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Protein sparing therapies were developed to mitigate the harms associated with protein-calorie malnutrition and nitrogen losses induced by either acute illness or hypocaloric diets in patients with obesity. We review the development of protein sparing therapies in illness and obesity with a focus on the pioneering contributions of George Blackburn, MD, PhD. He recognized that protein-calorie malnutrition is a common and serious clinical condition and developed new approaches to its treatment in hospitalized patients. His work with stable isotopes and with animal models provided answers about the physiological nutritional requirements and metabolic changes across a spectrum of conditions with varying degrees of stress and catabolism. This led to improvements in enteral and parenteral nutrition for patients with acute illness. Blackburn also demonstrated that lean body mass can be preserved during weight loss with carefully designed very low calorie treatments which became known as the protein sparing modified fast (PSMF). We review the role of the PSMF as part of the comprehensive management of obesity., (Copyright © 2017 Elsevier Inc. All rights reserved.)

It is now generally accepted that early feeding post injury exerts a positive effect on outcome in the critically ill, despite the fact that many of these patients are well nourished or even overnourished on admission. One possible mechanism is that early feeding post injury may have a positive influence on the duration and intensity of the systemic inflammatory response, especially when coupled with intensive insulin therapy to maintain normoglycemia at <150 mg/dL. Current clinical nutrition guidelines recommend early enteral nutrition providing full nutrition requirements in the critically ill patient; however, in the first week post injury, exclusive enteral feeding is typically inadequate, particularly in protein. A potentially new and different therapeutic goal to modulate the systemic inflammatory response might be more effectively accomplished for the first week post injury by hypocaloric feedings (~9-18 kcal/kg or 50%-75% resting metabolic expenditure) principally as intravenous dextrose but with at least 1 g/kg protein as intravenous amino acids to provide early metabolic support. This proposed regime, along with intensive insulin therapy to maintain glucose homeostasis, should promote the protein synthetic component of the postinjury inflammatory response while reducing net protein catabolism. A formal trial of early metabolic support in the acutely injured should be safe, easy to execute, and potentially efficacious, with subsequent improvement in the inflammatory state and, it is hoped, clinical outcomes.

This study determined the effects of chronic administration of the two principal proximate cytokines, interleukin-1 (IL-1) and tumor necrosis factor (TNF), on host wasting in rats. The effects were compared with those observed in a matched pair-fed group to distinguish the contribution from anorexia. Both TNF and IL-1 produced weight loss, net nitrogen loss, and skeletal muscle catabolism and increased liver weight. Such effects were independent from and additive to those resulting from semistarvation. However, under equivalent nutritional conditions, TNF infusion led to a greater effect on muscle protein catabolism and liver weight and caused liver protein anabolism, whereas only the group receiving IL-1 had altered glucose metabolism in the postabsorptive state. Tachyphylaxis was seen in the response of food intake over time after administration of IL-1. These actions define the two principal mechanisms for the development of protein calorie malnutrition that occur because of cytokine action, anorexia producing semistarvation and an increased net protein catabolic rate reducing anabolic efficiency.

GH has been shown to stimulate tyrosine phosphorylation of JAK2, several STAT proteins, insulin receptor substrate-1 (IRS-1), and SHC proteins in cultured cells. The goal of this study was to determine GH effects on protein tyrosine phosphorylation in liver and skeletal muscle of normal rats in vivo. Nonfasted male Sprague-Dawley rats (225-250 g) were injected with GH iv, and tissues were obtained after 5, 15, 30, or 60 min. At a maximally effective GH dose (1.5 mg/kg body weight), phosphotyrosine antibody immunoblots demonstrated marked stimulation of the tyrosine phosphorylation of JAK2 (maximal at 5 min) and a 95,000 Mr protein (maximal at 15 min) in both liver and skeletal muscle. The 95,000 Mr protein was recognized and immunodepleted by STAT5 antibody, but not by other STAT protein antibodies. Although basal tyrosine phosphorylation of IRS-1 and SHC was evident, GH did not stimulate tyrosine phosphorylation of either of these proteins in liver or skeletal muscle. In conclusion, GH stimulates the tyrosine phosphorylation of JAK2 and STAT5, but not IRS-1, SHC, or other STAT proteins in liver and skeletal muscle of normal rats. These results differ from findings in cultured cells and support the concept that selectivity for tyrosine kinase substrates is an important determinant of postreceptor signaling specificity in vivo.

