Leaky gut syndrome is a diagnostic entity popular in various branches of alternative medicine. Its proponents hypothesize that damage to the bowel lining, caused by antibiotics, toxins, poor diet, parasites or infection (e.g. with the yeast Candida albicans) can lead to increased permeability of the gut wall to toxins, microbes, undigested food, waste or larger than normal macromolecules. Some versions posit that these substances affect the body directly, while others postulate an immune reaction to these substances. While many practitioners maintain that leaky gut syndrome is a medical condition, the area of "gut problems" lies between conventional and alternative medicine, including other diagnostic titles such as Small bowel bacterial overgrowth syndrome or small intestine bacterial overgrowth (SIBO), and "yeast syndrome" or systemic candidiasis, remains controversial and scientifically unsettled.

The concept of increased gut permeability itself is well recognized in various conditions, such as hepatic encephalopathy. It has been discovered that Zonulin regulates intestinal permeability. Symptoms: diarrhea, pain in the upper and lower abdomen (centered near appendix and sometimes confused with appendicitis), fever, headaches, malabsorption leading to malnutrition, steatorrhea (excess fat in the stool), which results in pale, bulky stools that float, loss of energy, appetite and weight.

When the large intestine is involved, chronic bleeding often occurs which may lead to anemia, and after years increases the risk of colon cancer. If the ulcerated intestinal wall leaks, peritonitis can result. Fistulas (abnormal channels) can occur, creating an opening between the intestine and bladder. Mouth and anal sores often are present in about many when the disease is active. Strictures can occur that partially block the bowel. About a third of those with the disease, develop cracks in the lining of the mucus membrane around the anus.