The effect of gamma-tocopherol administration on alpha-tocopherol levels and metabolism in humans.

BACKGROUND

The bioavailability of gamma-tocopherol and metabolites of vitamin E after gamma-tocopherol administration is not well understood. We investigated the effect of gamma-tocopherol administration on the levels and metabolism of alpha- and gamma-tocopherol in healthy volunteers.

METHODS

We measured two metabolites of vitamin E (2,5,7,8-tetramethyl-2-(2'-carboxyethyl)-6-hydroxychroman (alpha-CEHC) and 2,7,8-trimethyl-2-(2'-carboxyethyl)-6-hydroxychroman (gamma-CEHC)) in plasma and urine by high-performance liquid chromatography with electrochemical detection (HPLC-ECD) during administration of gamma-tocopherol. Two groups of volunteers were enrolled. The gamma-tocopherol group received two gamma-tocopherol capsules (each containing 186.4 mg of gamma-tocopherol and 5 mg of alpha-tocopherol) for 28 days, while the control group received d-alpha-tocopherol at 5 mg/day, which was the same dose as that given to the gamma-tocopherol group. Blood and urine samples were obtained on days 0, 14, 28, 35, 42, and 56 after the initiation of gamma-tocopherol administration.

RESULTS

The plasma gamma-tocopherol concentration increased markedly during administration of gamma-tocopherol and the plasma gamma-CEHC concentration increased along with that of gamma-tocopherol. The plasma alpha-tocopherol concentration decreased significantly during gamma-tocopherol administration. The plasma concentration of alpha-CEHC decreased significantly and urinary excretion of alpha-CEHC tended to increase in the gamma-tocopherol group. Urinary sodium secretion was significantly increased at 1 week after the cessation of gamma-tocopherol administration, but there was no significant difference of urine volume between the two groups.

CONCLUSION

Metabolism of alpha-tocopherol is accelerated and the plasma alpha-tocopherol concentration is decreased during gamma-tocopherol administration.