Production of Glutathione (GSH) in a cell depends on the availability of glutathione precursors, three amino acids glutathione is made of.
These glutathione precursors are glutamate, glycine and cysteine. The first two, glutamate and glycine, are non-essential amino acids because they are manufactured by the body itself, however they can also be consumed and are readily available in the foods we eat.
The all-important cysteine is classified as a non-essential amino acid too because it can be produced in the body from another amino acid - methionine. But the interesting fact is that methionine is an essential amino acid because the body does not produce it.
Methionine enters the body exclusively with the food we eat, which makes cysteine an essential amino acid too in a way. In this way methionine may serve as a building block for glutathione as well, but it also has a tendency to convert into homocysteine, which increases the risk of heart disease. That is why excessive consumption of methionine is not advisable, and methionine cannot be viewed as a building block for glutathione.
For a cell to be able to manufacture glutathione these three GSH precursors - glutamate, glycine and cysteine - absolutely must be in a form that can be transported from the mouth, through the digestive system, into the blood and finally through the cell wall where they are combined into glutathione.
As I said above glutamate and glycine are readily available in most Western diets; cysteine, however, is a limiting factor of GSH production.
One reason cysteine is a limiting factor for GSH is that cysteine does not survive the trip to the cell through the digestive system very well. Most cysteine is broken down or altered somewhere along the long trip from the mouth to the cell. That is why it is important to consume cysteine in a form that resists alteration and is strong enough not to be broken down in the digestive tract.