In this study, we fed rats a 2% casein AIN 76 diet for 2 wk to produce protein malnutrition. We determined in these animals the effects of different concentrations of dietary protein refeeding (2% and 20% casein) on recovery and gut mucosal repletion and the potential role of type of dietary fat in the regulation of protein metabolism and mucosal growth by providing conventional long-chain triglyceride (LCT), a structured lipid composed of long-, medium- and short-chain fatty acids (SC/SL), or a physical mixture of the same components present in the structured lipid given as individual pure triglycerides (SC/PM) along with adequate amounts of protein and energy. The results confirmed that protein malnutrition can be reversed rapidly by protein refeeding, as indicated by an increase in body weight, positive nitrogen balance, liver growth and elevations in plasma concentrations of insulin-like growth factor-1, leucine and albumin. In the colon, crypt cell number, crypt depth and number of crypt cells in the rapidly proliferating fraction of the colon were greater in rats fed the higher protein diet. However, the general architecture of small intestinal mucosa, including duodenum, jejunum and ileum, was not affected by protein malnutrition. Although the number of colonic cells was similar with fat refeeding, there were significantly fewer displaying the proliferating cell nuclear antigen in the colonic epithelium when rats were fed SC/PM compared with SC/SL. Therefore, changes in colonic mucosal proliferation were only seen with repletion by adequate protein and by SC/SL feeding.

The effects of recombinant interleukin-1 (IL-1), given as a continuous infusion for 6 days, on host responses were determined in rats. The development of fever, change in food intake and body weight, and key components of the acute-phase response in energy and protein metabolism were assessed. The effects of IL-1 were compared with those observed in a matched pair-fed group (semistarved), to distinguish the contribution from anorexia, and in a group that received IL-1 for 4 h acutely. IL-1 significantly increased core temperature, plasma levels of IL-6, and acute-phase protein production and decreased food intake and the circulating zinc level. The catabolic effects of IL-1 on nitrogen loss and muscle protein breakdown were independent of, and additive to those from malnutrition. The changes in energy expenditure, cumulative urinary nitrogen, and hydroxyproline excretion in the chronic IL-1 group were increased over semistarved animals. Finally, changes in muscle protein kinetics were only seen with chronic IL-1 infusion, and the changes in acute-phase protein were greater.

In this study, 20 micrograms.kg.-1.h-1 of recombinant human insulin-like growth factor I (IGF-I) was infused into normal healthy rats to examine the effects of IGF-I on glucose, protein, and energy metabolism in either overnight-fasting or parenteral-feeding states. The fed state was maintained by continuous intravenous feeding of a complete diet containing 838 kJ.kg-1.d-1, 2 g N.kg-1.d-1, and no fat. At each nutritional state, one-half of the animals received IGF-I infusion while the other half received saline as control. [14C-1]leucine and [3H-6]glucose were used to determine the effects of IGF-I on protein and glucose kinetics during fed and fasting states. The results showed that 1) infusion of IGF-I at this amount did not alter plasma glucose appearance and only marginally decreased plasma glucose concentrations in both nutritional states; 2) during fasting, IGF-I did not show anabolic effects on protein metabolism either at the whole-body level or in individual tissues. However, during feeding, IGF-I significantly stimulated exogenous nitrogen utilization by the whole body; and 3) IGF-I reduced the thermogenic effect of feeding. The results suggest that parenteral feeding may be an important variable in the response of protein anabolism to IGF-I in vivo.

To determine the optimal fat intake and source in nutritional support, we measured the protein-sparing effects of a structured lipid (SL) derived from 60% medium-chain triglyceride (MCT) and 40% fish oil and a 50:50 soybean to safflower oil emulsion (long-chain triglyceride, LCT). Male Sprague-Dawley rats received an enteral diet for 7 d with either all nonprotein energy as dextrose (control diet) or 10% or 35% nonprotein energy as SL or LCT. The rats were burned on day 3. Indirect calorimetry and nitrogen balance were measured on day 2 (preburn) and days 4 and 6 (postburn). Respiratory quotient decreased postburn. There was a significant increase in total energy expenditure postburn, particularly with 35% LCT. Nitrogen balance was best without fat and 10% fat compared with 35% fat and with SL compared with LCT. These results confirm previous studies that fish oil-containing SLs possess protein-sparing effects in burn injury and that 10% SL seems optimal for nutritional support in burn injury.

