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IBS, SIBO, & Eating Disorders

Over the years, it has been suspected that eating disorder (ED) patients may have an additional diagnosis of small intestinal bacterial overgrowth (SIBO). To date, there has been a substantial amount of research on the gut microbiome and the digestive system of individuals with eating disorders, however, specific research on the prevalence of SIBO in this community is still lacking. The primary reason for this is due to it being particularly difficult to test eating disorder patients for SIBO. The SIBO breath test is fairly simple to perform; the patient drinks a solution of lactulose, and then a breath sample is collected and examined for the presence of hydrogen gas. The issue with testing ED patients, however, is that it requires the patient to fast prior to collecting a sample. Asking individuals with disordered eating to intentionally restrict their food intake is contradictory and potentially harmful to their recovery. That being said, there is a body of research focusing on the gastrointestinal function and the gut microbiome of individuals with eating disorders, outlining many of the similarities that the two diagnoses share. The research in this area primarily focuses on the eating disorder subtype; anorexia nervosa.

Prevalence of GI Symptoms in ED Patients

Patients with anorexia nervosa (AN) and bulimia nervosa (BN) frequently report symptoms of bloating, constipation and abdominal pain. Gastrointestinal (GI) symptoms, such as these, may be indicative of compromised GI function. Many individuals suffering from an eating disorder use disordered eating behaviors and purging mechanisms that affect the functionality of the GI tract. Some of the most common include vomiting, laxative abuse, and restrictive eating. Furthermore, delayed gastric emptying and slowed motility through the GI tract may aggravate feelings of fullness and abdominal distension, which can trigger more disordered coping mechanisms.

Compromised GI Function in ED Patients

Malnutrition caused by eating disorders dramatically impairs gastric motility, gastric emptying and intestinal transit, and it gives rise to GI disorders, such as IBS and Crohn’s disease. Other common GI disorders found in AN patients are defecatory disorders, characterized by an inability to successfully complete bowel movements. These findings have prompted numerous studies on intestinal transit times in eating disorder patients. One 12-month study compared whole-gut (colonic) and mouth-to-cecum (small bowel) transit times of anorexics, bulimics and healthy individuals. The lactulose breath test, a common testing mechanism for SIBO, was used to measure mouth-to-cecum transit. Overall, both intestinal transit times were found to be lengthened for both eating disorder sub-groups, in comparison to healthy individuals.

Gut Dysbiosis in ED Patients

It has been well documented that malnourishment alters the gut microbiome in both bacterial count and diversity. Severe malnutrition, such as that which often occurs in developing countries, has been shown to affect the gut microbiome, and vice versa. We see similar effects in the eating disorder community, as patients with AN demonstrate significantly lower bacterial diversity in comparison to healthy individuals. In regards to the species of bacteria present in these individuals, AN patients have displayed an increase in mucin-degraders (harmful bacteria which eat away at the mucosal lining of the intestines, resulting in “leaky-gut”) and a decrease in butyrate-producers (beneficial bacteria that digest dietary fiber). The chronic inflammation that arises from leaky-gut puts patients with AN at an elevated risk for autoimmune diseases, such as Crohn’s. Other beneficial species that were found to be lower in the anorexic microbiome include carbohydrate fermenters, and those which produce the short chain fatty acid, butyrate. The latter contributes to the maintenance of a healthy immune system. One study found microbial diversity to be the same in both the anorexic patients and the healthy controls. In this particular case, most of the anorexics reported following a diet that was low calorie, but high in fiber - so much so that the fiber content matched that of the higher calorie diets within the control group. Despite the difference in calories, the similar fiber intake may have protected the ED patients from a reduction in diversity. This finding supports the important role that fiber plays in maintaining a healthy gut microbiome.

Intestinal Permeability (Leaky Gut) in ED Patients

As aforementioned, patients with AN show an increase in mucin-degrading bacteria, which increase the permeability of the intestinal lining. Another factor that has been shown to correlate with this phenomenon is stress. Anorexic patients have demonstrated higher levels of the stress hormone, cortisol, as well as compromised cortisol suppression. This may help to explain why dysfunction of the intestinal barrier has been observed in individuals with anorexia.

Changes to the Gut Microbiome During Nutritional Rehabilitation

Studies have shown that the gut microbiome changes during nutritional rehabilitation and weight restoration in patients with AN, highlighting the important role that diet plays on gut health. Differences in macronutrient composition of rehabilitation diets for patients with anorexia have varying effects on their microbiome. Diets that are high in fat and protein, while being low in fiber may negatively alter the gut microbiota, and result in dysbiosis. Many anorexic patients maintain a low-fat, high fiber, vegetarian diet prior to treatment, and are then placed on a high-carb, high-fat diet during nutritional rehabilitation. This diet shift drastically alters the gut microbiome, and has the potential to introduce inflammation-inducing bacteria. Furthermore, when looking at common rehabilitation diets, many of them utilize nutritional supplements, consisting of triglycerides, corn syrup and emulsifiers, while lacking in fiber. Not only does the macronutrient composition of these supplements pose a potential threat to the gut microbiome, but the dietary emulsifiers they contain may also have adverse effects. These have been shown to perpetuate permeability and inflammation of the intestines, and increase anxiety. While nutritional supplements are an effective method for replenishing calories – a crucial part of ED recovery – their potential side effects may further complicate the process. Based on this information, it is suggested that the most optimal diet for weight restoration in AN patients is one that is primarily plant-based and high in fiber.

Animal-Based Diets vs. Plant-Based

It has been demonstrated repeatedly through animal studies that a high-fat diet causes an increase in gut permeability and inflammation throughout the body. Of particular concern are saturated fats, which are found primarily in animal-based foods. Both composition and diversity of the human gut microbiome is heavily dependent upon dietary patterns, with significant differences between animal-based and plant-based diets. The most abundant bacteria in those who follow an animal-based diet is one that is highly resistant to bile. This is due to the natural tendency of animal-based diets to be higher in fat.


Although there is no definite answer as to whether SIBO and eating disorders coexist, there are many similarities between the two conditions, suggesting a possible correlation. Altered composition of the gut microbiome, GI symptoms such as abdominal distension, and GI dysfunction such as lengthened transit time and slowed motility are all common occurrences between SIBO and eating disorders. SIBO and eating disorders also share the comorbidities IBS and Crohn’s disease. Due to these commonalities, it is evident that using a similar approach to the rehabilitation diet for each disorder may be beneficial. It is important to take into consideration the effect that diet composition has on the digestive system and the microbiome in treating individuals with eating disorders. If a rehabilitation diet can simultaneously help to alleviate some of the GI symptoms and digestive discomfort, it is likely to also improve the rate and success of the recovery for the individual.

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