by Dr. Max Bingham
Food Microbial Sciences Unit, Science and Technology Centre, Earley Gate, University of Reading, Whiteknights Road, Reading, Berkshire, UK. RG6 6BZ
Previous research into autism has raised the possibility that a link exists between the disorder and the human gut microflora. Research has remained limited due to the unwillingness of the orthodox medical establishment to adopt treatments suggested by this research. This short review aims to summarise the research completed to date and evaluate the relevance of a link between autism and a possible imbalance in the human gut microflora. It appears that yeasts (Candida species in particular) and clostridia may play an important role in the development of autistic symptoms. It is suggested that the control of the growth of these species may reduce the severity of autistic symptoms but is unlikely to offer a cure.
Previous research into autism has raised the possibility that a link exists between the disorder and the human gut microflora. However the relevance of this to sufferers of Autism has remained somewhat un-researched. As a consequence, it appears that the treatments suggested previously have remained unused and regarded by the orthodox medical establishment as irrelevant. This short review will consider the limited research completed to date.
The term autism is usually associated with the syndrome first described by Kanner (1943). In more recent years specific criteria have been set out to aid in the diagnosis of the disorder (Shaw et al, 1999). Autism typically develops early in childhood, however causality, explanation and treatment are often hotly debated. Symptoms can include (but not necessarily), hyperactivity, loss of eye contact, decreased vocalisation (i.e. loss of language), stereotypical behaviours, poor academic performance and other similar social deficits. Other similar disorders exist. These include Asperger Syndrome, Attention Deficit Hyperactivity Disorder (ADHD), Pervasive developmental disorder (PDD) and many others, where symptoms are similar to Autism but specific differences are demonstrated.
Yeast metabolites in the Urine of Autistic Children
Yeasts constitute only a very small proportion of the population of the gut (Holzapfel et al, 1998) under normal conditions – possibly kept from growing via competition from bacterial species and certain immune functions. However, Shaw et al (1999) has proposed that Autism (or at least many of the symptoms) may be a consequence of an overgrowth of candida species and a selective IgA deficiency. Following treatment with antifungal drugs and a gluten and casein free diet, a child rated as having severe autism improved such an amount as to be classed as a higher functioning individual with autism.*
It has been shown previously that children exhibiting autistic features have increased excretion of arabinose and the analogs of Krebs Cycle metabolites (including tartaric acid) (Shaw, Kassen & Chaves, 1995). Using Gas Chromatography/Mass Spectrometry (GC/MS) to test for urinary metabolites, it was found that the children had extremely high values of tartaric acid. The only source of tartaric acid is yeast (Shaw, 1999). Many reports have suggested that the onset of autism may be related to the occurrence in children of otitis media (Kontstantareas & Homatidis, 1987). It is common to treat otitis media with some sort of broad-spectrum antibiotic. Intestinal overgrowth of yeast and certain anaerobic bacteria are a well documented outcome of the administration of broad spectrum antibiotics (Kennedy & Volz, 1983; Danna et al, 1991; Ostfield et al, 1977; Kinsman et al, 1989; Van der Waaij, 1987; Samsonis et al, 1993, 1994a,b).
To evaluate this, it is known that Shaw (1999, 1996) has used GC/MS to test the urinary content of metabolites following the administration of Nystatin, an antifungal drug. Accordingly it was found that urinary tartaric acid declined to zero after about 60 days. When the Nystatin dose was cut to half, levels appeared to rise. Associated with this was seen an improvement in eye contact, a reduction in hyperactivity and an improvement in sleep patterns. It is unclear as to the method used, however these are interesting results. In other work, Shaw (1999) has evaluated the progress of the Herxheimer reaction of yeast die off. Values for the microbial metabolites described above increased dramatically during the first three days and began to normalise near day four. It is unclear as to the relevance of the point that many children take antibiotics for a whole series of illnesses, but do not develop autism.
