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Stop guessing. But also know what each test can and can’t tell you.

I get some version of this question almost every week. You feel awful, you suspect leaky gut, and you want a test that hands you a clear yes or no.

I understand the pull. After years of being dismissed, a number on a lab report feels like proof. Like permission to finally take yourself seriously.

So let me give you the honest version, the one I wish someone had given me.

There is no single, perfect, FDA-cleared “leaky gut test” you can take to confirm a diagnosis. What exists is a handful of tools, each measuring something slightly different, each with real strengths and real blind spots.

Knowing which is which saves you money, saves you false hope, and saves you from eliminating foods you never needed to remove.

First, what leaky gut testing is actually measuring

“Leaky gut” is the everyday name for increased intestinal permeability. Your gut lining is meant to be selective: it lets nutrients through and keeps bacteria, toxins, and undigested food particles out. When the tight junctions between those cells loosen, things slip through that shouldn’t.

Here’s the catch that trips up almost every test on the market. Increased permeability, inflammation, and food reactions are three different things. They often travel together. But a test that measures one is not measuring the others.

Keep that straight and the rest of this makes sense.

How to Test for Leaky Gut [Zonulin, Lactulose-Mannitol, Stool Testing, and Food Sensitivity Clues]

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1] The zonulin test

What it claims to measure: Zonulin is a protein that helps regulate your tight junctions. When it goes up, the junctions open. So in theory, high zonulin means a leakier gut. It’s usually run on blood or stool, and it’s the test most “leaky gut panels” lead with.

What it can tell you: In research settings, zonulin has been linked to conditions involving barrier dysfunction, including celiac disease, IBD, and type 1 diabetes. The concept is real.

What it can’t tell you: This is where I have to be straight with you. The commercial zonulin test most labs use has a serious problem. A 2021 paper put it plainly: the widely used commercial zonulin ELISA does not actually measure zonulin. It measures other, unidentified proteins, and the authors concluded it is not adequate to measure intestinal permeability.¹ A separate study in healthy adults found no correlation at all between zonulin and the gold-standard sugar absorption test.²

Translation: a high zonulin result on a consumer panel may not mean what the report says it means. I’m not telling you the protein is meaningless. I’m telling you the test being sold to you is shakier than it looks.

Make note: if a practitioner is building your entire protocol around one zonulin number, that’s a flag.

2] The lactulose-mannitol test

What it measures: This is the closest thing we have to a gold standard, and it’s clever. You drink a measured dose of two sugars: mannitol, a small molecule, and lactulose, a larger one. Then you collect your urine for several hours.

Mannitol is small enough to be absorbed normally through a healthy lining, so it tells you about your gut’s overall absorptive surface. Lactulose is too big to get through an intact barrier. If a lot of lactulose shows up in your urine, it slipped through gaps that shouldn’t be there. The ratio of the two is the actual measurement.³

What it can tell you: Of all the options, this one directly measures small intestinal permeability rather than guessing at it through a proxy protein. A 2024 review in Gastroenterology called it the test best positioned to become clinically available.

What it can’t tell you: It isn’t flawless. The validity of the underlying assumption has been debated, and the result is sensitive to how long your urine collection window runs. Different labs use different protocols, which makes results hard to compare.⁴ It also tells you about the small intestine specifically, not your whole digestive tract.

Of everything here, this is the test I’d trust most to answer the literal question:

Is my gut more permeable than it should be?

3] Stool testing

What it measures: “Stool test” covers a lot of ground, so let’s separate the useful from the oversold.

The genuinely validated stool marker is calprotectin. It’s a protein released by a type of white blood cell, and it’s an FDA-approved, sensitive marker of inflammation in the gut.⁵ Doctors use it to help tell inflammatory bowel disease apart from IBS.

The big commercial “gut health” stool panels also report things like bacterial balance, yeast, and digestive markers. Some of that is interesting. Some is still research-grade dressed up as diagnosis.

What it can tell you: Calprotectin answers a real and important question: is there active inflammation in your gut? That’s worth knowing, and an elevated result is a reason to see a doctor, not to start a supplement.

What it can’t tell you: Calprotectin measures inflammation, not permeability. They’re related but not the same. A stool test can flag that something is inflamed without telling you whether your barrier is leaking, and a leaky barrier won’t always light up as inflammation. Treat the microbiome-mapping portions of consumer panels as clues, not verdicts.

4] Food sensitivity testing

What it claims to measure: These are the popular IgG panels that test your blood against dozens or hundreds of foods and hand you a color-coded list of things to avoid. They’re marketed heavily to exactly the person reading this.

What it can tell you: Honestly? Mostly what you’ve been eating.

What it can’t tell you: Whether those foods are actually a problem. This isn’t my opinion alone, although I’ve described my exact experience with it. The American Academy of Allergy, Asthma & Immunology, the European Academy of Allergy and Clinical Immunology, and the Canadian Society of Allergy and Clinical Immunology all advise against using IgG testing to diagnose food sensitivities.⁶⁷

The reason is almost the opposite of what the test implies: a high IgG to a food usually signals exposure and tolerance, that your body has seen the food and made peace with it, not that it’s harming you.

So when you eliminate everything that lights up, you often cut foods you tolerate perfectly well, narrow your diet for no reason, and feed an already anxious relationship with eating.

Here’s what actually finds your food triggers: a structured elimination and reintroduction. Pull the common suspects, let things settle, then add foods back one at a time and watch your real symptoms.

It’s free, it’s low-tech, and it’s the method I trust over any blood panel. And HERE’S exactly how to do it.

So which test should you actually do?

If you want the most direct measure of permeability itself: the lactulose-mannitol test.

If you want to know whether your gut is inflamed: ask your doctor about calprotectin.

And if you want to find your trigger foods: a careful elimination and reintroduction, not an IgG panel.

If a practitioner you trust orders zonulin as one piece of a larger picture: fine, just know its limits and don’t let one number run the whole show.

And before you spend a dollar on any of it, the most underrated test is still the one most people skip: actually tracking your own symptoms. What you eat, how you sleep, your stress, your bloating, your energy, day by day, for a few weeks. The patterns that fall out of that are often more useful than a lab report, and they cost you nothing.

The honest bottom line

Testing can be a helpful piece. It is not the whole story, and for leaky gut specifically, the technology is still catching up to the marketing.

So stop guessing, yes. Order the test that answers your actual question. But don’t outsource your judgment to a number, especially a number from a test that hasn’t earned it.

You know your body. Track it. Test it where testing is solid. And work with a doctor you trust on the rest.

That combination beats any single panel I’ve ever seen.

What gut health question do you want me to tackle next? Drop it in the comments or contact me.

If you liked this article on Leaky Gut testing, you might also enjoy:

1. 3 Ways to Know Your Leaky Gut is Healing
2. How Long Does it Take to Heal Leaky Gut [Plus 9 Ways to Troubleshoot Your Diet]
3. Best Leaky Gut Supplements: Top 15 in 2026

Xox,

SKH

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC8355880/ ↩︎
2. https://pubmed.ncbi.nlm.nih.gov/32563954/ ↩︎
3. https://www.gastrojournal.org/article/S0016-5085(24)05416-7/fulltext ↩︎
4. https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2025.1633500/full ↩︎
5. https://www.aafp.org/afp/2021/0900/p303 ↩︎
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC3443017/ ↩︎
7. https://pubmed.ncbi.nlm.nih.gov/18489614/ ↩︎