The Causes of Intestinal Dysbiosis: A Review


The Causes of Intestinal Dysbiosis: A Review

Hawrelak JA, Myers SP

PhD Candidate, Intestinal Micro-ecology,
Southern Cross University’s School of Natural and
Complementary Medicine and the Australian Centre
for Complementary Medicine Education and Research.

School of Natural and Complementary Medicine,
Southern Cross University,
PO Box 157,
Lismore NSW, Australia 2480

Alterations in the bowel flora and its activities are now believed to be contributing factors to many chronic and degenerative diseases. Irritable bowel syndrome, inflammatory bowel disease, rheumatoid arthritis, and ankylosing spondylitis have all been linked to alterations in the intestinal microflora. The intestinal dysbiosis hypothesis suggests a number of factors associated with modern Western living have a detrimental impact on the microflora of the gastrointestinal tract. Factors such as antibiotics, psychological and physical stress, and certain dietary components have been found to contribute to intestinal dysbiosis. If these causes can be eliminated or at least attenuated then treatments aimed at manipulating the microflora may be more successful.


The gastrointestinal tract (GIT) is one of the largest interfaces between the outside world and the human internal environment. From mouth to anus, it forms a nine-meter long tube, constituting the body’s second largest surface area and estimated to cover approximately 250-400 m2. Over a normal lifetime, approximately 60 tons of food will pass through the GIT. [1] Food is obviously extremely important for well-being, but its passage through the GIT can also constitute a threat to health. While the GIT functions to digest and absorb nutrients, food also provides exposure to dietary antigens, viable microorganisms, and bacterial products. The intestinal mucosa plays a dual role in both excluding these macromolecules and microbes from the systemic circulation and absorbing crucial nutrients. [2]

As mentioned above, the mucosa is exposed to bacterial products – endotoxins, [3] hydrogen sulphide, [4] phenols, ammonia, and indoles [5] – that can have detrimental effects on both mucosal and host health. [5] The presence of many of these toxic metabolites is directly dependent on the type of fermentation that occurs in the bowel. In turn, this fermentation is dependent on the type of bacteria present in the bowel, as well as the substrates available for fermentation. Diets high in protein [6] and sulfate (derived primarily from food additives) 4 have been shown to contribute greatly to the production of these potentially toxic products. The production and absorption of toxic metabolites is referred to as bowel toxemia. [7]

The bowel toxemia theory has historical roots extending as far back as Hippocrates. In 400 B.C. he stated that, “…death sits in the bowels…” and “…bad digestion is the root of all evil….” [8] More modern proponents of the bowel toxemia theory have included naturopath Louis Kuhne in the late nineteenth century, [9] as well as naturopath Henry Lindlahr [10] and Nobel prize laureate Elie Metchnikoff in the early twentieth century. [11] Louis Kuhne proposed that excess food intake, or the intake of the wrong types of food, resulted in the production of intestinal toxins. Fermentation of these toxins resulted in increased growth of bacteria within the bowel and, subsequently, disease. He believed a predominantly vegetarian and mostly raw diet would prevent build-up of intestinal toxins and, hence, would prevent and even cure disease. [9]

Only a few years later, Metchnikoff popularized the idea that fermented milk products could beneficially alter the microflora of the GIT. He believed many diseases, and even aging itself, were caused by putrefaction of protein in the bowel by intestinal bacteria. Lactic acid-producing bacteria were thought to inhibit the growth of putrefactive bacteria in the intestines. Thus, yogurt consumption was recommended to correct this “autointoxication” and improve composition of the microflora. [11, 12]

The bowel toxemia theories eventually evolved into the intestinal dysbiosis hypothesis. The term “dysbiosis” was originally coined by Metchnikoff to describe altered pathogenic bacteria in the gut. [13] Dysbiosis has been defined by others as “…qualitative and quantitative changes in the intestinal flora, their metabolic activity and their local distribution.” [14] Thus dysbiosis is a state in which the microbiota produces harmful effects via:

(1) qualitative and quantitative changes in the intestinal flora itself;
(2) changes in their metabolic activities; and
(3) changes in their local distribution.

The dysbiosis hypothesis states that the modern diet and lifestyle, as well as the use of antibiotics, have led to the disruption of the normal intestinal microflora. These factors result in alterations in bacterial metabolism, as well as the overgrowth of potentially pathogenic microorganisms. It is believed the growth of these bacteria in the intestines results in the release of potentially toxic products that play a role in many chronic and degenerative diseases. [13]

There is a growing body of evidence that substantiates and clarifies the dysbiosis theory. Altered bowel flora is now believed to play a role in myriad disease conditions, including GIT disorders like irritable bowel syndrome (IBS) [15] and inflammatory bowel disease (IBD), [16, 17] as well as more systemic conditions such as rheumatoid arthritis (RA) [18] and ankylosing spondylitis. [19] Thus, knowledge of the factors that can cause detrimental changes to the microflora is becoming increasingly important to the clinician.

The Importance of Normal GIT Microflora

The microflora of the gastrointestinal tract represents an ecosystem of the highest complexity. [14] The microflora is believed to be composed of over 50 genera of bacteria [20] accounting for over 500 different species. [21] The adult human GIT is estimated to contain 1014 viable microorganisms, which is 10 times the number of eukaryotic cells found within the human body. [22] Some researchers have called this microbial population the “microbe” organ – an organ similar in size to the liver (1-1.5 kg in weight). [23] Indeed, this microbe organ is now recognized as rivaling the liver in the number of biochemical transformations and reactions in which it participates. [24]

The microflora plays many critical roles in the body; thus, there are many areas of host health that can be compromised when the microflora is drastically altered. The GIT microflora is involved in stimulation of the immune system, synthesis of vitamins (B group and K), enhancement of GIT motility and function, digestion and nutrient absorption, inhibition of pathogens (colonization resistance), metabolism of plant compounds/ drugs, and production of short-chain fatty acids (SCFAs) and polyamines. [14, 25, 26]

Factors that Can Alter the GIT Microflora

Many factors can harm the beneficial members of the GIT flora, including antibiotic use, psychological and physical stress, radiation, altered GIT peristalsis, and dietary changes. .