Have you heard the phrase “leaky gut” thrown around on the internet, but you’re not quite sure what it means?

Does it conjure up images of your intestines leaking out their contents, but you feel like that can’t be what everyone is talking about??

Don’t worry, it’s a little less dramatic (and a little more complicated) than that!

“Leaky gut” is actually a simpler term for “increased intestinal permeability” – a fairly common phenomenon associated with many chronic conditions (1).

In this article, we’ll provide a brief overview of what leaky gut is, why it occurs, and how medical professionals can actually test for it.

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What Is Leaky Gut?

The proper scientific term for leaky gut is “increased intestinal permeability” or “intestinal hyperpermeability” (2).

When the gut is “leaky”, it allows molecules that are normally kept out (like larger food particles or endotoxins) to pass through the lining of the intestines and into the bloodstream.

To understand why this might happen, first, we need to explore a little bit about the anatomy and physiology of the small intestine. We’ve broken it down into simple terms below.


The Anatomy of the Small Intestinal Barrier

Cells that make up the lining of the small intestine (where we absorb the bulk of our nutrients) are called enterocytes.

They form a barrier between the lumen (inside cavity) of the intestines and the rest of the body (3).

Their purpose is to regulate the absorption of vital nutrients and prevent anything dangerous (like bacteria, undigested food particles, or toxic substances) from being absorbed into the bloodstream (3).

In a healthy gut, most molecules (about 90%) are absorbed THROUGH the enterocytes, processed as needed, and then passed on into the bloodstream (4).

This is called the “transcellular” pathway and is tightly regulated to keep out anything harmful (5).

However, some molecules, especially small polar molecules like water, can also pass between the intestinal cells directly into the bloodstream (5).

This is known as the paracellular pathway.

Understanding the Paracellular Pathway

Enterocytes are held together by connections known as tight-junctions, adherens junctions, and desmosomes (6).

Normally, the permeability between enterocytes is very low and not many molecules are allowed to pass.

However, the permeability of these junctions can be influenced by a variety of factors, including regulatory proteins, nutrients, inflammatory cytokines, and bacteria (7).

When these factors cause tight junctions to stay open for too long, substances that otherwise wouldn’t be allowed to cross the intestinal barrier are able to cross, which can trigger an inflammatory immune response (4, 8).

And ultimately, when our bodies are exposed to inflammation over a long period of time, we are more likely to develop a variety of chronic diseases (9, 10, 11).

Demonstration of Trans and Paracellular Transportation Pathways


Causes of Leaky Gut

Potential causes of leaky gut that are currently being investigated include:

  • Alcohol consumption (12, 13)
  • Food components and additives (14)
  • High-fat diets (15)
  • Infections in the gut (16, 17)
  • Inflammation (18)
  • NSAID use (19, 20)
  • SIBO (21)
  • Stress (22, 23)
  • Vitamin A, Vitamin D, or zinc deficiency (24, 25)

9 Potential Causes of Leaky Gut - Functional Nutrition Answers


Conditions Linked to Leaky Gut

There are many conditions associated with leaky gut, but it is still debated whether increased permeability is a cause or consequence of these conditions.

These are some of the conditions that have been associated with leaky gut:

  • Alcohol-related liver disease (25)
  • Alzheimer’s disease (26)
  • Ankylosing spondylitis (27)
  • Asthma (28)
  • Autism (29)
  • Celiac disease (30, 31)
  • Chronic fatigue syndrome (32)
  • Critical illness or trauma (33, 34)
  • Depression (35, 36)
  • Fibromyalgia (37)
  • Food allergies or sensitivities (38, 39)
  • Inflammatory bowel disease (40, 41)
  • Irritable bowel syndrome (42)
  • Multiple sclerosis (43, 44)
  • NASH (45, 46)
  • Obesity (47, 48)
  • Parkinson’s disease (49)
  • Polycystic ovary syndrome (50, 51)
  • Rheumatoid arthritis (52)
  • Type 1 diabetes (53, 54)

What Types of Tests Are Available?

There are a few different ways to measure intestinal permeability, so let’s review the most common methods below.

1. Differential Sugar Test (Lactulose & Mannitol Test)

The differential sugar test has been used since the 1970s and is still the most popular way to test intestinal permeability (55).

There are technically a variety of sugars that can be used in this test, but lactulose and mannitol are most common and will be the focus of this review.

How does it work?

To perform the test, a sweet solution containing lactulose and mannitol is consumed after an overnight fast.

Then, all excreted urine is collected for 6 hours in order to measure the amount of lactulose and mannitol it contains (56).

Lactulose is a larger sugar, so it is not normally absorbed whole. If lactulose is found in the urine, it is assumed to have passed between the cells, indicating a “leaky” gut (56).

