Dietary glutathione intake in humans and the relationship between intake and plasma total glutathione level

Glutathione may function as an anticarcinogen by acting as an antioxidant or by binding with cellular mutagens. Orally administered glutathione increases plasma glutathione levels, and plasma glutathione is also synthesized in the liver. To investigate the associations between glutathione intake and plasma glutathione level, we compared dietary intake estimates from food frequency questionnaire data and measured concentrations of plasma total glutathione and other serum antioxidants in 69 white men and women. Daily glutathione intake ranged from 13.0 to 109.9 mg (mean 34.8 mg). Fruits and vegetables were found to contribute over 50% of usual dietary glutathione intake, whereas meats contributed less than 25%. Small negative correlations were observed between dietary and plasma glutathione and, although they were usually not statistically significant, they were generally consistent by different time periods of dietary intake assessment. Adjustment for sex, age, caloric intake, and dietary intake of the sulfur-containing amino acids methionine and cystine did not alter the observed associations. The correlations appeared to be modified, however, by serum vitamin C concentration, with little or no association between dietary and plasma glutathione among those with lower levels of serum vitamin C and stronger negative correlations among those with higher serum vitamin C levels. These findings indicate that factors regulating plasma glutathione concentration are complex and not simply related to dietary glutathione intake or supply of precursor amino acids.