Does leaky gut actually exist?
Leaky gut is a measurable biological phenomenon. Probiotics are the best-supported way to improve the gut barrier; the evidence for glutamine and special diets is far less convincing.
Increased intestinal permeability, better known as 'leaky gut', exists as a measurable biological phenomenon. Gut cells are connected by tiny sealing rings. When those rings loosen, bacterial substances and other molecules can pass through the gut wall and enter the bloodstream. Researchers measure this using several substances, including zonulin (a protein that controls the sealing rings), lipopolysaccharides (substances from bacteria that normally stay inside the gut), and a ratio of two sugars that tests how easily things pass through.
In liver disease and fatty liver, the gut barrier is demonstrably disrupted. Higher levels of zonulin in stool were associated with more severe liver fat, although the predictive value was modest. Whether a leaky gut makes liver disease worse, or the other way around, cannot be determined from this type of research.
Probiotics improve measurable signs of a leaky gut: zonulin fell in several studies, harmful substances leaking from the gut into the blood fell, and the gut wall became better at keeping things out. At the same time, markers of inflammation such as CRP also fell. The confidence in this evidence varies considerably: for the reduction in harmful substances in the blood, the researchers themselves judged the confidence to be very low; for zonulin, confidence was moderate. High variation between studies makes it difficult to say which probiotics work best and for whom.
Glutamine, popular as a gut supplement, showed no demonstrable effect overall on gut permeability. Only at doses above 30 grams per day, taken for a short period, was there a modest measurable effect. Standard supplement doses have not shown this. Butyrate (a substance gut bacteria produce from dietary fibre) convincingly improved the gut barrier in animal studies, but this has barely been tested in people. A low-fermentable-carbohydrate diet, well known for irritable bowel syndrome, did not change measured zonulin levels, although participants did report feeling better.
Zonulin is the most commonly used measure, but its reliability is a matter of debate. The threshold values used in laboratories may differ from what is meaningful in a specific population. And sometimes symptoms improve while zonulin does not change, or the other way around. That does not mean the measurement is useless, but it does mean you cannot rely on it blindly.
This answer draws on several large summaries of previously published studies, supplemented by two smaller studies in people and one study following participants over time. The evidence for the existence of increased gut permeability and its measurability is reasonably well supported. For the effects of probiotics, the researchers themselves rated the confidence in the evidence for the reduction of harmful substances leaking into the blood as very low, and for zonulin reduction as moderate; there was also high variation between the individual studies. For butyrate, the vast majority of findings come from animal studies, not from people. The evidence for glutamine is limited and predominantly negative at standard doses.
Are abdominal pain and bloating signs of an unhealthy gut?
Abdominal pain and bloating are a sign that your gut needs attention, but on their own they say little about how "unhealthy" that gut is. If you have prolonged or severe complaints, first have other causes ruled out and seek guidance on diet or treatment.
Can stress really make your gut sick?
Stress can really make your gut sick, from abdominal pain and unpredictable bowel movements to stomach complaints. If you have chronic gut symptoms, stress is a factor worth discussing with your doctor.
Can an unhealthy gut contribute to skin problems such as acne or eczema?
A disrupted gut microbiome is associated with acne and eczema, although that association has not yet been proven to be cause and effect. Eating more fibre and plant-based foods is a sensible first step; for targeted probiotics the evidence is still too mixed for a clear recommendation.
Do probiotic pills actually help your gut?
Probiotics genuinely help, but not for everything and not equally with every pill. The evidence is strongest for antibiotic-associated diarrhoea and irritable bowel syndrome; for Crohn's disease they demonstrably do not work.
Does eating flaxseed help your gut?
Through its fibre and lignans, flaxseed has a plausible positive effect on the gut, but for the gut as a specific target the evidence in humans is still limited. If you want to take concrete action, opt for at least 30 grams of ground flaxseed per day.
Are fermented foods and probiotics good for your gut?
Fermented foods, particularly yoghurt and kefir, show positive indications for gut health; for most other products such as kimchi and kombucha, this has not yet been demonstrated in people.