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The Role of Fasting in Gut Health

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The Role of Fasting in Gut Health

In recent years, numerous books and studies have increased the public's interest in fasting. Fasting is not new, as we have evidence that fasting has been practiced for thousands of years for religious, ethical, and health reasons. Christians, Buddhists, and Muslim religions all incorporate some type of fasting, and it has been documented that philosophers Plato and Socrates fasted and attributed many health benefits to fasting.  

Another popular health topic of recent is the gastrointestinal, or gut, microbiome. The gut microbiome is an extensive collection of microbes that play a beneficial role in our health. The gut microbiome is quite expansive; the genes from the microbes in our gut outnumber the number of genes we have by 150:1.

Researchers have started connecting the two, assessing the impacts that fasting may have on gastrointestinal health, including the microbiome.  


What is Fasting?

Fasting is not a diet; rather, it is an eating pattern. Fasting is an umbrella term that encompasses many different types of eating patterns.  

Time Restricted Feeding (TRF): food intake is restricted to 8-12 hours per day

  • 16/8 is the most common TRF, with 16 hours of fasting and 8 hours of eating.
  • 12/12 is also standard practice, with 12 hours fasting to 12 hours eating.
  • It is thought that caloric restriction during eating windows is not necessary because limiting the eating window is enough to induce caloric restriction naturally

Intermittent Fasting (IF): IF is a type of TRF with intentional addition of a caloric reduction.  There are two primary types of IF:

  • Alternate Day Fasting (ADF): refraining from food every other day or on certain days of the week. On non-fasting days caloric intake is not monitored. 5/2, or five days of normal eating with two days of water fasting only, usually on non-consecutive days, is the most common form of ADF.
  • Modified Alternate Day Fasting (MADF): Similar to ADF, except 15-25% of caloric intake is allowed on fasting days. The 5/2 method, as described above, is the most common, again with 15-25% of calories allowed on the fasting days.

Prolonged Fasting (PF): fasting for four to seven days with water only

It is worth mentioning the Fasting Mimicking Diet. While technically not a fast, the program, created by longevity researcher Dr. Valter Longo, allows your body to reap the benefits of fasting without actually abstaining from food.

Symptoms of Poor Gut Health

Poor gut health can manifest in the traditional gastrointestinal symptoms

  • Bloating
  • Burping
  • Flatulence
  • Diarrhea
  • Constipation
  • Cramping
  • Acid reflux

However, poor gut health can also manifest in other ways that may seem unrelated to the gastrointestinal system. In fact, the following diseases may be connected to poor gut health:

  • Autoimmune conditions, including Lupus and Rheumatoid arthritis
  • Neurodegenerative disorders
  • Cancers
  • Metabolic diseases, including obesity and type II diabetes
  • Acne
  • Cardiovascular disease

How Does Fasting Support Gut Health?

Fasting may help with numerous conditions, but fasting's effect on gut health seems to take shape in its impact on the microbiome. The gut microbiome contains trillions of microorganisms that live within our intestines, including bacteria, fungi, parasites, and viruses. The microbiome works synergistically with our bodies, meaning we benefit from it, and it benefits from us; it is because of this that some consider the microbiome an organ.

Everyone's microbiome is unique and determined by their DNA, although outside influences such as diet and environment can shape the microbiome's composition. In fact, the microbiome begins to form in utero, with the birthing process and nursing playing a significant role in its development.  

The microbiome has numerous functions:

  • Aids in the breakdown and digestion of foods via bile acid production, enhancing gut motility and absorbing minerals
  • Eliminates toxins
  • Production of vitamins, amino acids, and active metabolites
  • Develops and regulates immune cell production within the GI  
  • Produces fuel for cells of the colon which in turn help to feed the bacteria in that region
  • Creates neurotransmitters and other signaling molecules that communicate with the brain and other organs.

While the majority of microbes found in the microbiome are beneficial, there are some that, when in large quantities, can become pathogenic. Dysbiosis, or disturbances in the balance of good and bad microbes in the microbiome, is a causative factor for diseases.  

We also know that healthier people tend to have a more diverse microbiome, while those with diseases tend to have less diversity, and studies have shown that diversity can be increased with fasting.

Fasting has shown to increase the number of good microbes in the GI tract which can lead to many other health benefits including:

  • Lactobacillus plantarum has been shown to increase memory and enhance cognitive functioning while also helping to protect the gastrointestinal lining.
  • Lactobacillus rhamnosus may improve anxiety. Clostridium butyricum can improve gastrointestinal functioning and also may have a positive impact on memory.
  • The Lactobacillus species as a whole helps to aid in the metabolism of fats and carbohydrates. Both Lactobacillus and Bifidobacterium can assist in maintaining a healthy weight.
  • Faecalibacterium prausnitzii lowers inflammation.  
  • Oxalibacterium formigenes can help to regulate oxalic acid levels and thus help to prevent kidney stones.

A study published in Frontiers of Microbiology showed increases in the anti-inflammatory bacteria Bifidobacteria and Lactobacillus with intermittent fasting for 16 hours per day for 27 days. These bacteria may also play a role in maintaining a healthy weight. A similar study with 16 hours of fasting for 25 days showed increased microbial diversity with increased populations of Bacteroidetes and Prevotellacea bacteria.  

Functional Medicine Labs to Test for Gut Health

GI Map: GI Map is a test that uses DNA to detect various bacteria, viruses, and parasites in the stool, representing the microbiome. It also measures markers of acidity as well as inflammation to give a comprehensive view of how the gastrointestinal tract is functioning.

GI Effects Gut Pathogen Profile: This test focuses on finding numerous pathogenic, or harmful, bacteria, fungi, and parasites and also gives the estimated amounts present in the stool, again representing the microbiome.

TriSmart SIBO test: The small intestine should have a minuscule amount of microbes to aid digestion. However, certain causes can amplify the presence of microbes in the small intestine, called Small Intestine Bacterial Overgrowth (SIBO), which can lead to many issues. This breath test will show if SIBO is present and what type of bacteria is causing the problem.


Fasting has been practiced for thousands of years, and for good reasons. Fasting can positively affect numerous body systems, especially the gastrointestinal system. With functional medicine testing, we have greater insight into gastrointestinal health, how the GI is functioning, and the microbiome composition, which we know can play a role in various diseases. Functional medicine GI testing can help to assess what type of microbes are within our GI tract, and fasting can act as a tool to help diversify and balance the microbiome.  

Lab Tests in This Article

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