Temporal relationships between dietary, plasma, hepatic, and adipose tissue lipids after short-term feeding of safflower oil to rats.

The objectives of this research were to study effects of dietary neutral fat on the concentration and the fatty acid composition of triglyceride (TG) and other lipid classes in plasma, liver, and adipose tissue and, particularly, to study the temporal and precursor-product relationship among those various lipid pools. Accordingly, male rats were administered 1.5 ml. of safflower oil by gastric intubation at 0, 4, and 8 hours after a 16-hour fast. Samples of plasma, liver, and adipose tissue were collected from groups of rats before fat feeding (0 time, controls), during the period of feeding (4 and 8 hours), and at several times thereafter up to 36 hours (28 hours after the last meal). The esterified lipids in all lipid fractions of liver and plasma became enriched with linoleic acid (18:2) during the experiment. The enrichment of plasma and hepatic TG, and of plasma FFA with 18:2 was substantial, reaching a maximum 8 hours after the third meal, whereas only a modest enrichment of adipose tissue TG was observed. The enrichment with 18:2 of various lipid fractions proceeded in the following sequence: total plasma TG became enriched first with 18:2 of chylomicron TG fatty acids during the period of active absorption of fat. Second, plasma FFA were enriched with 18:2 derived presumably from metabolism of chylomicron TG. Subsequently, enrichment of hepatic TG with 18:2 was observed. After maximum enrichment was attained, the percentage of 18:2 in these various lipid pools decreased in the following order. The percentage of 18:2 in plasma FFA fell rapidly, followed by a slower decrease of the 18:2 content of plasma TG and, finally, by an even slower decline of the 18:2 content of hepatic TG. All lipid fractions except plasma FFA contained an increased content of 18:2 even 28 hours following the last fattly meal. It can be deduced from these temporal relationships that plasma TG is derived indirectly from dietary TG long after active absorption of dietary fat has ceased. The composition of hepatic TG is altered by dietary neutral fat, through the intermediary uptake of plasma FFA derived from metabolism of chylomicron TG; this hepatic TG, which reflects dietary fat, is a significant precursor pool of the plasma TG in the postabsorptive state.