Leaky Gut: What Intestinal Permeability Is, and What the Research Actually Supports

"Leaky gut" has a reputation problem. Search it and you'll find it blamed for nearly everything, with a supplement for sale at the bottom of every page. That's made a lot of doctors dismiss the whole idea. They shouldn't. Increased intestinal permeability is real and measurable. It's just more specific, and less of a cure-all, than the internet suggests.

What intestinal permeability actually is

Your intestinal lining is a single layer of cells, held together by structures called tight junctions. Its job is selective: let nutrients and water through, and keep bacteria, bacterial fragments, and partly digested food on the right side. A layer of mucus and a population of immune cells add more protection.

When that barrier loosens, more of what's in the gut can cross into the tissue beneath it, where the immune system responds. That's increased intestinal permeability, the medical term for what people call leaky gut. A low-grade, ongoing immune response is the concern, not a literal hole.

What the research supports

A thorough 2019 review in Gut by Dr. Michael Camilleri at Mayo Clinic is the clearest summary I know of. The key points:

• Increased permeability is present in inflammatory and ulcerating gut diseases, and it has been linked to many other conditions.
• Endurance exercise, NSAIDs like ibuprofen, pregnancy, and some food additives such as emulsifiers can all increase permeability.
• Dietary factors can reverse leakiness in those non-inflammatory stress states.
• No disease has been cured simply by restoring the barrier, and it's still unproven that fixing permeability on its own improves symptoms.

In other words, a leaky barrier is best seen as one contributor and a reasonable target, not the single root cause of everything. That's the frame I use.

Why it matters for women in midlife

Estrogen plays a role in maintaining the gut lining, and its decline in perimenopause and menopause is one of several reasons digestion changes in your 40s. Stress hormones, poor sleep, alcohol, regular NSAID use for headaches or joint pain, and a less diverse microbiome all push in the same direction. Many women are dealing with several of those at once.

The problem with zonulin tests

Zonulin is a protein involved in opening the tight junctions, and blood or stool zonulin tests are widely sold as a leaky gut test. The trouble is that the tests may not measure zonulin at all. A 2019 study in PLoS ONE analyzed what two commercial zonulin assays were actually detecting and found mostly other proteins, including complement C3 and haptoglobin. The authors advised caution in using them as a marker of gut barrier function.

So a zonulin result on its own shouldn't drive treatment. The better approach is to look at the whole picture: symptoms, what's driving them, and testing for the causes we can measure well, like inflammation, infections, or an imbalanced microbiome. A comprehensive stool test covers several of those at once.

What helps the gut lining

The basics carry most of the weight, and they're well supported:

• Remove what's irritating it. Cut back on regular NSAID use where you safely can, along with alcohol and heavily processed foods with emulsifiers. Address real food triggers if you have them.
• Feed the microbiome. Fiber from a wide range of plants feeds the bacteria that make butyrate, a fatty acid that fuels the cells of the gut lining.
• Add fermented foods. In a 17-week Stanford trial published in Cell in 2021, a diet high in fermented foods steadily increased microbiome diversity and lowered inflammatory markers.
• Treat what's underneath. SIBO, gut infections, celiac disease, and unmanaged stress all keep the barrier under pressure. Fixing the cause matters more than any supplement.
• Sleep and stress. Both directly affect gut barrier function, which is why they're part of every gut protocol I build.

Targeted nutrients such as glutamine, zinc, or specific probiotics can play a supporting role. They work best after the basics are in place. Professional-grade options are in my Fullscript dispensary.

When to look deeper

If you have ongoing bloating, irregular stools, food reactions that keep multiplying, skin flares, or joint aches with no clear cause, it's worth investigating your gut properly rather than guessing. And blood in the stool, unintentional weight loss, or symptoms that wake you at night need a medical evaluation first.

References

1. Camilleri M. “Leaky gut: mechanisms, measurement and clinical implications in humans.” Gut, 2019. View source
2. Ajamian M, et al. “Serum zonulin as a marker of intestinal mucosal barrier function: May not be what it seems.” PLoS ONE, 2019. View source
3. Wastyk HC, et al. “Gut-microbiota-targeted diets modulate human immune status.” Cell, 2021. View source