There is a lot of talk these days about the gut and how a compromise of the gut lining can affect one’s health. Health blogs and postings online are claiming that everything from intestinal disease (like celiac disease, irritable bowel syndrome) to food allergies, migraine, chronic fatigue syndrome, asthma, eczema, multiple sclerosis and even autism are linked to a disruption of integrity of the intestinal lining. The reason for disruption of the gut lining (leaky gut) is blamed on bacterial or candida overgrowth, poor diet or antibiotic overuse.
Most of the sites tell us that undigested food particles, bacterial toxins and even whole bacteria are able to pass the disrupted lining of the gut and enter the bloodstream where they trigger immune response and cause chronic inflammation. But is this scientifically proven? Is leaky gut fact or fiction? Is it really possible for large molecules or microbes to pass through the intestinal lining to enter the blood stream? And finally, is there any solid proof to the claims of candida being responsible for a leaky gut (if it exists at all)? Let us try and understand our intestinal biology and discover if what we are told has a solid foundation.
Our intestines play a role not only in absorption of nutrients, but also act as a barrier to prevent disease causing organisms and toxins from entering the body to cause disease. It produces some secretions that have various other functions apart from immune response.
The wall of the intestine has four specialized layers:
- Mucosa: This is the innermost layer. This is where nutrients are absorbed. This layer in turn is made up of 3 layers:
- Epithelial cells: This layer faces the inside of the gut. It is a single layer of glandular cells attached to a basement membrane. There are specialized cells called the goblet cells which secrete gel-forming mucins that form mucus.
- Lamina propria: This is the middle layer and is made up of connective tissue and lymph nodes. There are specialized cells in this area called the plasma cells which produce IgA type antibodies.
- Muscularis mucosae: This is the third layer which is a sheet of muscle cells.
- Submucosa: This is the second layer. It has different inflammatory cells, lymphatics, nerve fibers, and nerve cell clusters called ganglions. It is this layer that is also a branching and distribution zone for arteries and veins (the blood vessels).
- Muscularis propria: This is a layer of muscle cells. The muscle cells are arranged in two layers between which are nerve fibers and clusters of ganglions. This layer functions to propel the food through the gut through contraction waves (called peristaltic waves). The peristalsis (process of propelling the food in the gut) is started and controlled by nerve transmission and hormones.
- Serosa: Most of the intestine is lined by this protective layer. Serosa is made up of a muscle cell layer followed by a thin sheet of connective tissue which is followed by a sheet of epithelial cells (mesothelium). Serosa prevents the spread of inflammation if it occurs at all.
As we can see, the intestine isn’t single layered and the many layers by themselves are a barrier against unwanted molecules and microbes to pass through to the blood supply added to which are the mucus secretions and antibodies that prevent this even further. However, the intestinal barrier is a much advanced system than this; let us find out what the components of this barrier are.
Components of intestinal barrier
We unknowingly eat microbes and toxins along with our food and drinks. While some of the microbes may be essential for our functioning, toxins and pathogenic microbes can cause disease. There are not one, but four components to the intestinal barrier. These protect our body against disease causing microbes and toxins that we eat along with our food. If the large molecules and microbes must pass these, all these 4 barriers need to be crossed.
- Physical barrier: This is the first line of defense. It is made up of the epithelial cell lining of the mucosa (described above). Additionally, there are junctions (closely associated areas) between these cells. Tight junctions are the primary barriers. These are formed by proteins spanning across the space between two cells of gut epithelium.These allow the passage of fluids, electrolytes and small molecules, but prevent the passage of large molecules – this means that food that is digested into small molecules, vitamins, minerals and water can pass through these junctions, but larger, undigested molecules and bacteria are prevented from passing through into the blood.
- Chemical barrier: Mucus forms the chemical barrier of the intestine. It is a slippery secretion that covers all of the intestinal epithelium. Not only is mucus a physical barrier against the diffusion of unwanted substances into the cell, but also prevents the cells from drying out and also acts as a lubricant that prevents the friction between digesting food and the epithelial cells of intestine.
In 2008, Johansson and colleagues from Sweden showed that colonic mucus in mice has two layers – an outer and an inner layer. While the outer layer is movable and is colonized by bacteria, the inner layer is tightly packed and has no bacteria at all. In humans, a thicker inner mucus layer separates bacteria from the tissue.
