starts to move into this area.

To end, an oldie but goodie which was the soundtrack to a favourite film of mine. Cue Richard Dreyfuss.. "I never had friends later on like the ones I had when I was 12...".

* Clayton TA. Metabolic differences underlying two distinct rat urinary phenotypes, a putative role for gut microbial metabolism of phenylalanine and a possible connection to autism. FEBS Letters. February 2012.
DOI: 10.1016/j.febslet.2012.01.04


  1. This doesn't begin to knock any brains cells lose in my simple mind. I can't begin to fathom any of it.

    I like pictures, though. I want you to see this one, of a woman who was treated for PKU as a newborn, on her wedding day, with her sister who was not diagnosed until after she was a year old.

    A picture tells a thousand words.

    My son's second speech therapist had 2 sisters who died in institutions before they were 20 years old of untreated PKU. To think that some forms of mental disease can't be diet related is ignorant. You and I and the children and adults who suffer from a magnitude of genetic/metabolic diseases that are caught at newborn screenings know that.

    I hope you never give up. Brett Miller, father of an autistic child, had a cool tweet today. It went: "The adjacent possible, liquid networks, the slow hunch, serendipity, errors, exaptation, platforms. Where Good Ideas Come From ". I especially like serendipity...hope you meet up with it!

  2. Dr Folling, who as you know, first described PKU is in my mind one of the greatest pioneers in medicine. He opened the gates to a variety of in-born errors of metabolism to be diagnosed some of which are still being discovered.

    I agree that the paper from Dr Clayton is a complex one but between a few reads and some initial contact with him, I kinda get what he is suggesting: these little building blocks of proteins interact with various bacteria in our gut to produce some 'interesting' compounds, some with quite potent effects. The difference seems to lie in what bacterial composition makes you.

    Added to that fact that these amino acids share things like enzymes and cofactors, the ripples from issues in one system might very well extend further than any of us realised.

  3. Dr. Folling was a chemist, wasn't he? It's beyond amazing, what he did. I studied PKU 30 years ago in college as a special education major. I have had an interest for all that time.

    The link leads to The Montreal Children's Hospital Hyperphenylalaninemia (PKU) Resource Booklet for Families, to encourage the difficult regimen that families must undergo to help treat their child's disease. Years ago I had heard that a PKU child had went to college to be a teacher, and I thought it was outstanding. But to marry and have children...the same disease that would have killed the same child at an early age, just astounds me.

  4. This may be off the subject. Can I pick your brain? What is the difference between indole, indican, and indolyl-3-acryloylglycine (in simple terms)? I am reading some of Christian Herter's work, over 100 years old, and he talks alot about indole (very very interesting). My son's organix profile was very high in Indican and Sulphate and high in Hippurate. Is his indican similar to what Herter is discussing and what your paper is discussing with the indolyl-3?

  5. Thanks for the comment Mrs. Ed.

    Difference between indole, indican and IAG: tryptophan degrades to indole via bacteria. Indican is an indole metabolite. Indican represents a potential marker for how good protein digestion is working and also for the possible presence of certain bacterial species (among other things). IAG figures in this same process possessing an indole ring in its chemistry.

    Without wishing to give any clinical advice (not allowed to so please don't take it as such) - if the profile was based on a urine sample, high sulphate levels in the urine would not be an uncommon finding in autism (see Rosemary Waring's work on sulphate); high hippurate - normally you would need to see how this figures against benzoate (which converts to hippurate via conjugation with glycine). The ratio between benzoate:hippurate gives an indication of what might be going on and why.

    Quite a good review article is here:

    The whole amino acid-bacteria connection is an interesting if complicated area. Yes, recently not so much work has been done looking at the various pathways and what they might mean but thinking back to conditions such as Hartnup disease:

    there are some potentially interesting lessons to be learned.

  6. Thank you. I am actually trying to figure this out not for clinical advice, but for writing project purposes. I am attempting to delve into the history of all of this and certain things need clarification. It's interesting that you mention Hartnup. I found something that suggested PKU and Hartnup had high indole prescence that cleared up when conditions were controlled with diet, etc. The research history of gut microbes, their products and diet is fascinating. It appears as though science is now starting to pick up where it left of about 100 years ago (thinking of Herters autointoxication). I have found there may have been some slight interest in the 1970's, but really between Herter and present day there isn't much, or at least I haven't stumbled upon it yet.

  7. Thanks again.

    I guess it's all about research which waxes and wanes (as per sulphation and autism, which is at the moment, in a bit of a trough compared to the peak 10-15 years ago).

  8. You ARE heading in the right direction. I'm hoping that you won't discount feedback from parents as non scientific. My ADHD/Aspergers labeled 7 year old daughter has just had the best two weeks of her entire life...and the only change has been the elimination of all lactose in her diet. She's never taken meds although symptomatic from birth...because I wanted to address the core neurological problems if possible. Lactose elimination is the ONLY change in her environment. She just slept through the night last night for the first time in her life!. She's gone from highly op positional, very anxious, very impulsive, unable to focus, doing very poorly in school(though quite intelligent), not looking people in the eye or talking to most people, etc, being a lovely cooperative,almost normal's just been two weeks but her teacher has emailed saying she's now focusing in school and finishing her work early (the teacher has not been notified of the change in diet).I'm very curious to see what the next few weeks/months bring. She is a completely different child.

  9. @Cedros Homes. Many thanks for the comment - nobodys opinion is discounted on such posts; indeed parents very often provide the best source of feedback when it comes to their child.

    Lactose intolerance is something that has cropped up in autism research circles before, most recently with reference to this paper by Kushak and colleagues:

    It's an area that quite a few people would like to see more thoroughly investigated (and tested for).


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