The use of xylitol as an alternative carbohydrate calorie source in total parenteral nutrition may offer unique pharmacologic and nutritional properties in the therapy of the thermally injured. Male Sprague-Dawley rats (250 g) received a 15-second dorsal scald injury (25-30% BSA) and were parenterally fed isovolemic diets (60 ml/day) that provided 200 kcal/kg/d, 9.68 g of amino acids/kg/d, and 23.5% nonprotein calories (NPC) as fat for 3 days. The balance of NPC were provided as dextrose (Dex) or 50% xylitol:50% dextrose (Xyl/Dex). Rectus muscle and liver fractional protein synthetic rates (FSR, %/day), whole body leucine appearance (Flux), oxidation (OX), protein breakdown (PB), and synthesis (PS) were estimated using a 4-hour iv infusion of [1-14C]leucine on day 3. Mean values (+/- SE) for leucine kinetics (mumol leucine/hr/100 g), cumulative nitrogen balance (mg N) and plasma insulin concentration (Table I). (microU/mL). The partial replacement of dextrose calories with xylitol did not significantly alter whole body and tissue leucine kinetics, daily and cumulative nitrogen balance, insulin concentration, and energy expenditure (indirect calorimetry). These data indicate that xylitol may be useful as an alternative carbohydrate calorie source in parenteral nutrition to avoid possible deleterious side effects of glucose overfeeding in the critically ill but did not improve protein metabolism under the conditions of this study.

In order to study the mechanisms by which nutrients influence post-trauma metabolism, Sprague-Dawley rats received a 25% full-thickness burn and were randomly assigned to receive 12.5 g kg-1 bodyweight (BW) per day amino acids (AA) only, AA and 14.7 g hydrous glucose kg-1 BW per day or AA and 14.7 g hydrous xylitol kg-1 BW per day. After 4 days of hypocaloric feeding, rats receiving xylitol had a cumulative nitrogen balance of +213 +/- 82 mg N, which was significantly (P less than 0.001) better compared with either the AA with only -493 +/- 61 mg N or the AA and glucose group (P less than 0.01) with -160 +/- 101 mg N. During glucose infusion reduction of insulin-mediated fat oxidation was partially compensated by an increase in glucose oxidation. Xylitol infusions resulted in increased glucose oxidation compared with the amino acid only group without simultaneously reduced fat oxidation. Although glucose and xylitol are calorically similar, their protein sparing properties and metabolic action after injury cannot be based entirely upon their caloric equivalent.

The influence of dietary lipid manipulation with menhaden or safflower oil on changes in protein metabolism in rats receiving recombinant interleukin-1 beta (IL-1 beta), tumor necrosis factor alpha/cachectin (TNF), or both combined (COINF) was examined. Whole-body protein kinetics, energy expenditure, nitrogen excretion, and liver and muscle protein synthesis were studied using tracer quantities of L-[1-14C]-leucine. Rats fed menhaden oil, high in omega-3 fatty acids, had significantly lower rates of leucine oxidation compared to safflower-fed rats after monokine infusion (P less than .05). However, muscle protein synthetic rates and the specific activity of free leucine in plasma and muscle indicated greater net muscle-protein breakdown in animals fed fish oil or receiving monokines. Rats fed omega-3 fatty acids had significantly larger livers as percent of body weight and more total liver protein than safflower oil controls (P less than .0001). Liver weight was further increased by monokines, particularly TNF and COINF (P less than .001) in both diet groups, suggesting that net hepatic anabolism occurred at the expense of net skeletal protein catabolism. Monokines as a group and COINF significantly decreased whole-body leucine flux and incorporation into protein; no effect of menhaden oil was noted. In addition, monokines increased nitrogen excretion during the 24-hour experimental period (P less than .05), and total energy expenditure rose significantly in all groups receiving IL-1 beta and COINF. The recombinant monokines IL-1 beta and TNF, particularly when coinfused, are able to reproduce many of the protein anabolic and catabolic consequences seen following infection and injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Whole-body leucine kinetics and rectus muscle synthetic rates were evaluated in postabsorptive rats fed semipurified diets that varied in the casein content. Rats were allowed to consume ad libitum a 2% casein diet or were pair-fed or ad libitum-fed 6, 20, or 40% casein diets for 14 days. After overnight starvation, rates of whole-body leucine kinetics and rectus muscle synthetic rates were determined with a 2-h constant intravenous infusion of L-[1-14C]leucine. The postabsorptive response to inadequate protein intakes included a significant reduction in the release of leucine from whole-body protein degradation as well as subsequent reutilization for protein synthesis. In contrast, dietary protein intake at levels greater than required for maximal growth were not associated with any increases in leucine incorporation into whole-body protein or muscle fractional synthetic rates. Rates of whole-body leucine oxidation based on plasma leucine specific radioactivities underestimated total oxidation by 22-27%, and this was relatively constant as the protein component of the diet was varied. In addition, the muscle acid-soluble leucine specific radioactivity was similar to the plasma alpha-ketoisocaproate enrichment, regardless of dietary protein intake.