In contrast, mannitol is a smaller sugar alcohol that can be absorbed through enterocytes, particularly via small pores at the top of the villi of the small intestine.

The purpose of including mannitol in the drink is two-fold:

  1. It controls for any extraneous factors that might affect absorption or excretion (like gut motility or kidney function) since those factors would affect both types of sugars equally (57, 58).
  2. It can detect villous atrophy in the small intestine, since mannitol is primarily absorbed there. Poor mannitol absorption and excretion (but normal or high lactulose) suggests probable damage to the small intestinal villi.

This test is typically performed at hospitals or clinics, but health practitioners can also order at-home test kits for patients. Genova Diagnostics is a popular choice for these home testing kits, as well as Doctor’s Data.

Interpreting the Results.

The test results are usually expressed as a lactulose-to-mannitol ratio (LMR), in which the percentages (compared to the original oral dose) of lactulose and mannitol present in the urine are compared to each other (LMR = %lactulose / %mannitol) (57).

A higher LMR is associated with increased gut permeability (aka leaky gut).

It is important to note that this test may not be accurate when someone has SIBO (small intestinal bacterial overgrowth) because bacteria in the small intestine can metabolize these sugars before they are absorbed (59).

This could lead to low levels of lactulose and mannitol in the urine and potentially false-negative results.

Is there evidence to support this test?

Numerous studies have shown that LMR correctly identifies increased intestinal permeability, especially in patients with gastrointestinal diseases like Celiac disease and Crohn’s disease (60, 61, 62, 63).

However, lactulose is still a relatively small molecule (molecular weight of 342 Da), so it may not actually be a good indicator for how larger food antigens or bacterial toxins (which usually have a molecular weight of 5,000+ Da) would pass through the gut (64).

It is also important to note that other areas of the digestive tract can also become “leaky”, but different types of molecules are used to test the permeability of those regions (sucrose for the stomach, sucralose for the colon, or PEG or EDTA for the whole tract) (2459).

The Verdict: The lactulose & mannitol urine test is a popular way to evaluate permeability of the small intestine and is backed by decades of research. However, it’s important to note that this test may not be accurate for people with SIBO.

2. Serum Zonulin Testing

Zonulin is a regulator protein that basically acts like a gatekeeper of the intestines.

When zonulin levels rise, the tight junctions between enterocytes open slightly in order to allow nutrients and other molecules to move between them (1).

Since the discovery of the regulatory protein zonulin in the year 2000, scientists have been measuring serum zonulin levels to determine the likelihood of increased intestinal permeability (30).

How does it work?

Testing for zonulin is a much simpler process than the differential sugar test.

If you choose to have this test done, your blood will be drawn and sent to a lab where it will be evaluated for the amount of zonulin it contains.

High levels of zonulin in the blood means too much zonulin has been released in the gut, which probably signifies increased permeability (1).

There are several labs that offer zonulin testing:

  • The “Advanced Intestinal Barrier” test from Dunwoody Labs evaluates serum zonulin, DAO, and histamine levels.
  • The popular “GI-Map” from Diagnostic Solutions Laboratory is a stool test designed to evaluate your gut microbiome. It also offers fecal zonulin as an add-on.

Is there evidence to support this type of test?

While many studies have found high zonulin levels in patients with conditions associated with leaky gut, there are a few issues with using this lab test as a way to diagnose leaky gut (3065, 66).

Problem #1: Unstable zonulin levels.

Zonulin levels are prone to large fluctuations because zonulin is targeted by the immune system.

Once released into the bloodstream, macrophages target and destroy zonulin, so blood levels will vary for each person from day to day, based on their immune response (67, 68).

Problem #2: Measurement error.

Another issue is that most labs use a technique called ELISA (enzyme-linked immunosorbent assay) to measure zonulin levels in blood samples.

However, a study released in 2018 found that ELISA often improperly identified other types of proteins as zonulin, leading to falsely elevated results (69).

Problem #3: Poor correlation with differential sugar tests.

In one study, researchers actually tested patients using both lactulose/mannitol and zonulin testing.

The results of the two tests didn’t correlate very well, so some scientists have questioned the usefulness of serum zonulin testing (65, 67).

Zonulin Antibodies – A better choice?

Testing for zonulin antibodies might be a better alternative.

A recent study found that zonulin antibody testing had better reproducibility (meaning that it consistently showed the same results for the same patient) than serum zonulin levels (67).

Basically, when zonulin is released into the bloodstream, it triggers certain immune cells to produce specific antibodies (also called immunoglobulins) that recognize zonulin.

Because these zonulin antibodies are more stable and remain in the bloodstream longer, they are thought to be a better measurement of zonulin release than serum zonulin  (70).

The Verdict: Testing for zonulin antibodies (through labs like Cyrex or Vibrant Wellness) is more accurate than testing serum zonulin alone. However, it’s unclear whether this is better than lactulose/mannitol testing.

