Nitrogen balance in patients receiving either fat or carbohydrate for total intravenous nutrition

Computational modeling to predict nitrogen balance during acute metabolic decompensation in patients with urea cycle disorders

Nutritional management of acute metabolic decompensation in amino acid inborn errors of metabolism (AA IEM) aims to restore nitrogen balance. While nutritional recommendations have been published, they have never been rigorously evaluated. Furthermore, despite these recommendations, there is a wide variation in the nutritional strategies employed amongst providers, particularly regarding the inclusion of parenteral lipids for protein-free caloric support. Since randomized clinical trials during acute metabolic decompensation are difficult and potentially dangerous, mathematical modeling of metabolism can serve as a surrogate for the preclinical evaluation of nutritional interventions aimed at restoring nitrogen balance during acute decompensation in AA IEM. A validated computational model of human macronutrient metabolism was adapted to predict nitrogen balance in response to various nutritional interventions in a simulated patient with a urea cycle disorder (UCD) during acute metabolic decompensation due to dietary non-adherence or infection. The nutritional interventions were constructed from published recommendations as well as clinical anecdotes. Overall, dextrose alone (DEX) was predicted to be better at restoring nitrogen balance and limiting nitrogen excretion during dietary non-adherence and infection scenarios, suggesting that the published recommended nutritional strategy involving dextrose and parenteral lipids (ISO) may be suboptimal. The implications for patients with AA IEM are that the medical course during acute metabolic decompensation may be influenced by the choice of protein-free caloric support. These results are also applicable to intensive care patients undergoing catabolism (postoperative phase or sepsis), where parenteral nutritional support aimed at restoring nitrogen balance may be more tailored regarding metabolic fuel selection.

Nutrition in critical care

Critical care has evolved from a prolonged recovery room stay for cardiac surgery patients to a full medical and nursing specialty in the last 5 decades. The ability to feed patients who cannot eat has evolved from impossible to routine clinical practice in the last 4 decades. Nutrition in critically ill patients based on measurement of metabolism has evolved from a research activity to clinical practice in the last 3 decades. The authors have been involved in this evolution and this article discusses past, present, and likely future practices in nutrition in critically ill patients.

Effect of high-lipid high-nitrogen intravenous nutrition on total body nitrogen, visceral protein synthesis and nitrogen balance

The impact of high-lipid intravenous nutrition (IVN) on selected indices of nitrogen retention following major surgical resection was studied. Twenty-two patients, randomly allocated to two equal well matched groups, received either high-lipid IVN (75 per cent non-protein calories supplied as lipid) or isocaloric isonitrogenous glucose IVN (100 per cent non-protein calories supplied as glucose). Total body nitrogen (assessed by in vivo neutron activation analysis), nitrogen balance and levels of circulating proteins were measured. Mean(s.d.) total body nitrogen and fat-free mass decreased (P = 0.04) in patients receiving high-lipid IVN, -109(36) gN and -1.7(0.4) kg respectively, but not in those given glucose-only IVN, 8(43) gN and 0.1(1.0) kg. This small loss of body protein does not appear to be clinically significant because postoperative hospital stay, complication rates and the acute-phase protein response (immunological and visceral) were similar in the two groups.

Protein-sparing effect of substrate infusion in surgical patients is governed by the clinical state, and not by the individual substrate infused

We have determined iostopically the effect of either glucose or lipid infusion on rates of glucose turnover, glucose oxidation, and net protein catabolism (NPC) in three groups of surgical patients. Kinetic measurements were performed using the primed constant infusion of 3H-glucose, and either 14C-glucose or 14C-urea. The three groups included patients with: (1) sepsis and/or trauma (ST); (2) upper gastrointestinal cancer (UGI); (3) lower gastrointestinal cancer (LGI). In each patient group the effect of either glucose infusion (approximately 4 mg/kg.min) or lipid infusion (20% Intralipid lipid emulsion infused to provide calories approximately isocaloric to the glucose infusion) was assessed. The infusion of calories as either glucose or lipid was equally effective as a means of suppressing NPC in each individual patient group, and the degree of response was governed by the clinical disease state. In the LGI and ST patients the infusion of either glucose or fat resulted in a significant suppression of NPC (p less than 0.005) of approximately 15%. However, the ongoing rate of NPC that occurred despite substrate infusion was more than twice as great in the ST patients as in the LGI patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Energy and protein requirements of general surgical patients requiring intravenous nutrition