The effects of burn and first-pass splanchnic leucine extraction (FPE) on protein kinetics and energy expenditure were assessed by measuring O2 consumption, CO2 production, nitrogen balance, leucine kinetics, and tissue fractional protein synthetic rates (FSR-%/day) in enterally fed rats. Anesthetized male rats (200 g) were scalded on their dorsum with boiling water (25-30% body surface area) and enterally fed isovolemic diets that provided 60 kcal/day and 2.4 g of amino acids/day for 3 days. Controls were not burned. An intravenous or intragastric infusion of L-[1-14C]leucine was used to assess protein kinetics on day 3. FPE was taken as the ratio of intragastric to intravenous plasma leucine specific activity. There was a 69% reduction in cumulative nitrogen balance (P less than 0.001) and a 17-19% increase in leucine oxidation (P less than 0.05) and total energy expenditure (P less than 0.01) in burned rats. A 15% decrease in plasma leucine clearance (P less than 0.05) was accompanied by a 20% increase in plasma [leucine] (P less than 0.01) in burned rats. Burn decreased rectus muscle FSR from 5.0 +/- 0.4 to 3.5 +/- 0.5 (P less than 0.05) and increased liver FSR from 19.0 +/- 0.5 to 39.2 +/- 3.4 (P less than 0.01). First pass extraction of dietary leucine by the splanchnic bed was 8% in controls and 26% in burned rats. Leucine kinetics corrected for FPE showed increased protein degradation with burn that was not evident without FPE correction. This hypermetabolic burn model can be useful in the design of enteral diets that optimize rates of protein synthesis and degradation.

Changes in protein content and function were investigated in man Sprague-Dawley Crl:Cd rats that were either fasted, consumed a diet containing 1.5 g fat/100 g initial body weight per day or received ad libitum an 85% carbohydrate, minimal nitrogen diet. Fasted rats became moribund after 9--10 days and were both hypoglycemic and hypoketonemic. In contrast, rats fed the fat-only and high carbohydrate, minimal nitrogen diets survived the 28-day study period. Although rats consuming the high carbohydrate, minimal nitrogen diet lost less body weight and nitrogen than either fasted animals or rats receiving the fat-only diet, they also had a loss of delayed hypersensitivity recall to tuberculin antigen and a 55% decrease in plasma essential amino acid concentrations. Rats fed the fat-only diet maintained delayed hypersensitivity recall and near normal concentrations of plasma essential amino acids despite a greater loss of body nitrogen than was observed in terminally fasted animals. Plasma albumin concentrations and total liver protein content showed comparable decreases in rats consuming the two incomplete diets. We concluded that nonprotein calories prolong survival in the rat and fat or predominantly glucose calories are sufficient at preventing mortality. However, the source or quantity of nonprotein calories consumed during periods of protein deprivation can markedly alter individual tissue protein status and function.

The rate of protein synthesis in vivo was assessed in tumor tissue, skeletal muscle, liver, and the whole body of rats bearing either the Yoshida sarcoma or Novikoff hepatoma after 18 days of tumor growth and compared to tumor-free controls. Changes in size of the whole animal and tumor (i.e., growth) were measured, and fractional rates of growth, synthesis, and degradation were estimated. Muscle protein synthesis and whole-body growth were significantly reduced in both groups of tumor-bearing rats after 18 days of tumor growth. In addition to reductions in muscle protein synthesis, whole-body protein synthesis was significantly reduced in the Yoshida tumor-bearing group (587 +/- 36 versus 401 +/- 40 mg/h; mean +/- SEM; control versus Yoshida group, respectively, P less than 0.01). Tumor protein synthesis was not statistically different between the Yoshida tumor (76 +/- 21 mg/h) and the Novikoff tumor (50 +/- 8) after 18 days of growth despite the fact that the Yoshida tumors were significantly larger (33.9 +/- 4.2 g versus 11.9 +/- 1.2 g; P less than 0.01). The fractional synthesis rate (Ks) was, in fact, significantly slower in the Yoshida versus the Novikoff tumor (36.8 +/- 7.6 versus 55.1 +/- 4.8%/day). Tumor growth (Kg) followed first order growth rates for both tumor types (r = 0.945, 0.869; Kg = 17.2 +/- 1.6, 15.5 +/- 1.9%/day; Yoshida and Novikoff, respectively). The fractional degradation rate of tumor protein (Kd) was determined as the difference between the two first order rate constants Ks and Kg. The tumor protein degradation rate was significantly reduced in the Yoshida tumors compared to the Novikoff tumors (19.6 +/- 8.2% versus 39.6 +/- 4.2%/day, respectively). The greater size in the Yoshida sarcoma can be attributed to reduction in fractional protein degradation rather than change in synthesis rates, which supports the theory that some tumors can regulate their growth by alteration in tumor protein degradation rates (J. A. Tayek et al., Cancer Res., 46:5649-5654, 1986).

