Changes of serum apolipoprotein levels after oral administration of fat in human subjects.

Changes of serum apolipoprotein levels were studied every hour for 6 h after the administration of 55 g butter to 8 healthy male subjects. The mean serum apo A-IV level was significantly increased at 4 h (P less than 0.05) after fat ingestion compared to the mean initial level, although no significant changes of levels in other apolipoproteins (apo A-I, A-II, B, C-II, C-III, and E) were observed. The mean apo A-IV level in the triglyceride-rich lipoprotein (TRL) fraction (d less than 1.006) increased progressively over 6 h. In all 8 subjects, the time of peak concentration of apo A-IV in TRL fraction was delayed by 1-2 h compared to that in whole serum. On the other hand, the mean apo B-48 level in the fraction reached a peak at 4 h. These results raise the possibilities that some apo A-IV, newly synthesized or already existing in intestinal cells, may be directly secreted into the venous circulation and that apo A-IV and apo B-48 may distribute differently in different sizes of chylomicron. Alternatively, the amount of each apolipoprotein synthesized may depend upon the content of ingested fat. It is suggested that apo A-IV production by intestinal cells does not appear to be regulated by the rate of fat transport, and that apo A-IV does not play an important role in chylomicron formation compared to apo B-48.