Supplementing has been difficult, as research suggests that glutathione taken orally is not well absorbed across the GI tract. In a study of acute oral administration of a very large dose (3 grams) of oral glutathione, Witschi and coworkers
found that "it is not possible to increase circulating glutathione to a clinically beneficial extent by the oral administration of a single dose of 3 g of glutathione."
. However, tissue and serum glutathione concentrations can be raised by increased intake of the precursor cysteine
. Glutathione precursors rich in cysteine include Acetylcysteine
(NAC), undenatured whey protein 
and N-acetyl-cysteine 
have been shown to increase glutathione content within the cell. N-acetylcysteine
is a generically available supplement which has been demonstrated to increase intracellular reduced and total glutathione by 92% and 58% respectively. 
All of the published clinical studies using bioactive whey proteins mentioned in the references above used a form of a bioactive whey protein and bonded cystine
dietary supplement derived from lactose
milk (whey protein) called Immunocal. This whey protein is clinically proven to increase glutathione levels within the lymphocytes
of the immune system by 35.5% while increasing peak power
and muscular performance by 13%.