Fat metabolism in injury and stress

Fat metabolism is a complex mechanism involving energy production as well as various physiological and biochemical effects involved in a large spectrum of both normal and pathological phenomena. Many different paths are currently being investigated by different medical and basic science disciplines. These include lipoproteins, prostaglandins, leukotrienes, and more. This chapter is limited to the energy related facets of fat metabolism, though in the future interrelations may be found between the different fields of fat research and serve not only for better understanding of the human organism, but also better treatment in hypermetabolic states. Much has been learned about fat metabolism both in normal and stressed man. The finding that, in most hypermetabolic states, fat is being oxidized and used as the main caloric source is definitely a cornerstone in improving nutritional therapy in such cases. The development of safe fat solutions, together with the studies showing that exogenous fat is efficiently utilized, has led to increased use of fat in TPN regimens as well as in enteral nutrition. The possibility of tilting the energy source in nutritional therapy from a 50:50 ratio of carbohydrates:fat to a fat-dominant regimen in the steady flow state (thus approaching the actual metabolic preference in stress) requires further studies and may prove to be beneficial. On the other hand, an increasing number of observations show that in late severe stress states--the so-called multi-organ failure syndrome--significant alterations in fat metabolism occur both in liver and in muscle. Recognizing the situations in which fat oxidation is decreased, leading to liver failure with fat accumulation, necessitates a different mode of treatment. Providing glucose or amino acids, and possibly more BCAA, is one method of treatment to be pursued. On the other hand, the prospect of new fat solutions containing medium chain triglycerides, may lead to improved nutritional support and decreased complications in these extreme situations. Other treatment modalities, like carnitine, need further research as well. Furthermore, if and when the relationship between fatty acids used in nutritional support and the various mediators involved in trauma and sepsis can be elucidated, better treatment of critically ill patients may be possible and offered in a more comprehensive way.