The mucus layer also contains antimicrobial peptides which destroy bacteria through different mechanisms. This controls both good and the bad microbes from reaching the intestinal cells. In a review published in 2012, Gallo and Hooper from USA discussed these antimicrobial proteins, the mechanisms by which these protect against bad microbes, and how these also regulate the balance of the gut microbial flora. They also describe how microbes have failed to develop resistance to these antimicrobial proteins over evolutionary timescales.
- Immunological barrier: As mentioned earlier, plasma cells in the lamina propria layer of the mucosa produce IgA antibodies. The intestinal immune system performs two tasks: a) immune defense against pathogenic organisms, and b) avoiding reaction against food antigens and normal flora of the intestine.
In a 2012 review, Miron and Cristea from Romania described how the intestinal mucosal immune system needs to be able to differentiate between harmful things that it must react to and the harmless things that it should not react against. They described research by various scientists on how the intestinal cells maintain the equilibrium between activation and tolerance in the gut mucosa by reacting intelligently to the large number of antigens that the gut comes across. The intestinal cells use a variety of mechanisms that involve special protein molecules called receptors, anti-microbial peptides, and cell communication using small secreted proteins called cytokines. However, this field is still growing and a lot still remains to be discovered.
- Microbiological barrier: Research has showed that the good microbes living in our guts have the ability to suppress our gut immune system so that it does not attack them. On the other hand, research has also showed that these good microbes that reside in our gut increase our immunity against the bad, disease causing microbes. A review article by Andrew S. Neish from USA, published in 2008, discussed the multiple factors that allow the good microbes to make us resistant against the disease causing ones. They suggest that the normal flora may compete for access to the binding sites on the gut epithelial surface, or may stimulate secretion of mucus. The short chain fatty acids (SCFAs) produced by the good microbes may stop the growth of other harmful bacteria either directly or by increasing acidity. Neish cites 2007 research by Corr and colleagues from Ireland who showed that some good microbes like Lactobacillus salivarius also produce small peptide molecules called bacteriocins which inhibit the growth of other disease causing microbes like listeria or even kill them.
In 2015, a very interesting research published by Johansson and colleagues from Sweden and Norway showed that genetically similar laboratory mice with slightly different microbiota had very different inner mucus inner layers. To their surprise they found that the free-living mice who had more viral and parasitic infections compared to the laboratory mice had an even more developed mucus layer. They found that mice with better mucus barrier had a particular type of supposedly beneficial bacteria. However, when change in food increased the number of these bacteria, the gut was found to be more permeable. Mice with more permeable gut, on the other hand, had a different type of bacteria – bacteria of the type that are associated with irritable bowel disease and Crohn’s disease.
Thus, it is likely that the good microbes not only prevent bad microbes from growing, but they also may be involved in modulating the health of the physico-chemical barrier, the mucus.
Understanding the gut biology and the barriers that it poses, it seems like it is not really easy for toxins, microbes, undigested food, etc. to pass through to the blood circulation. These substances and organisms need to pass through not one but four different barriers. So, can “leaky gut” really occur? If so, under what circumstances? And can it cause the range of conditions it is claimed to cause?
Leaky gut – is it fact or fiction?
While, it is widely accepted that in HIV infection the final immune deregulation is driven by increased gut permeability where the microbes from the intestine end up in the blood circulation, there is much debate regarding the involvement of leaky gut in causing a wide range of diseases. There is contradictory information floating around regarding leaky gut. On the one hand, National Health Services, UK considers “leaky gut” to be a “proposed condition” and a theory that is supported largely by the practitioners of complementary and alternative medicine who link it to a wide range of other conditions from allergies to autism. NHS, UK finds little evidence to suggest that these conditions could be caused by leaky gut and warn the patients to be “wary of treatments offered by people who claim to be able to cure leaky gut syndrome”.
On the other hand, scientists like Allesio Fasano, director of Mucosal Immunology and Biology Research Center at Massachusetts General Hospital in Boston who spent 30 years studying gut and its role in human health told the Time Magazine early in 2016:
We now believe that under certain circumstances, fissures open up in the intestinal barrier that may allow things to get through.
Zonulin, a gut protein which regulates the permeability of the tight junctions was discovered by Fasano and his team in 2001. According to Fasano’s 2012 review article, people of a certain genetic makeup are more susceptible to a leaky gut. In such individuals, there is a deregulation of the zonulin pathway. Why only some with such genetic makeup develop the syndrome and not others may depend on environmental factors that they are exposed to.
