Robert Silverman, DC has written a compelling article describing how 
It’s an interesting concept considering how many people have delayed recovery of this common, but not well-understood condition. So how does this occur?
First, we need to know that traumatic brain injury (TBI) is something that does not just occur with football players. It can happen in a car accident, slipping and falling, or impact with any contact sport. Immediately following a head injury or several weeks after the event, a patient may experience a variety of symptoms. These can include headaches, vomiting, lethargy, and irritability. TBI can also have long-term effects on the musculoskeletal system, gastrointestinal tract and the immune system. These problems sometimes takes months to emerge from the initial accident.
After a head injury, the brain becomes inflammed. We are not sure why, but this leads to inflammation in the gut. It also happens very quickly…within hours. This gut inflammation increases intestinal permeability which allows large, immunogenic (substances that are able to produce an immune response) dietary proteins and bacterial toxins into the bloodstream. Inflammation in the bloodstream follows which keeps the blood-brain barrier (BBB) open and fuels the inflammation in the brain. The intestinal mucosa is the most nutrient-dependent, toxin vulnerable organ in the body. So when the intestinal villi are destroyed due to inflammation after a TBI, then it compromises the patients ability to absorb important nutrients for healing and repair. Also, the compromised BBB allows entry of more circulating substances that provoke the immune system, fueling the neurological inflammation. It’s a double whammy!
Knowing what structures are breaking down in the gut is important. Blood tests for certain bacterial toxins such as Lipopolysaccharides (LPS) and distending toxin-B (CdtB) are recommended to determine the state of a patients gut health. The sooner you test the better, because delays identifying this cycle results in greater neurological tissue damage and reduced quality of life.
Reference: Silverman, R. “Gut Dysfunction Frequently Follows Traumatic Brain Injury”. Holistic Primary Care, Spring 2018, p. 8