Glutathione is exclusively made inside cells and is produced in two steps:
- The first makes Glyteine from the amino acids glutamate and cysteine
- The second adds glycine to the Glyteine to make glutathione.
During aging and in many chronic illnesses, our cells lose the capacity to make enough Glyteine, which means cells do not produce sufficient glutathione to protect against oxidative stress .
Supplementation with glutathione will not increase cellular glutathione for one simple reason. In the tissues of the body, there are two very different environments. One is the fluid inside cells (intracellular, about 70% of the total fluids) and the other is the fluid outside cells (extracellular, about 30% of total fluids).
The intracellular environment, which is bound by cellular membranes, is where most of the essential reactions such as protein synthesis and energy production occur. Many of these reactions generate free radicals that can, if not controlled (neutralized), cause damage inside the body in the form of oxidative stress. To counter this, cellular glutathione levels must be maintained at an optimal concentration (homeostasis) and in a tightly controlled manner.
The extracellular environment, such as blood plasma, allows the transportation of nutrients to the cells and removal of waste products which in turn are processed in the liver and kidneys. The glutathione concentration found in this extracellular environment (micromolar) is lower than that found intracellularly (millimolar) by about a thousand-fold. This large concentration difference means that there is an insurmountable concentration gradient that prevents extracellular glutathione from entering cells.
So, when you take a glutathione supplement orally or by injection, it will ‘hang around’ in the extracellular environment but will not be able to enter cells intact to combat oxidative stress.
But what happens to all of that extracellular glutathione? Well, glutathione is made up of some valuable amino acids, and cells have a system of scavenging them before they are recycled. Most cells in the body have an outer membrane bound enzyme (gamma-glutamyltransferase)  which starts this process by breaking glutathione down to its constituent amino acids (protein building blocks).
Several researchers have confirmed this thesis. For example, one group  concluded that “dietary glutathione is not a major determinant of circulating glutathione, and it is not possible to increase circulating glutathione to a clinically beneficial extent by the oral administration of a single dose of 3g of glutathione”. Similarly, authors  of the first double-blind, randomized, placebo-controlled clinical trial of oral glutathione supplementation performed in healthy adult humans concluded that “despite the biochemical plausibility, optimal dose, recommended timing of administration and appropriate choice of outcome measures, no significant changes were observed in oxidative stress biomarkers or erythrocyte glutathione concentrations following 4 weeks of oral (2 x 500mg daily) glutathione supplementation”.
Glyteine, on the other hand, does not have this concentration gradient problem. It is found in roughly the same low concentration, both intracellularly and extracellularly. Once ingested, however, the extracellular Glyteine concentration increases, and therefore it can quickly diffuse through the cell membrane to the inside of cells. Once inside, it is immediately used to produce glutathione .
In summary, the problem in aging and most chronic disease is that our cells lose the ability to make enough Glyteine which results in insufficient glutathione production. Glutathione supplements cannot enter cells as there is an insurmountable concentration gradient across the cell membrane and, although there are well-understood membrane transporters for transporting glutathione from the inside to the outside of the cell, therefore most cell types, there are no transporters for transporting extracellular glutathione in the reverse direction i.e. from outside to inside the cell. However, Glyteine does get readily transported into the cell [5, 6].