During the Absorptive Phase, Triglyceride Accumulation in Adipose Tissue Occurs by Two Mechanisms: Uptake from Very-Low-Density Lipoproteins and Direct Lipid Synthesis from Glucose
TrigIyccride fatty acids arc transferred from chylomicrons and VLDLs to adipose tissue by the action of lipoprotein lipase (LPL). This enzyme resides on endothelial surfaces of capillaries and, when activated, binds to chylomicrons and VLDLs, catalyzing the hydrolysis of fatty acids from their core triglycerides and allowing the transfer of those fatty acids to the surrounding tissues.
The sensitivity of LPL to specific hormones varies in different tissues. Adipose tissue LPL is stimulated by insulin; thus, during the absorptive phase, fatty acids from chylomicrons and VLDLs are selectively transferred to adipose tissue. Therefore, under the influence of insulin, excess carbohydrate and amino acids are converted to fatty acids in the liver, and those fatty acids are subsequently transported, via VLDLs, to the adipose tissue. Similarly, chylomicron triglyceride arising from intestinal fatty-acid absorption is also selectively transported to adipose tissue, under the influence of insulin.Adipose tissue fatty acids may also arise from direct synthesis in addition to uptake from chylomicrons and VLDLs. Adipose tissue cells are metabolically active, and under the influence of insulin, they take up glucose. Within the adipocytes, glucose can be converted to fatty acids by the same metabolic mechanisms by which fatty acids were synthesized in the liver. In addition, acetate from fermentative digestion also can serve as a substrate for fatty-acid synthesis in adipose tissue (see later discussion of the special fuel considerations of ruminants). Thus there are two major sites of fatty-acid synthesis in the body: liver and adipose tissue. The relative importance of these sites varies with species.