after oral ingestion of a capsule and a sustained-release product. Their results indicated that the extent of absorption from the sustained-release product was almost twice that from a regular capsule. In addition, -98% of the sustained-release dose was absorbed compared to an intravenous dose. The latter value is unusually high and we are not aware of any study where an oral dose of ascorbic acid is virtually completely absorbed. Another discrepancy is that the ascorbic acid half-life was determined (27) to be -11 hr prior to saturation and 29 hr after saturation. These values are considerably greater than those reported for the half-life of the vitamin after exogenous administration (11,17,26,30). The reason for the substantial differences in half-life and availability from the timed-release products reported here and by Zetler et al. ( 27) is not known.
The results of this study indicate that ascorbic acid absorption is incomplete after oral ingestion and that there is considerable intersubject variation in the extent of absorption. In addition, absorption of the vitamin is considerably less efficient from the timed-release capsule examined here compared with the other oral forms. These findings have several implications. From a practical point of view, efficient oral therapy with the vitamin can be achieved by dissolving powdered ascorbic acid in water. In addition, for the specific manufacturers' products examined in this study, tablets and chewable tablets appear comparable to a solution of the vitamin. This conclusion may not apply to tablets made by all manufacturers but that can only be determined from the evaluation of other products in a manner used in the present study. The timed-release capsule examined here appears to be a more expensive and less reliable means of providing oral vitamin therapy compared with other more conventional dosage forms. This conclusion may apply to similar dosage forms which attempt to delay or sustain the release of the vitamin; therefore, bioavailability studies for such forms are essential.
A question that remains to be answered is to what extent does variation in ascorbic acid absorption influence the results of large-scale trials designed to examine the clinical effects of the vitamin? To the authors' knowledge no consideration has been given to absorption as a parameter potentially influencing the findings of such studies. Investigators pursuing such trials should give some consideration to the possible influence of variation in ascorbic acid absorption on clinical outcome.