The metabolic response to yellow fever immunization was investigated in four obese subjects who were consuming a protein-sparing modified fast plus 100 g glucose (PSMF + 100 g) for 3 wk. Fasting, 1/2, 1, and 2 h postprandial values for insulin, glucagon, glucose, lactate, beta hydroxybutyrate, acetoacetate, and free fatty acids were assessed before and the 1st, 3rd, and 5th day postimmunization. The hormone and substrate response to dietary protein plus glucose was a substantial rise in insulin (p less than 0.005), glucose (p less than 0.005), and fall in free fatty acid and ketone levels (p less than 0.005). These results are distinctly different from the response to dietary protein ingestion only (PSMF) determined in a previous study which was a slight rise in insulin and glucose and slight fall in ketone bodies, but a substantial rise in glucagon. The only significant (p less than 0.05) change following immunization in the PSMF + 100 g was a rise in serum glucagon on day 1. During the PSMF significant elevations in insulin and lactate and falls in serum ketones were noted postimmunization. The metabolic response to mild infection is dependent on the diet consumed. These differences may have a clinical impact when the infection is severe, but this possibly requires study in an experimental infection of greater severity.

The ability of human endogenous pyrogen (EP) harvested from malnourished patients (marasmic or kwashiorkor-like) to induce fever and a simultaneous hypoferremia in rabbits is described. Human peripheral blood leukocytes were obtained from malnourished adult patients before total parenteral nutrition support, and after 1 and 7 days on this therapeutic regime; the leukocytes were stimulated to produce EP in vitro and EP was injected into each rabbit. EP obtained from the leukocytes of patients with predominant protein deprivation syndromes (kwashiorkor) before nutritional support produced an attenuated fever (0.23 degrees C over 4 h) and a relatively unchanged plasma iron concentration (delta Fe = -15 microgram/100 ml over 4 h) in the rabbits. When EP was harvested from these same patients after 7 days of nutritional support therapy and was injected into rabbits, normal 4 h fevers (1.10 degrees C) and reduction in plasma iron levels (delta Fe = -97 micrograms/100 ml) occurred in the rabbits. Human EP obtained from patients with marasmus (predominant calorie deprivation syndromes) produced relatively normal fevers and changes in plasma iron levels in the rabbits, regardless of whether the samples were taken pre-total parenteral nutrition or after 1 and 7 days of total parenteral nutrition. These observations suggest that the synthesis of endogenous pyrogen by human peripheral leukocytes is sensitive to the availability of key substrates and that nutritional support therapy restores the capacity of these cells to produce EP in vitro.

