A single 3 g dose of glutathione by mouth did not raise blood plasma glutathione
This 1992 human study gave seven healthy volunteers a single dose of glutathione by mouth and followed their blood for four and a half hours. Plasma glutathione did not rise significantly, and the authors concluded that glutathione swallowed in one dose is barely absorbed into the circulation.
What they did
Seven healthy volunteers took one dose of glutathione by mouth, 0.15 mmol per kilogram of body weight, which the authors describe as 3 g. The researchers measured glutathione, cysteine and glutamate in plasma, the liquid part of blood, before the dose and over the following 270 minutes.
What they found
Before the dose, plasma glutathione was 6.2 µmol/L, cysteine 8.3 µmol/L and glutamate 54 µmol/L. None of the three rose significantly in the 270 minutes after the dose.
The authors concluded that the systemic availability of oral glutathione, the share that reaches the circulation, is negligible in people. They put this down to glutathione being broken apart in the gut and liver by an enzyme called gamma-glutamyltransferase, and said a single 3 g dose cannot raise circulating glutathione to a clinically useful level.
What it can't tell us
This was a single dose in seven people, followed for under five hours. It cannot say what happens with daily doses over weeks or months, which is what Richie 2015 later tested. It measured plasma only, not glutathione inside blood cells or in tissues such as cartilage.
The gut and liver breakdown is the authors' explanation; the abstract does not report measuring it directly.
Why it matters
This is the study most often cited for the view that glutathione taken by mouth is not absorbed. That view shaped decades of research towards supplying glutathione's building blocks instead. A later six-month trial (Richie 2015) came to a different conclusion with daily dosing, so the two are best read together. They differ in design: one dose versus months of daily use, and plasma alone versus several blood compartments.
The paper
- Witschi A, Reddy S, Stofer B et al. The systemic availability of oral glutathione. Eur J Clin Pharmacol 1992.
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