Although there is less research into it than l-glutamine, there is rapidly growing interest into the effects of creatine supplementation on dysfunctions of the epithelial barrier (which would include intestinal permeability aka "leaky gut") [1]. The upshot is that it is an well studied supplement that is very well tolerated by almost everyone.
Zinc carnosine is another one that can be helpful, if tolerated. However it is believed that enteric-coated formulations are required to be effective (which may be hard to find) due to very high absorption rates and susceptibility to stomach acid prior to reaching the intestines [2].
[0] https://www.mayoclinic.org/drugs-supplements/glutamine-oral-...
[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8145094/
[2] https://www.sciencedirect.com/science/article/pii/S221074012...
- Gastroparesis
- Thiol intolerance (impaired sulfur metabolism) - could try molybdenum and maybe Boron supplementation (ask doctor if ok)
- Leaky gut -> undigested proteins enter blood stream -> autoimmunity (eg via viral mimicry; all kinds of viruses can do that, but COVID and EBV come to mind). Water fasting could help, maybe digestive enzymes
- Mechnical interaction, eg dysbiosis -> bloating -> cervical spine misalignment -> more bloating due to vagal nerve pinching (try massaging back of the neck vigorously)
- Auto-brewery syndrome
All of those can be downstream issues of systemic issues, like hypothyroidism (which in turn is often caused by eg microbiome imbalance; I wouldn’t just down the next best yogurt though, but look at studies around Bifidobacterium Longum, which looks promising; probably others too).