Gupta et al (1996) have studied some possible modulating factors in Autism. These include immunodeficiencies and probable differences in biochemical detoxication factors which appear to be very common in autism,. It has been estimated that a high percentage of autisitic children have a significant immune dysfunction that may include myeloperoxidase deficiency, a genetic deficiency that impairs the action of white blood cells on yeast cells, IgA deficiency, complement C4b deficiency, IgG deficiency or IgG subclass deficiency (Gupta et al, 1996). In the above study, a complete remission of autisitic symptoms was achieved by infusions of gamma globulin in one case. Shaw (1999) has suggested that it seems increasingly likely that the immune system takes an inventory of bacteria and yeast cells present in the gut soon after birth. This inventory is performed by CD5+ B-cells. These cells may play a role in regulating the secretion of IgA, the antibody class that is secreted into the intestinal tract and which may select which microorganisms are tolerated in the gut. Continuing, it is suggested that the eradication of the normal gut flora when antibiotics are administered repeatedly during infancy may cause the CD5+ cells to reject normal cells and award immune tolerance to species that potentially could cause harm. Either antibiotic use in infancy or yeast infection of the mother during pregnancy may result in later tolerance to yeast. This would possibly extend any remission from symptoms induced by methods of treatment suggested for autism.
Yeast and Tartaric Acid Toxicity
Shaw, Kassen & Chaves (1995) noted in their initial study of the two brothers with autism, that in addition to their autistic characteristics, they exhibited severe muscle weakness. Both of these brothers excreted large amounts of tartaric acid in their urine. It is known that tartaric acid is an analog of malic acid. Malic acid is a key intermediate in the Krebs cycle which is responsible for the extraction of most of the energy from food substrates. Tartaric acid is presumably seen as toxic since it would inhibit this pathway and limit energy production. This may be the reason for the muscle weakness seen in the two autistic brothers. It has also been shown that Candida albicans produce gliotoxins (Shah and Larsen, 1991 and 1992) and immunotoxins (Podzorski et al, 1989; Witken, 1985), which may further impair the immune system. This would have relevance in terms of promoting yeast growth and increasing the chances of additional infections from bacteria leading to antibiotic usage again.
Arabinose, Yeast and Autism
Shaw, Kassen & Chaves (1995), also identified high levels of arabinose in the urine of the autistic children. The exact biochemical role of arabinose is unknown, but a closely related yeast alcohol, arabitol has been used as a biochemical indicator of invasive candidiasis (Kiehn et al, 1979; Wong et al, 1990, Roboz & Katz, 1992). It is thought that arabitol produced by the yeast in the gut is absorbed into the portal circulation and then converted to arabinose by the liver. Elevated protein-bound arabinose has been found in the serum glycoproteins of schizophrenics (Varma & Hoshino, 1980) and in children with conduct disorders (Varma et al, 1983).
Arabinose reacts with the epsilon amino group of lysine in a wide variety of proteins and may then form cross-links with arginine residues in an adjoining protein (Sell & Monnier, 1989). Arabinose has therefore been implicated in protein modification via cross-linking the proteins and altering both biological structures and functions of a wide variety of proteins. Shaw et al (1999) has proposed that this may include proteins involved in the interconnection of neurons. Decreased clinical symptoms of autism after antifungal treatment might be due to decreased arabinose and pentosidine formation (cross-linked proteins described above). This would result in fewer random neural connections and increased numbers of neural connections that are oriented to the child’s environment. The influence of a number of vitamins on candida activity and the operation of many metabolites of candida has been discussed (Mahler & Cordes, 1966). It is commonly found that children with autism experience improved symptoms following removal of gluten and casein from the diet (Shattock et al 1990, 1991, 1996, 1997; Reichelt et al, 1981, 1986; Knivsberg et al, 1990). It is unclear whether there is a link with arabinose and pentosidine formation, however given the wide distribution of these proteins in foods, there may be a relevant process involved with respect to autistic symptoms. Research into this may be useful.