Fasano says that there still is a lot of debate over how large molecules that cause allergies or autoimmune disease got into the bloodstream from the gut, but it does seem like “gaps may sometime form between the cells that may allow larger molecules to pass through.” According to him, leaky gut syndrome is legit and it is likely that it plays a role in food allergies, type-1 diabetes, celiac disease, Crohn’s disease, IBS and other issues related to digestion. However, he also said that the scientific community has refused to accept the concept of leaky gut due to the use and abuse of the term “leaky gut” by some alternative medicine practitioners who, without any evidence, attributed everything from autism to cancer to be caused by a leaky gut.
There definitely is evidence that problems with intestinal gut permeability can cause issues related to digestion. There is some preliminary evidence that leaky gut may also be related to other non-digestion related issues like multiple sclerosis (MS) as was shown in 2014 by Lavasani’s research group from Sweden. However, this was a study on mouse models and lot more studies will be needed to establish a clear link between leaky gut and MS in humans.
However, Quigley from USA, in his 2016 review provides arguments against the concept that leaky gut can cause a plethora of diseases and says that the evidence is rather flimsy. He maintains that;
Strictly speaking, this term [Leaky gut] should be restricted to those situations where epithelial tight junctional function is impaired resulting in increased flux across the paracellular route; a phenomenon, though accessible to measurement in vivo in man, has little to do with the diseases and disorders in which a ‘leaky gut’ is thought to play a role.
His main arguments are:
- Changed gut permeability could be a phenomenon that is secondary to something else like inflammation, dietary components, bile acids, allergens, stress and physical activity. The question he is asking is, for example – does leaky gut lead to inflammation, or does inflammation lead to leaky gut? In other words, does leaky gut cause celiac disease or celiac disease cause leaky gut? However, he does mention that there is some “circumstantial” evidence that gut permeability may have a more primary role.
- Experiments in animals have not been able to show that a leaky gut by itself results in disease. However, as Fasano mentioned the impact of a leaky gut may be seen only in combination with other factors. This does not per se negate the effect of leaky gut on disease causation.
- It is not necessary that increased permeability is bad. The research that he cites for this argument shows that when the gut permeability was artificially increased in mice using ethanol or cholera toxin, the immune system was activated that protected the mucosa from inflammation. However, it is natural for the gut to respond in this manner – gut is after all also an immune barrier. One could imagine that a constant abuse of the intestinal lining by the toxin or ethanol will overcome the immune barrier. This work by Boirivant and colleagues from Italy in fact mentions that only a mild or transient breach in epithelial barrier would cause protective effects – which means that if there is a constant leaky gut the protective function of the gut immune system will be overwhelmed.
- There is no convincing evidence that an intervention that restores or improves barrier function can improve the disease. There are some studies on mice, however, that show that certain probiotics can prevent the development of some diseases. It is true though that there are no human studies yet that show the same effect. We can only hope that more research in humans on these lines will lend weight to this point.
Overall, leaky gut is an actual phenomenon and it does seem like a possibility that some if not all diseases could be caused due to this in combination with other factors.
Overcoming the barriers
When there is evidence that leaky gut is an actual phenomenon, what happens then that allows the toxins, microbes, undigested food etc. to cross the four intestinal barriers? The question is how can these things overcome not one but four barriers? Do these things overcome the barriers themselves or do they get help from some other factors?
- Microbes: A 2014 study by Johansson and colleagues from Sweden found that normally the inner mucus layer of our gut cannot be penetrated by bacteria. In case of ulcerative colitis, this layer allows the bacteria to penetrate and reach the epithelial cells. As described earlier in the “barriers section”, the microbes themselves may modulate our mucus barrier – make the layer thicker and impenetrable or thinner and more penetrable by microbes and other molecules. Antibiotic usage can result in the loss of good microbes that help the mucus to stay healthy and impenetrable, resulting in thin layers of mucus that are more penetrable. Also, as described in a 2009 review article by Groschwitz and Hogan from USA, an associated overgrowth of disease causing microbes will result in these secreting toxins and protein degrading compounds (proteases) that help overcome the gut barrier by damaging the gut cells and causing cell death, or by changing the ion transport, or by breaking down the tight junctions, or a combination of these. Apart from this, these microbes also may disrupt intestinal barrier by binding to cell surface molecules and inducing changes in the proteins that are involved in forming the tight junctions.
