A new study on oral glutathione:
...Witschi et al. (1992) have observed no increase in plasma GSH levels after a single oral supplementation of GSH to healthy human volunteers at 0.15 mmol/kg body weight.17 The present study confirmed these results [Figure 5(a) and (b)]. Based on these results, it has been suggested that the oral supplementation of GSH does not affect blood GSH levels.It has been demonstrated that plasma proteins, including albumin, can bind to low molecular weight thiol-compounds through a disulfide bond.21,22 Therefore, there is the possibility that supplemented GSH may be transported as a conjugate of protein in the blood, and this has not been examined. In the present study, the effects of the supplementation of GSH on plasma protein-bound GSH levels were examined......The present study also demonstrated that only a negligible amount of GSH was bound to plasma protein before the supplementation of GSH. However, the protein-bound GSH significantly (P < 0.01) increased from 60 to 120 min after the oral supplementation of GSH. This is the first report to demonstrate an increase in GSH in the human blood fraction by the oral supplementation of GSH. The protein bound form GSH level in plasma after supplementation of GSH is much higher (>1000 times) than other food-derived peptides such as Val-Tyr 25 and Ile-Pro-Pro 26, but less than the food-derived collagen peptides in human blood.It has been thought that orally administered GSH is successively degraded to cysteinyl-glycine, cysteine, and glycine by γ-glutamyl-transferase and peptidase.17,28,29 Cysteine could be used for GSH synthesis in cells. Increased levels of protein-bound GSH might be derived from the newly synthesized GSH. The present study also detected fragment peptide (Cys-Gly) and precursor peptide (γGlu-Cys) as protein-bound form in human blood, which suggests some GSH is synthesized from degradation products of GSH.However, an early study by Kubo (1968) that used 35S-labeled GSH and paper electrophoresis has suggested that GSH could be directly absorbed from the small intestine into rat portal blood.30 Therefore, there is a possibility that supplemented GSH is directly absorbed into human blood and bound to plasma protein. To solve these problems, further studies on the metabolic fate of supplemented GSH that use 13C-labeled GSH are in progress...
And, this was another study from earlier in the month: