Cholesterol balance studies were carried out in 11 patients with various types of
hyperlipoproteinemia to determine the mechanism by which
unsaturated fats lower plasma
cholesterol.
Unsaturated fats produced no increase in fecal endogenous neutral
steroids in 10 of 11 patients and no decrease in absorption of exogenous
cholesterol in 5 patients who received
cholesterol in the diet. In 8 of 11 patients no changes occurred in excretion of
bile acids during the period on
unsaturated fat when plasma
cholesterol was declining. However, in 3 of 11 patients small but significant increases in
bile acid excretion were found during this transitional period; in 2 others increases also occurred after plasma
cholesterol had become constant at lower levels on
unsaturated fat.Since the majority of patients showed no change in
cholesterol or
bile acid excretions during the transitional period, we propose that when excretion changes did occur they were probably not the cause of the plasma
cholesterol change. Furthermore, turnover data and specific activity curves suggested that
cholesterol synthesis was not influenced by exchange of
dietary fats. Thus, excluding changes in excretion and synthesis, we conclude that it is most likely that
unsaturated fats cause plasma
cholesterol to be redistributed into tissue pools. We have also examined the possibility that
cholesterol which is redistributed into tissues could be secondarily excreted as neutral
steroids or
bile acids. In at least 5 of 11 patients excretion patterns were consistent with this explanation. However, we cannot rule out that excretion changes may have been due to alterations in transit time, to changes in bacterial flora, or to transitory changes in absorption or synthesis of
cholesterol or
bile acids. Our conclusion that
unsaturated fats cause a redistribution of
cholesterol between plasma and tissue pools points to the necessity in future to explore where
cholesterol is stored, to what extent stored
cholesterol can be mobilized, and to define the factors governing these fluxes.