3. Other Types of Antibody Testing

In addition to testing for zonulin antibodies, other types of antibodies are being used as indirect measures of gut permeability.

These include:

  • LPS antibodies: Lipopolysaccharide is a type of sugar found on the cell walls of some gram-negative bacteria. LPS molecules are released when the bacteria die, but since they are relatively large in size they do not normally pass through the lining of the small intestine. However, when the gut is “leaky” some LPS can pass between the cells and stimulate the immune system. Therefore, the presence of LPS antibodies in the blood is an indirect marker of gut permeability (71).
  • Occludin antibodies: Occludin is one of the proteins that make up the tight-junctions between enterocytes. If occludin breaks down, the gut becomes more permeable, and if occludin fragments cross the intestinal barrier, they can stimulate the immune system to form antibodies against them. The presence of occludin antibodies in the blood suggests leaky gut (72).
  • Actomyosin network antibodies: Similarly, actomyosin proteins are found between the cells of the small intestinal lining and play a role in opening up the tight junctions. If the actomyosin network begins to break down and antibodies are formed, it suggests increased gut permeability as well (72, 73).

Testing options:

  • Cyrex Labs has a test called “Array 2” that analyzes zonulin antibodies as well as antibodies for occludin and LPS.
  • Vibrant Wellness has a test called “Wheat Zoomer” that checks for zonulin, actin, and LPS antibodies and also includes serum zonulin levels.

It is important to note that any testing that involves antibodies can be blunted by steroids or immunosuppressants.

While these tests don’t directly measure intestinal permeability, if larger molecules like LPS are able to pass through the intestinal barrier and be flagged by the immune system, chances are that other large molecules like food antigens are able to pass through as well.

4. Intestinal Biopsy

Technically, the most direct way to test for leaky gut is to have an intestinal biopsy done (24).

After removing a sample of intestinal tissue, a device called an “Ussing chamber” is used to measure the transport of molecules across the lining of the intestines (74).

Using this information, it can be determined whether there is actually increased permeability.

Biopsy was the first method used when scientists began studying the concept of intestinal permeability. However, since this an expensive and invasive method, it is not typically used in clinical practice.

Instead, scientists used the information they learned from studying intestinal permeability to create less invasive tests, like the differential sugar test (75).

The Verdict: Intestinal biopsies for the evaluation of leaky gut may be accurate, but this method is far too invasive, expensive, and labor-intensive to be used regularly. It is typically only used in research settings.


Should You Get Tested for Leaky Gut?

Well…maybe not.

Many practitioners believe that testing for leaky gut is a waste of time and money for several reasons.

Downside #1: Leaky gut is probably pretty common.

We know that leaky gut is linked with MANY health conditions (see above), so if you have any of them, there’s a very good chance you have leaky gut as well.

Downside #2: It doesn’t get to the root cause.

Sure, testing for leaky gut can tell you whether or not you have increased intestinal permeability, but many practitioners believe that leaky gut is more of a symptom than a root cause.

Ultimately, we need to know what actually triggered the leaky gut so that we can address that directly.

That’s why many practitioners prefer to evaluate or test for things that are likely to trigger leaky gut, like infections, stress, nutrient deficiencies, or alcohol and medication use.

Downside #3: Knowing if you have leaky gut will probably not affect your treatment.

Knowing whether or not you have leaky gut will probably not affect your treatment protocol.

Testing for leaky gut doesn’t get you any closer to figuring out the root cause (which can then be addressed directly).

Someone on a limited budget is probably better off requesting more targeted tests to determine why their gut might be extra permeable.

However, some practitioners do like to test for leaky gut to evaluate the effectiveness of their interventions.

So, for example, if intestinal permeability is high at the beginning of treatment and then reduced after therapy, this suggests that the treatment protocol was probably effective.

If you do choose to get tested for leaky gut, the best options are the differential sugar test (lactulose/mannitol test) and zonulin antibody testing.


Final Thoughts

Leaky gut (or, more technically, intestinal hyperpermeability) is a real phenomenon associated with many health conditions.

It occurs when the tight junctions between the cells that line our small intestines open up slightly, allowing larger molecules to “leak” through into the bloodstream.

When the immune system encounters these unexpected molecules, it can trigger an inflammatory response, which over time, is believed to increase the risk of many chronic illnesses.

However, leaky gut is probably more of a symptom than a root cause. Digging deeper to find the things that trigger leaky gut may be a better use of time and resources.

Because of this, most healthcare practitioners don’t specifically test for leaky gut unless they plan to use it as a marker of treatment effectiveness.

If you do plan on testing for leaky gut, the differential sugar test or serum zonulin antibodies are the preferred methods.

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How to Test for Leaky Gut - Functional Nutrition Answers