[Energy substrates in parenteral nutrition].
The most appropriate nutriment for total parenteral feeding (TPF) must be nutritionally efficient, safe and easy to use. Glucose is the most used carbohydrate as it has most of these qualities, as well as a high rate of metabolism by all tissues. It has not been clearly demonstrated that the administration of exogenous insulin with glucose improves nitrogen retention. Substitutes for glucose, such as fructose, maltose, galactose or polyols (xylitol, surbitol, glycerol) are not really superior to glucose itself. On the other hand, they have major side-effects. Therefore, they are not much used as energy substrates for TPF, at least not for long term TPF. Intravenous fat emulsions have taken an important place as a source of energy during TPF. Fat emulsions containing long chain triglycerides (LCT) supply essential fatty acids (EFA) (linolenic and linoleic acids), thus preventing EFA deficiency. The metabolism of fat emulsions is influenced by various factors: age, metabolic and nutritional status, the amount of glucose intake, insulin deficiency, sepsis, heparin therapy. Recently, medium chain triglycerides (MCT) have been proposed as an alternative energy source. The latter are cleared more rapidly from the blood, and are therefore less liable to be deposited in the liver and adipose tissue; they are also oxidized more quickly and more completely. MCT are safe to use at a rate of less than 0.12 g.kg-1.h-1 and with a MCT/LCT ratio less than 3 to 1. The simultaneous administration of glucose prevents an acceleration of ketogenesis. MCT/LCT emulsions are a safe and effective source of calories. It is important that those patients for whom such nutriment may be of particular interest should be identified. Fat emulsions associated with glucose seem to be more efficient in terms of nitrogen sparing effect than glucose alone. They also avoid the problems due to the infusion of large amounts of glucose (excessive carbon dioxide production, fatty infiltration of the liver), while there is no EFA deficiency. If the infusion of TPF nutriment must be continuous in intensive care patients, or during the postoperative period, cyclic nocturnal parenteral nutrition over a 12 or 16 hour period may be used in patients who are not in a catabolic state, or only mildly so. This is a safe and efficient method of nutritional support, which reduces the incidence rate of TPF-induced cholestasis.