Recent developments in lipid emulsions: relevance to intensive care.

For years, intravenous lipid supply has been considered as a means of providing an efficient fuel to many tissues of the body and of preventing or correcting essential fatty acid deficiency. The potential for lipid emulsions to modulate cell function via their content of specific fatty acids and of liposoluble vitamins has not received much attention yet. Soybean [long-chain triglycerides (LCT)] emulsions provide a valuable source of energy, but they are excessively rich in omega-6 essential fatty acids (FAs). Their infusion is associated with an accumulation of linoleate and a reduction of long-chain (> or = C20) omega-6 and omega-3 fatty acids in cell membrane phospholipids, as well as with a depletion of antioxidant status, associated with a reduction of alpha-tocopherol in plasma lipoproteins. Infusions of the mixed medium-chain triglycerides (MCT)/LCT (50%:50%; wt:wt) largely bypass these disadvantages. In addition, plasma elimination of MCT/LCT is faster than that of LCT. Recent advances indicate a great potential for omega-3 FAs incorporated into membrane phospholipids to modulate cell response to various stimuli and to influence several intracellular metabolic processes. Furthermore, some of these FA directly influence the production and the action of important mediators, the eicosanoids. In practical terms, an increased intake of omega-3 FAs may reduce inflammatory and thrombotic responses while protecting tissue microperfusion and immune defenses. Such properties may find interesting applications in several types of intensive care unit patients, provided that omega-3 FA incorporation takes place promptly. We recently had the opportunity to study in vitro and in vivo the metabolism of emulsions made of a mixture of MCT, soybean LCT, and fish oil triglycerides. Plasma elimination of such preparations appeared to be very fast, and their infusion was not associated with a prolonged residence of emulsion particles. In addition, uptake of remnants enriched with omega-3 FAs and liposoluble vitamins was fairly fast and occurred in several types of cells, leading to an efficient incorporation of omega-3 FAs in cell membranes within a few hours. The understanding that remnant uptake plays a significant role in the delivery of components included in lipid emulsions opens new areas of investigation and is likely to find several conditions of applications for new types of preparations.