Could the Gut Microbiome Impact Anorexia Recovery?

By Bhavya Singh, MSc Candidate in Chemical Biology

Only a handful of studies have explored the gut microbiome of eating disorder patients. However, some experts argue that gut bacteria could play an important role in the road to recovery, particularly for those affected by anorexia nervosa. 

The Gut Microbiome

There are 100 trillion bacterial cells in the human gut – that’s 100 times more than the number of human cells. Over time, we have developed a relationship with these bacteria. They help us digest food that we can’t digest by ourselves, and play important roles in nutrient absorption, metabolism, immune function, and even depression and anxiety.

Since these microbes primarily digest dietary fibers—which are found in complex carbohydrates such as legumes, grains, fruits, potatoes, and vegetables—diets lacking in these specific carbohydrates can often cause some grumbling within our gut communities. In particular, a restrictive diet can prevent bacteria from producing important metabolites, such as short-chain fatty acids (SCFAs). This is important because SCFAs are required for proper gastrointestinal function, host metabolism, regulating blood pressure, and regulating the circadian rhythm

An example of such end products is the SCFA butyrate. As the preferred energy source for colon cells, butyrate contributes to colonic mucosal health. It has been implicated in fighting inflammation, suppressing tumors, promoting growth of normal colonic cells, encouraging the absorption of water and sodium in the intestine, and impacting host immunity. In addition to butyrate, there are many more microbial metabolic products that impact our bodies every single moment. 

The Gut Microbiome Could Impair Recovery and Weight Gain in Anorexia Recovery

Anorexia nervosa is an eating disorder associated with food restriction, increased physical activity, and fear of gaining weight. Out of all psychiatric illnesses, anorexia has one of the highest mortality rates. For the most part, treatment involves nutritional rehabilitation and family-based therapy. 

The gut flora of anorexia nervosa patients has been shown to be significantly different from those of healthy controls. Anorexia is known to impact the gut microbiota of patients, but at the same time, the microbiota can also contribute to the pathophysiology of the disease. While this makes it difficult to determine causality, it is also one of the reasons why it is important to investigate this relationship.

These patients have less bacteria that produce beneficial SCFAs like butyrate and propionate, and more bacteria that degrade intestinal mucin. Butyrate concentrations are inversely correlated with anxiety, meaning that individuals with reduced amounts of butyrate often report higher anxiety. Furthermore, the reduced propionate concentrations are correlated with reduced fat cell formation, and could hinder the necessary fat gain required during anorexia treatment. 

A recent study in mice demonstrated that food restriction alone causes gut microbial dysbiosis and disrupted metabolic pathways, including a reduction of bacterial metabolic activity. Specifically, certain gut bacteria and their important metabolic byproducts are decreased in anorexic individuals, and are increased only after weight restoration.  

Importantly, caloric restriction reduces the amount and diversity of important bacteria in the gut. In general, there is a decrease in gut microbiota richness in anorexia patients, which may lead to patients having a reduced ability to extract calories from food.Strikingly, this has led researchers to hypothesize that a disrupted gut microbiota could make it more difficult for patients to gain weight in recovery, which is an essential and life-saving part of treatment for most people. 

Should the Gut Microbiota be Considered in Anorexia Nervosa Nutritional Rehabilitation?

Malnutrition in anorexia nervosa negatively impacts the gut microbiome of patients, which could  the pathophysiology of the disease in a number of ways. Since the gut microbiome in anorexia patients may be playing a role in both the psychological and physiological components of the disorder, a number of studies have speculated upon its role in treatment and recovery.  

Currently, anorexia treatment plans are focused on increasing caloric intake for weight restoration, in addition to extremely personalized family-based therapy. Given the psychological, physical, and social complexities of the disease, it is necessary for patients to consume a wide variety of foods, including foods that are often labeled “unhealthy”, and not necessarily “optimal” for the gut microbiota. While fiber-rich foods are typically considered good for gut health, they are suboptimal for weight gain since they are high in volume and low in calories. The fiber can also negatively affect patients who have more severe gastrointestinal problems, which is a common symptom in the pathophysiology of the disease. 

However, given the importance of the gut microbiome and bacterial metabolites on nutrient and calorie absorption, it may be beneficial for patients to consume some fiber-rich foods while still maintaining a calorically adequate diet. Another potential option is to supplement patients with prebiotics to encourage the growth of important bacteria.

Fecal Microbiota Transplantation – Could it Help?

Previously, it was shown that transplanting mice with  fecal microbiome samples of human anorexia patients impaired their ability to gain weight. Not only did these mice gain less weight, but they  also absorbed fewer calories, and had lower serotonin levels. In a nutshell, healthy mice exhibited the symptoms and physical manifestations of anorexia nervosa when their microbiota was changed., This also means that the opposite might also be true.  Could patients be treated by receiving a microbiota transplant from a healthy donor? Potentially, yes. 

In a 2019 study from the Netherlands, scientists were able to show successful weight gain in an anorexia nervosa patient after receiving a fecal microbiota transplant from a healthy subject. While this was a case study  on only one patient, it does suggest that fecal microbiota transplants may be a promising avenue for recovery in patients who are not responding to the regular treatment protocols. 

Given these recent developments in the field of anorexia and the gut microbiota, it is possible that gut bacteria may play an important role in both the pathophysiology and recovery process of anorexia nervosa. Although most treatment protocols do not currently take the gut flora into account, it may be a promising supplement to nutritional rehabilitation in the future. 

*If you would like to learn more about eating disorders, linked below are some useful resources:

  1. National Eating Disorders Association (NEDA)
  2. The NEDA helpline can be accessed here
  3. Eating Disorder Treatment and Recovery

**Cover photo references:

  1. Photo by Aleksandar Pasaric from Pexels
  2. Photo by cottonbro from Pexels

Peer-edited by Meredith Edelman, MPH/RD Candidate in Nutrition

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