General surgical patients require intravenous nutrition either because their gastrointestinal tract is blocked, too short or inflamed or because it cannot cope. Such patients can be grouped into four nutritional/metabolic categories: normal and unstressed; normal and stressed; depleted and unstressed; depleted and stressed. The energy requirements of patients in each of these groups vary according to their energy expenditure. Normally nourished and stressed patients have the highest energy expenditure and therefore require the highest energy input (45-55 kcal.kg-1day-1). Other groups of patients rarely require more than 40 kcal.kg-1day-1. Energy can be given mainly as dextrose although calories needed above 40 kcal kg-1day-1 should be given as fat (unless lipogenesis is desirable). In very stressed patients high rates of glucose infusion can themselves constitute a metabolic stress and fat may play a bigger role as a calorie source. For long term feeding, 1 litre of 10 per cent fat emulsion should be given weekly to avoid essential fatty acid deficiency. The level of nitrogen intake required to maintain a positive nitrogen balance is a lot higher in surgical patients than the suggested recommended dietary allowances for normal subjects. It is dependent not only on the nutritional and clinical state of the patient but also on the levels of energy and nitrogen intake given. When energy intake is below energy needs, normally nourished patients cannot retain nitrogen, although depleted patients can. When energy intake exceeds energy needs, both normally nourished and depleted patients retain nitrogen at levels of nitrogen intake ranging from 250 mg kg-1day-1 (depleted and unstressed) to over 400 mg kg-1day-1 (stressed). Depleted patients can maintain a positive nitrogen balance at lower levels of calorie and nitrogen intake than normally nourished patients and in this respect are analogous to a growing child. In all surgical patients, energy and nitrogen intakes can be manipulated to provide for a controlled maintenance or restoration of either wet lean tissue and/or fat. There is little place for protein sparing therapy or the use of insulin and anabolic steroids to promote nitrogen retention in surgical patients requiring intravenous feeding.

Comparison of body protein metabolism during total parenteral nutrition using glucose or glucose and fat as the energy source

Direct measurement of body protein metabolism using L-(1-14C) leucine during total parenteral nutrition following cholecystectomy demonstrated no significant difference between isocaloric isonitrogenous regimens containing glucose alone (n = 5) or with a fat emulsion (n = 5) as the energy substrates. It would appear that providing the obligatory requirement for glucose is met (about 150g d(-1)), fat and carbohydrate calories are inter-changeable with regards to fueling protein metabolism in the early period following moderately severe trauma.

Studies with a safflower oil emulsion in total parenteral nutrition

The prevention of essential fatty acid deficiency and the provision of adequate amounts of energy are two major concerns in total parenteral nutrition. Since earlier preparations of fat emulsion used to supplement the usual regimen of hypertonic glucose and amino acids have widely varying clinical acceptability, a new product, a safflower oil emulsion available in two concentrations (Liposyn), was evaluated. In four clinical trials the emulsion was used as a supplement to total parenteral nutrition. In five surgical patients 500 ml of the 10% emulsion infused every third day prevented or corrected essential fatty acid deficiency; however, in some cases in infusion every other day may be necessary. In 40 patients in severe catabolic states the emulsion provided 30% to 50% of the energy required daily: 10 patients received the 10% emulsion for 14 to 42 days, 9 patients received each emulsion in turn for 7 days, and 21 patient received the 20% emulsion for 14 to 28 days. All the patients survived and tolerated the lipid well; no adverse clinical effects were attributable to the lipid infusions. Transient mild, apparently clinically insignificant abnormalities in the results of one or more liver function tests and eosinophilia were observed in some patients. Thus, the safflower oil emulsion, at both concentrations, was safe and effective as a source of 30% to 50% of the energy required daily by seriously ill patients.

Development of fat emulsions

The studies on the possibility of preparing IV fat emulsions began in the 1920's; since then, a very large number of emulsions with various fats and emulsifiers have been investigated. However, none of the early fat emulsions could be used safely in man. By the trial and error technique, a method was finally found to prepare soybean oil and egg yolk phospholipids to make a fat emulsion that had a low incidence of adverse reactions. A newly devised biological test system based on nutritional concepts was of crucial importance. It meant that any energy supplying nutrient should be tested in animals in relation to the energy requirement of the species used and not identical to the human dose/kg body weight as in the "classical" toxicity and tolerance tests. With the development of well-tolerated IV fat emulsions it has been possible to devise a TPN regimen that is adequate from a nutritional point of view as well as an adequate alternative to the ordinary oral food. This regimen, which can be administered by the peripheral route, means that the original aims for developing IV fat emulsions were finally achieved.

Indirect calorimetry as a guide to caloric replacement during total parenteral nutrition

Fifty patients were studied by indirect calorimetry to assess caloric needs. Seventeen patients received total parenteral nutrition (TPN) in a fixed dose of 45 +/- 3 kcal/kg. Thirty-three patients were studied after acute injury. Indirect calorimetry and the basal energy expenditure equation (BEEE) were compared. In male patients receiving TPN, indirect calorimetry more closely approximated caloric needs than did the BEEE X 1.75. In female patients, the BEEE X 1.75, indirect calorimetry value, and calories infused were more equivalent, and positive nitrogen balance was consistently achieved. In thirty-three trauma patients, indirect calorimetry and the BEEE were compared. Indirect calorimetry consistently predicted higher caloric expenditure than did the BEEE X 1.75. In assessing caloric requirements in acutely catabolic patients, the BEEE X 1.75 appears to be inadequate. The BEEE does not take into account changes in temperature or degree of illness. Indirect calorimetry is easy to perform and gives more pointed information about the patient's caloric needs.