While some penetration of the intestinal cells after passing through the mucus layer can be effectively handled by the gut immune system, if the immune system is over-stimulated by too much stuff passing through constantly, it can lead to chronic inflammation of the gut – which is what happens in cases of Inflammatory bowel disease (IBD), a disease with proven link to a leaky gut.
- Intestinal motility: The peristaltic movement of our intestines affects the intestinal barrier by propelling microbes and toxins. Peristalsis makes sure that the bacteria and toxins, along with the digested food material, moves ahead through the intestine. This ensures that the microbes and toxins do not stay for a long time in the same place, thus preventing them from having any adverse impact on the lining of the gut. The gut microflora is also responsible for regulation of intestinal motility – they secrete various substances that can influence intestinal movement. This means that any deregulation of microflora can reduce gut motility leading to slow movement of digested food, microbes and toxins through the intestine. This would allow more time for these to be able to overcome some if not all barriers.
- Stress: The role of stress in increasing intestinal permeability is well documented – Söderholm and Perdue have described the extensive body of research on the role of stress in intestinal permeability issues in their review from 2001. There is evidence from animal studies that acute and chronic stress can change gut permeability. Chronic stress also seems to worsen colitis, enhances reactivation of disease, and reduces mucus production by colon apart from increasing gut permeability. The stress can be either physical such as trauma or surgery, or can also be psychological.
- Alcohol: Intestinal permeability is found to be increased on moderate and chronic intake of alcohol via acetaldehyde and nitric oxide formation. In 2006, Ferrier and colleagues from France showed in rats that the gut microflora converted the ethanol quickly to acetaldehyde which increased gut permeability. They also found that a special group of immune cells called the mast cells get activated by ethanol and cause inflammation resulting in gut permeability impairment. Apart from this there are some other mechanisms that are thought to be responsible for ethanol mediated leaky gut, although the exact mechanism is not completely understood yet.
- Diet: Diet affects the gut microflora which in turn affects intestinal permeability. Amit-Romach and colleagues from Israel showed with their experiments in 2009 that a vitamin A deficient diet increased the total amount of bacteria in the gut and changed the gut microflora. Such diets caused a reduction in proportion of lactobacilli (good bacteria) and increased the proportion of E.coli (bad bacteria). They also showed that vitamin A deficiency caused loss of integrity of the mucosal barrier.
In 2008, a study by Kong and colleagues from USA showed that similar effects could be observed in cases of vitamin D deficiency which increased the risk for IBD. Although vitamin D is mostly obtained from sunlight, a part of vitamin D also comes from diet.Fermentation of undigested dietary carbohydrates by bacteria in the colon produces short chain fatty acids (SCFA) like acetate, propionate, butyrate and valerate. Butyrate is particularly important for maintaining the intestinal barrier. In IBD, there is a deficit of butyrate which increases gut permeability as was shown by Plöger and colleagues from Germany in 2012.
A diet high in fats has been found to increase intestinal permeability. A study on 8 healthy people by Pendyala and colleagues, published in 2012, found that when these people were given Western-style diet that is rich in fats and carbohydrates for one month it changed their gut barrier function. The scientists also say that the effect could also be due to a change in the composition of gut microbes – which in turn change gut permeability.
- Exercise: Gut complaints are common in endurance athletes. Bill Rodgers, the marathon legend once said “More marathons are won or lost in the porta-toilets than at the dinner table”. Breakdown of mucosal barrier and ischemic colitis apart from other symptoms has been observed after long distance running. In a 2014 review, de Oliviera and colleagues from Brazil have detailed the different effects of high endurance exercise on the gut which includes reduced blood circulation to the gut, increased gut permeability, mechanical damage due to posture, and nutrition.
- Non-steroidal anti-inflammatory drugs (NSAIDs): There is sufficient evidence showing that NSAIDS (a type of pain killers) like aspirin, ibuprofen and naproxen can increase intestinal permeability. For example, a study by Bjarnason and colleagues from the UK showed in 1987 that NSAIDS could induce inflammation in us humans and that this inflammation can last for up to 16 months. In 1997, Lanas and colleagues from Spain found strong association between use of NSAIDS and gastrointestinal perforation – a hole that is formed in the gut wall and needs emergency treatment. However, it has been found that intestinal damage caused by NSAIDS is a multistage process and involves a number of mechanisms which would mean that only a regular intake of these pain killers (possibly also along with other factors) will increase gut permeability.
