Background: The uptake and biotransformation of γ-tocopherol (γ-T) in humans is largely unknown. Using a stable isotope method we investigated these aspects of γ-T biology in healthy volunteers and their response to r-T supplementation.
Method: A single bolus of 100mg of deuterium labeled γ-T acetate (d2-γ-TAC, 94% isotopic purity) was administered with a standard meal to 21 healthy subjects. Blood and urine (first morning void) were collected at baseline and a range of time points between 6 and 240h post-supplementation. The concentrations of d2 and d0-γ-T in plasma and its major metabolite 2,7,8-trimethyl-2-(b-carboxyethyl)-6-hydroxychroman (γ-CEHC) in plasma and urine were measured by GC-MS. In two subjects, the total urine volume was collected for 72h post supplementation. The effects of γ-T supplementation on a- concentrations in plasma and α-T and γ-T metabolite formation were also assessed by HPLC or GC-MS analysis.
Results: At baseline, mean plasma α-T concentration was approximately 15 times higher than γ-T (28.3 vs 1.9µmol/l). In contrast, plasma γ-CEHC concentration (0.191µmol/l) was 12 fold greater than α-CEHC (0.016µmol/l) while in urine it was 3.5 fold lower (0.82 and 2.87 µmol, respectively) suggesting that the clearance of α-CEHC from plasma was more than 40 times that of γ-CEHC. After d2-γ-TAC administration, the d2 forms of γ-T and γ-CEHC in plasma and urine increased ,but with markedly inter-individual variability, while the d0 species were hardy affected. Mean total concentrations of γ-T and γ-CEHC in plasma peaked, respectively, between 0-9, 6-12 and 9-24h post supplementation with increases over baseline levels of 6-14 fold. All these parameters returned to baseline by 72h. following challenge, the total urinary excretion of d2-γ-T equivalents was approximately 7mg. Baseline levels of γ-T correlated positively with the post-supplementation rise of (d0 + d2) –γ-T and γ-CEHC level in plasma, but correlated negatively with urinary levels of (d0+d2)-γ-CEHC. Supplementation with 100mg γ-TAC had minimal influence on plasma concentrations of α-T and α-T related metabolite formation and excretion.
Conclusion: Ingestion of 100mg of γ-TA transiently increases plasma concentrations of r-T as it undergoes sustained catabolism to CEHC without markedly influencing the pre-existing plasma pool of γ-T nor the concentration and metabolism of α-T. These pathways appear tightly regulated, most probably to keep high steady-state blood ratios α-T to γ-T and γ-CEHC to α-CEHC.