Measurement of the rate of fat absorption.

The rate of fat absorption has generally been measured either directly by determining the amount of lipide remaining in the alimentary tract after feeding fat, or indirectly by following the appearance of fat in the blood stream. The latter measurement is known to be influenced by factors other than the actual absorption of fat, such as the rate of removal of the lipides from the blood. This rate, of course, is determined by the speed of lipide utilization and deposition, as well as any mobilization from the tissues. Hence, the direct recovery by an adaptation of the Cori (1) method, introduced by Irwin et al. (2) and later modified by Deuel and coworkers (3), seems to have been preferred in studies of fat absorption with animals. The findings of the present study indicate that the results obtained with the direct method may also be influenced by factors other than the absorption of fat. Lipide recovered from the alimentary tract shortly after feeding C14-labeled fat contained considerably less activity per mg. of lipide than did the fed fat. A similar finding has been reported by Borgstrom (4). These observations suggested that it might be worth while to compare the results obtained with a number of different methods which have been used for determining, either directly or indirectly, the rate of fat absorption. Then, too, it would be advantageous to have additional information regarding the limitations of some of these indirect methods since the direct procedure is necessarily limited to studies with animals. The various determinations studied have involved the appearance of the ingested labeled fat in the blood and tissues, its disappearance from the alimentary tract, and its utilization. The rate of appearance of the fat in the blood was measured by both the chylomicrograph and the activity of the blood, whereas its disappearance from the alimentary tract was determined by the amount as well as the activity of the fat recovered. The excretion of the labeled carbon dioxide was employed as a measure of utilization.