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The Connection Between Autism Spectrum Disorder and The Gut Microbiota



Gastrointestinal (GI) dysfunction and symptoms are commonly reported among children with Autism Spectrum Disorders (ASD). Forty seven percent of children with ASD have GI issues and are nearly four times more likely to have frequent, recurring GI issues when compared with those without. (1, 2) Symptoms often include chronic constipation, diarrhea, abdominal pain, gastroesophageal reflux (GERD) and/or vomiting. (1)


These GI symptoms are frequently associated with a disruption in the GI microbiome, or dysbiosis. Simply put, dysbiosis is when there are more “harmful” bacteria than “healthy” bacteria living in your intestines. There are well established links between disruptions in the microbiome and a number of mental disorders including depression, anxiety, mood, cognition, and neurodevelopmental disorders, such as ASD. (3)


Dysbiosis can cause inflammation in the intestines and can damage the cells lining the GI tract. If these cells are damaged, they are unable to protect the rest of the body, including the brain, from potentially harmful molecules or microbes.

The Gut-Brain Axis

Research has proven that the gut microbiome is connected to the central nervous system in a bidirectional communication pathway called the gut-brain axis. This acts like a telephone line between the GI tract and brain. This line is heavily influenced by the health of the cells that line the GI tract and gut neurotransmitter levels. Bacteria have been shown to consume and produce a wide range of neurotransmitters, such as dopamine, norepinephrine, serotonin, or gamma-aminobutyric acid (GABA). (4)


The health of gut cells highly depends on the types and the amounts of bacteria living in the gut. Imbalance between “healthy bacteria” and “harmful bacteria” can trigger an immune response and inflammation. Inflammation is known to damage the cells lining the GI tract. Damage to GI cells can interfere with neurotransmission and the gut-brain axis which can eventually impact brain function and behavior. (2,3)

Dysbiosis

More about dysbiosis. Although the bacteria in the gut, or microbiota, are involved in the absorption, metabolism and storage of nutrients, the primary role of microbiota is to digest nutrients from foods that are indigestible (5) Different types of bacteria are responsible for digesting specific nutrients.


Due to these specific bacteria-nutrient relationships, the number of and types of gut microbiota differ based on diet patterns. For example, diets rich in prebiotics that are high in fiber, and low in simple carbohydrates (sugars) promote the growth of healthy bacteria. Diets that are high in simple sugars and low in fiber can lead to bacterial overgrowth.


Leaky-Gut (Intestinal Permeability)

As gut bacteria digest nutrients, they release break-down products called metabolites. Harmful bacteria release metabolites such as enzymes and endotoxins that damage the cells lining the intestinal wall. When excess bacteria, their metabolites or damaged GI cells are detected by the immune system, an immune response is activated. This causes more immune cells to be sent to the intestines to help fight the bacteria and repair the damaged GI cells.

This immune response eventually causes inflammation which can actually cause more damage to the cells that line the intestinal wall. Normally, these cells are tightly packed like soldiers standing shoulder-to-shoulder protecting our blood and body from any harmful bacteria, toxins or other molecules in the gut.


As the gut becomes inflamed, little channels between these cells start to open up. Bacteria, toxins, and other molecules from the gut can “leak” through these channels into circulation. A term that is often associated with leaky-gut is “intestinal permeability”. Increased intestinal permeability simply means there are more channels for molecules to leak through from the gut into circulation.


The bacteria, toxins, and other molecules that leak out of the GI tract travel through the lymphatic system and the blood to other organs. When these molecules are detected throughout the body, immune responses are constantly triggered which can lead to systemic inflammation. Not only does this disrupt a number of bodily functions, but it is especially dangerous for the brain. If these harmful molecules reach the brain, another localized inflammatory response is triggered. Brain cells are very sensitive to inflammation and are easily damaged. This damage causes cell dysfunction and thereby disturbances in behavior and cognition.


The Connection To Autism

Despite the prevalence of ASD, the cause and reliable treatments are still not well understood. Early behavioral and educational interventions have shown to be effective for treating some autistic symptoms, but there are still very limited medical therapies