Clostridia and Autism
Bolte (1998) outlined the possibility of a subacute, chronic tetanus infection of the gut as the underlying cause for symptoms of autism observed in some individuals. Here it is postulated that a percentage of individuals with autism have a history of extensive antibiotic use. As above it is known that oral antibiotics significantly disrupt protective gut microflora, creating a favourable environment for colonisation by opportunistic pathogens. Clostridium tetani is a ubiquitous anaerobic bacillus that is known to produce a potent neurotoxin. The normal site of binding for tetanus neurotoxin is the spinal cord, yet the vagus nerve is capable of transporting tetanus neurotoxin, providing a route of ascent from the intestinal tract to the central nervous system and bypassing the spinal cord. The result would be the typical symptoms of a tetanus infection would not be evident. Once in the brain the tetanus neurotoxin disrupts the release of neurotransmitters. This may explain the wide variety of behavioural deficits apparent in autism. Bolte (1998) presents evidence of lab animals exhibiting many of these behaviours after being injected in the brain with tetanus neurotoxin, and also of children with autism showing a significant reduction in sterotyped behaviour following treatment with antimicrobials effective against intestinal clostridia.
Shaw (1999) has developed a similar technique to yeast metabolite analysis by GC/MS for clostridia metabolites. While it has not been fully identified it has been shown that tyrosine derivative, a compound very similar but not identical to 3,4-dihydroyphenylpropionic acid is raised in the urine of children with various conduct disorders. It has also been pointed out that tyrosine, a substrate used by the body for the production of neurotransmitters, might imply that this unidentified product is important in altering key biochemical pathways for neurotransmitters in the brain. The relevance to autism remains unclear. Clearly a treatment to reduce the numbers of clostridia in the gut might help to reduce the effects of this inhabitation. However, clostridia are spore forming meaning recolonisation is possible even after treatment. The relevance of probiotic and prebiotic treatments has not been researched, yet this appears a reasonable approach to aid suppression of the clostridia species.
Human Gut Flora and Gluten and Casein Intolerance in Autistic Children
Yeasts, including Candida albicans are known to secrete a number of enzymes. These may include phospholipase, which will break down phopholipids and proteases such as secretory aspartate protease which break down proteins. These enzymes may partially digest the gut membranes and lining itself. Furthermore, it is known that the mycelium and chlamydospore are capable of tissue invasion (Nolting et al, 1994). It is likely that such factors could increase the permeability of the gut. This has important consequences in terms of food absorption and digestion. In terms of autism symptoms this may be highly relevant. It has been shown that incompletely broken down portions of gluten and casein may be crossing the gut into the blood and having an opioid effect in autistic children. Their symptoms being a consequence of this opioid action (Reichelt, 1981; Shattock et al, 1990). While many mechanisms have been suggested for this incomplete breakdown, it seems key that a yeast and/or clostridia overgrowth would affect this in some way.
Products of the human gut microflora in relation to autism and its symptoms appear to have been largely ignored in the past. However they appear to be relevant certainly in terms of yeast and clostridia species. Abnormal bacterial metabolites of tyrosine appear to be elevated in autism and this seems related largely to an overgrowth in clostridia species. Treatments to control this have resulted in significant clinical improvement or complete remission of symptoms in some cases. The issue of whether probiotic and prebiotic treatments are relevant, needs to be researched. Elevation of yeast metabolites such as tartaric acid and arabinose appears to be even more common in autism. The arabinose appears to be involved in abnormal protein binding which may adversely affect neuron connection and this may have relevance to the appearance of autistic symptoms. Given the intolerance autistic children appear to have to gluten and casein, an involvement of arabinose may be important. However, the exact biochemical role of arabinose remains unclear. Tartaric acid from the yeast overgrowth may have a direct toxic effect on the muscles and is a key inhibitor of the Krebs Cycle that supplies raw materials for gluconeogenesis. The implications of this appear to be detrimental to autistic function.
While it is clear that abnormal metabolites from the human gut flora may contribute to autistic symptoms, it is certain that many other systems and pathways are involved. It is possible that many are functioning simultaneously but at varying levels between individuals. This may give rise to the differences in symptoms exhibited by sufferers. This may also explain the similarity of autism and other chronic behavioural disorders such as Asperger Syndrome, PDD, ADHD, ADD and Rett syndrome. While other pathways may exist, research into the role of human gut flora and the full identification of species involved may allow for more directed treatments. While it will not cure the disorder, modifications of gut flora function might improve symptoms significantly.
* The Childhood Autism Rating Scale, an observational measure of various aspects of autism, for the child in question decreased from 43 (severely autistic) prior to introduction of these therapies to a value of 29 (non-autistic) after therapy.