Parenteral fat emulsions are used clinically as a source of polyunsaturated fats to prevent or treat essential fatty acid deficiency in total parenteral nutrition, or as an energy source for partial replacement of calories which would otherwise be given as glucose in total parenteral nutrition.' During intravenous infusion of lipid emulsion, a transient increase in plasma triglyceride (TG) levels routinely occurs. Depending on the dosage level of fat administered, the degree of starvation, and the clinical status of the patient, the duration and magnitude of hypertriglyceridemia are variable. However, serum TG values generally approximate preinfusion levels within a few hours after fat infusion is terminated.
CLEARANCE OF FAT EMULSIONFat emulsion is cleared from the bloodstream in a manner similar to natural chylomicrons' and is dependent on lipoprotein lipase (LPL) activity. The rate-limiting step is the enzymatic hydrolysis of circulating protein-bound TG by LPL.' This reaction takes place at or near capillary endothelial cells, primarily in muscle and adipose tissue. The free fatty acids (FFAs) so released enter these tissues and, in the case of adipose tissue, are reesterified to TG and stored, while in muscle they can be readily oxidized for energy. In addition, an appreciable portion of the released FFAs recirculate bound to albumin, causing an increase in plasma FFAs which are readily utilizable as metabolic fuel in the liver, heart, or skeletal muscle. In the liver, circulating FFAs are also converted to very low density lipoprotein (or pre-{3-lipoprotein) and secreted into the plasma4 (Fig. 1).Hallberg2.5-8 investigated the pharmacokinetic parameters of soybean oil emulsion elimination from plasma in man. The removal rate was found to be similar to enzyme, or Michaelis-Menten, kinetics. That is, at very low concentrations of fat emulsion, the rate of removal from the plasma is dependent on the concentration of LPL present. However, when the concentration of emulsion is increased to a certain critical level representing satura-tion of the binding sites on LPL, a maximum elimination capacity is reached. In normal adults, this maximum elimination rate is about 3.8 g fat/kg body weight/24 hr, which corresponds with 35 kcal/kg body weight/24 hr. This maximum rate is increased after trauma and during periods of starvation. For example, the maximum clearance capacity has been shown to increase by 250% following abdominal surgery and by 50% following a 39-hr fast.Edgren and Meng9 showed that the elimination halflives of chylomicrons and soybean oil from the plasma of dogs are similar; the values were approximately 14 and 15-16 min, respectively. The half-lives of safflower oil and soybean oil emulsion have also been studied by other investigators 10 (Table I).The metabolism of soybean oil emulsion has been studied in neonates. Andrew et al (11) studied 27 neonates during the first 48 hr of life. The infants were administered the emulsion at 1 g fat/kg body weight/4 hr. Differences in metabolism of the emulsion w...