One point to note here is that taking pain killers after high endurance exercise can potentially deal you double damage in terms of increased gut permeability – so regular high endurance exercise combined with regular intake of NSAID pain killers will be doubly bad for your gut and for your body.
- Pollution: An unexpected link was discovered between air pollution and changes in gut permeability! A 2011 article by American scientists (Mutlu and colleagues) describes their study on mice which found a link between exposure to air pollution and increased gut permeability and inflammation. The air pollution in form of particulate matter can cause death of gut epithelial cells, disrupt the tight junctions, increase inflammation and gut permeability. Kish and colleagues from Canada found in their mouse study in 2013 that the particle matter in air can alter our gut microflora and induce acute and chronic inflammation in the intestine accompanied by increased gut permeability.
- Disruption of body clock: Our body clock regulates a number of functions. Disruption of body-clock can happen in people who work shifts, those who have irregular sleep-wake cycles or irregular eating times, in people who fly long distance regularly, etc. The genes involved in our body clock also regulate the physiology and function of the gut. In 2013, it was shown in mice by Summa and colleagues from USA that when the body clock is disrupted, it can increase gut permeability. They also found that disruption of body clock increased alcohol mediated gut leakiness. They suggest from their studies that the increased gut permeability could be caused due to a reduction in the number of tight junction proteins that are bound to the cell membrane. What this means is that although the number of tight junction proteins is not reduced per se by a disrupted body clock, there is a reduction in the number of proteins that are bound to the cell membrane – and unless these proteins are bound to the cell membrane, they cannot form tight junctions!
Intestinal permeability and Candida
So, is there any role that candida can play in increasing gut leakiness? From the information above, the following logical conclusion can be made:
Candida is a microbe that is clearly implicated in dysbiosis – Candida could easily overgrow in the gut when the normal microflora is disturbed as in case of antibiotic and NSAID overuse, alcohol abuse, imbalanced diet and vitamin deficiencies, etc. . Dysbiosis seems to play a huge role in increasing gut permeability (of course, along with other factors). If you have understood this, you will be able to see the possibility that candida overgrowth could very likely cause intestinal permeability.
It sounds logically fine, you would say, but is there any scientific evidence to this? Are there any studies conducted at all that show a link between candida and leaky gut? Surprise, surprise, there are a few studies in animals conducted by scientists that implicate candida in increase of gut permeability.
Yamaguchi and colleagues from Japan showed with their mouse study in 2006 that gut colonization by candida promotes food allergy partly by increasing gut permeability which is mediated by activation of mast cells that causes inflammation. Once the guts of mice were colonized with candida, there was an increase in antibodies in blood against food proteins which indicated that the food protein antigens had entered the blood stream from the gut. The authors however are careful to mention that the situation could be different in humans and that human studies will clarify whether or not the same holds true for humans as well.
A 2013 article by Carol Kumamoto from USA reviews the research that provides proof of gut colonization by candida and inflammation. She found that candida colonization in patients with gut problems is documented in various conditions like Crohn’s disease, ulcerative colitis, gastric ulcers, duodenal ulcers etc. She finds that these patients along with their healthy relatives have a leaky gut. She also cites research in mouse and rat models that showed that candida colonization inhibits healing of inflammatory lesions. On the other hand she also provides research evidence that inflammation promotes gut colonization by candida. She suggests that this, along with lowered healing, can lead to a vicious cycle where inflammation promotes candida colonization and candida does not allow inflammation to heal, which again works in the favor of candida, and so on.
Invasive candida infection has emerged as a life-threatening disease in immune-compromised, critically ill patients. In such patients, candida is able to enhance colonization and penetration of the cells by changing from yeast to hyphal form and take advantage of reduced immunity to become invasive. Another 2013 article by Yan, Yang and Tan from China review the studies on virulence factors of candida albicans and how mucosal barrier works against these factors and how candida works its way through these barriers. Candida penetrates the mucus barrier by producing special protein breaking enzymes that break down the mucus layer and distorts cell membranes. This helps candida in binding to the cells and invading them. Additionally, candida can “contact sense” weak areas and invade these. It also produces certain enzymes that impair the tight junctions, break down the epithelium and increase permeability.
Thus, there is some evidence that candida can cause increased gut permeability that would allow large molecules, toxins, and small undigested food proteins to pass through the intestinal lining into the blood-stream causing inflammation. However, we need to remember that there are a variety of factors working in combination to cause leaky gut and more studies need to be carried out in humans to find out what exactly happens in human gut and if it is any different from the mouse and rat gut.