ABC’s Four Corners recently aired a controversial documentary (The Autism Enigma) on how gut bacteria might affect autism. Here we review some of the science surrounding what it suggested caused autism - the involvement of gut (or enteric) bacteria.
The human microbiome includes viruses, fungi and bacteria, their genes and their environmental interactions, and is known to influence human physiology. To suggest, as The Autism Enigma does, that the microbiome is altered in autism and that a prescription of antibiotics to modulate gut bacteria can “cure” autistic characteristics is premature.
Our knowledge of the microbiome is in its early stages, and two recent landmark studies (here and here) show that it is so complex that scientists can’t yet match gut flora profiles between healthy subjects.
There’s very broad variation in these bacteria in different people and that severely limits our ability to create a “normal” microflora profile for comparison among healthy people and those with any kind of health issue, including autism.
How the gut works
Let’s step aside from the puzzles of the microbiome for a moment to consider the basics of how our digestive tract functions. It may surprise some to hear it referred to as a mini brain, but our gut contains 500 million neurons and can go about its business independently of the brain, so the title is apt.
These enteric (gut) neurons are arranged in two web-like layers (plexuses) sandwiched between the two tubes of muscle that form the digestive tract. The neurons control the gut, contracting the muscle to propel or mix its contents, as well as triggering secretion of water and enzymes to aid digestion and absorb nutrients.
Although it’s too early to suggest that the microbiome is where the cause of autism can be found, some of the issues covered in the documentary are solidly supported by scientific literature. Gut bacteria interact with enteric neurons and the mucous membrane of the digestive tract, for instance, and changes in these bacteria can alter anxiety-like behaviours and memory function.
Signals produced by the metabolic products of the gut flora can indeed travel to the brain via several different routes, including via vagal and spinal sensory nerves, and alter function in the central nervous system. And changes in brain gene expression have been observed in mice fed some bacterial strains.
The role of fatty acids
Gut bacteria produce many products (including fatty acids) that affect neuronal function. One of the researchers interviewed in The Autism Enigma showed that rats became less social when their brains were injected with one particular fatty acid (propionic acid).
The documentary suggested that some fatty acids may cause autism; in particular regressive autism, where children forget skills learnt during their early years. It suggested that this might happen when fatty acid levels are altered due to dietary changes when families migrate to a new country and adapt to new foods. The specific example it provided was of Somali families with a traditionally high-fibre diets that migrate to Canada and adapt to a Western high-sugar diet.
The Autism Enigma specifically focused on propionic acid, a short-chain fatty acid product of bacterial fermentation in the gut. This is one of the three short-chain fatty acids (the others are acetic and butyric acids) that are the main fermentation products of bacteria in the gut. The main fuel for this fermentation process is dietary fibre.
Short-chain fatty acids have several roles that promote gastrointestinal health. They are the predominant energy source for the mucous membrane of the colon and important for secretion and absorption. And they maintain an acidic environment in the colon, which reduces the growth of pathogenic organisms.
The researcher studying rat behaviour suggested that excessive sugars in the diet contribute to increased propionic acid and gut problems. But given that dietary fibre is processed by the metabolic pathway that produces propionic acid and that common dietary sugars are digested and absorbed nearly three metres away in the upper small intestine, singling out a high-sugar diet as the culprit may be a simplification.
Gut bacteria are largely confined to the third portion of the small intestine, the pouch connected to the junction of the small and large intestines (caecum) and the adjacent portion of the colon. Sugars, proteins and fats are digested well before food reaches the colon, and are largely in the form of fibre by the time they reach the colon, not carbohydrates as suggested in The Autism Enigma.
Increased levels of propionic acid production are likely to be due to increased dietary fibre, not sugars per se. So, increased propionic acid production would logically correspond with a high-fibre diet, rather than a high-sugar diet. This contrasts with the documentary, which suggests diets high in carbohydrates and sugars would feed the gut bacteria. In fact, the original diet of Somali families with high levels of autism includes many fermented foods, which would also be high in propionic acid.
There are many theories about the causes of autism, and a role for the microbiome is only one of them. Early intervention behavioural therapy is, for now, the only treatment scientifically shown to improve behavioural symptoms of autism.