You might want to revise your understanding, especially with regard to reduced glutathione.
I beg to differ. I don't think whether it is reduced or not matters in the case of the bioavailability of ingested glutathione, for several reasons:
1) Both the reduced form and the oxidized form of glutathione will be hydrolyzed in the digestive tract.
2) Even if some small portion of the ingested glutathione manages to escape the stomach and reach the small intestines before being hydrolyzed, whether it is found in its reduced or oxidized form will depend much more on the overall redox environment in the digestive tract (which will favor either form) rather than on what form was ingested.
3) Even if the ratio of reduced to oxidized glutathione were higher in the intestines after ingestion of the reduced form compared with the oxidized form, that still might not matter much because it may well be that only one of the forms is absorbed. F.ex. if only the reduced form were absorbed then the oxidized form would have to be converted to the reduced form anyways, before being absorbed, and the effect would be the same regardless of which form you consumed.
4) Regardless of what form is consumed, upon absorption, once the glutathione is inside the intestinal cells, the ratio of reduced/oxidized glutathione within the cell depends on the total redox environment inside the cells, wich usually strongly favors the reduced form. This is important because during absorption it isn't absorbed directly into the blood but has to enter the intestinal cells first.
See here--Reversal of age-associated decline in immune responsiveness by dietary glutathione supplementation in mice.
And here--Enhanced Glutathione Levels in Blood and Buccal Cells by Oral Glutathione Supplementation
Thanks for the references, and this goes for everyone else in this thread that provided peer reviewed references, I always appreciate that. However, while these two studies show increased glutathione concentrations in cells and blood of animals and humans, that does not prove it is bioavailable. It might as well have been hydrolyzed to it's corresponding amino acids, and after being absorbed the amino acids were then used by cells to produce glutathione. To determine this we would need to see a study administering radiolabeled glutathione or something similar to accertain that the glutathione that is found in the blood or tissues is the glutathione that was ingested not that which was merely produced in the body.
There's conflicting information on oral glutathione.
Thanks for the references. I don't have time to go over them in detail, but more comments on this below.
It seems like one would have to ingest many grams of oral glutathione to have any sort of effect, due to intestinal hydrolysis. Being an expensive substance, ingesting grams doesn't seem like a cost-effective, and therefore practical, method of raising glutathione levels. I have to agree with Olafur that these positive experiences seem to me to be the product of placebo.
I agree. One would likely need several grams of glutathione for it to have significant benefits, and that certainly is not cost effective.
There's conflicting information on oral glutathione.
No kidding! These groups of researchers have come to nearly opposite conclusions. And with the study you referenced, the p=.7 suggests that there was no difference at all between the two groups in gluthathione concentration. Which makes no sense, especially with the obvious benefits many are seeing from it.
I think you're being a bit too quick on judgement here. I don't see the info being very conflicting. I didn't have time to read through all the studies so correct me if I am wrong, but it appears to me that none of the studies used any method (such as radiolabeling) to measure whether the raised glutathione was caused by actually absorbed glutathione or merely by glutathione produced by amino acids supplied by the ingested glutathione. If you ingest glutathione and all of it were to be hydrolyzed an then large part of the amino acides absorbed and becoming available as substrate for glutathione production, how much glutathione ends up being produced and whether it reaches significance depends on several factors such as how healthy the animal or person is and whether the animal or person is already getting plenty of the amino acids needed for glutathione production or not. So it is not strange to me if some studies show a rise in glutathione upon ingestion while others do not.
Overall I still think glutathione is likely to have very poor bioavailability in humans, and because of this it is mostly worthless. However there are clearly some studies showing raised glutathione levels after oral ingestion of it and I think this is caused by it being produced by the amino acids that were formed by digesting the ingested glutathione with little to no part of this being caused by it being absorbed intact. Please note that I didn't have time to read over all the studies in this thread (I'm way too busy working on the Live120Plus project) but my points above are based on my prior knowledge on the metabolism of ingested compounds, the redox environment of various compartments of the body, and the general absorption mechanisms of various compounds. I think I will leave it at this. I hope this helped.