There are metabolic limitations to the infusion of large quantities of dextrose in critically ill patients receiving total parenteral nutrition. Of the alternative, nonprotein lipid sources, medium chain triglycerides (carbon chain length 8 and 10) are more rapidly oxidized and are deposited in the adipose tissue at rates much less than long chain triglycerides. In rats with burn injury receiving hypocaloric (dextrose and amino acids) parenteral feeding, we studied the changes in protein metabolism as a result of increasing the caloric intake by 33% by the addition of either dextrose, a soybean oil emulsion, a medium chain triglyceride emulsion, or a structured lipid emulsion of triglycerides synthesized from safflower oil (40%) and medium chain triglycerides (60%). Changes in body weight, blood glucose concentration, beta-hydroxybutyrate, lactate, amino acids, insulin, albumin, liver protein content, and nitrogen balance were measured during three days of feeding. Whole body leucine kinetics and muscle protein fractional synthetic rate were evaluated using a constant intravenous infusion of L-[1-14C]leucine. The addition of dextrose or soybean oil emulsion produced a significant increase in body weight and liver nitrogen but did not change albumin concentrations or leucine kinetics compared to those of the hypocaloric feeding group. Rats receiving medium chain triglycerides and structured lipid emulsions showed a reduction in branched chain amino acid concentrations and an improvement in serum albumin levels. However, the rats receiving the structured lipid emulsion also showed increased body weight, had a significant decrease in leucine oxidation, and showed a three day cumulative nitrogen balance significantly greater than zero.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma amino acid kinetics were determined in hospitalized patients receiving one of three intravenous solutions: isotonic amino acids, isotonic sodium chloride, or total parenteral nutrition. Whole body amino acid appearance, oxidation and incorporation into protein were estimated with two different isotopically labelled amino acids: L-[1-14C]leucine and L-[U-14C]tyrosine. A positive correlation was obtained between whole body amino acid appearance, oxidation and incorporation into protein with the two isotopically labelled amino acids. Derivation of whole body protein kinetics with L-[U-14C]tyrosine consistently gave higher values than those obtained from L-[1-14C]leucine, presumably due in part to the contribution of phenylalanine hydroxylation to plasma tyrosine appearance. However, the percentages of amino acid appearance oxidized and used for protein synthesis were similar. It can be concluded that estimates of whole body protein kinetics are qualitatively similar when measured with L-[U-14C]tyrosine and L-[1-14C]leucine in hospitalized patients receiving either isotonic sodium chloride solutions or balanced amino acid intakes.

Changes in protein metabolism 8 hr after anesthesia and femur fracture were studied in healthy rats fasted or receiving either intravenous glucose or crystalline amino acids. Whole body rates of amino acid turnover (flux) and release from protein (breakdown) as well as fractional synthetic rates of mixed muscle, liver, and plasma protein were measured using the constant infusion of L-(I-14C)-leucine. Injury resulted in a 24% increase in the synthesis of liver (p less than 0.05) and plasma proteins (p less than 0.01), respectively. Amino acid infusions in the injured animals further increased the synthesis of liver protein (from 36.6% to 44.3%/day, p less than 0.05) and increased muscle protein synthesis (from 7.0% to 9.3%/day, p less than 0.05) without altering rates of protein breakdown. Glucose infusions, in contrast, reduced whole body protein breakdown 36% (p less than 0.05) when compared to fasting, and depleted the plasma essential amino acid pool (p less than 0.05). The usual increases in liver protein synthesis observed in fasted rats following injury were not seen when the animals were receiving intravenous glucose. The nitrogen-sparing mechanism of these two infusions are different. Protein-free glucose infusions impair the normal response to injury aimed at increasing visceral protein synthesis and maintaining plasma essential amino acid concentrations.

The metabolic response to yellow fever immunization was investigated in five obese patients who were consuming a protein-sparing modified fast for 3 wk. Fasting 1/2, 1, and 2 h postprandial values for insulin, glucagon, glucose, lactate, beta hydroxybutyrate, acetoacetate, and free fatty acids were assessed before and the 1st, 3rd, and 5th day after immunization. The hormone and substrate responses to meat ingestion in patients well adapted to a protein-sparing modified fast and prior to infection was a rise in glucagon, no change in insulin, lactate, or ketone bodies and slight increase in serum free fatty acid and glucose levels. Over the entire period a significant (p less than 0.01) postprandial rise in glucagon was noted. Significant increases in insulin (p less than 0.01) and lactate (p less than 0.01) and falls in beta hydroxybutyrate (p less than 0.01) and acetoacetate (p less than 0.01) were noted in the postinfection period. There was also a significant interaction of diet and infection to increase insulin levels (p less than 0.05). The clinically mild infection produced by yellow fever immunization elicits a metabolic response which thus may be useful to investigate intermediary metabolism in the hospital setting.

This study examines the effect of a purified leukocyte-derived endogenous mediator (LEM) on protein metabolism during liver dysfunction and after sham surgery and the role of a nonsteroidal, antiinflammatory drug (indomethacin sodium trihydrate) in modifying this response. Febrile response, protein kinetics, urinary end products of protein metabolism, and plasma acute phase protein levels were studied in rats given a pyrogenic dose of LEM or saline solution, and these same indicators were studied after the administration of the same dose of LEM plus 2 mg/kg indomethacin 3 weeks after a portacaval shunt (PCS) or sham operation. In both surgical groups given LEM, a maximum of 1.1 degrees C fever was observed. LEM increased protein turnover and urinary excretion of nitrogen and urea in sham-operated rats but not in the PCS animals. Administration of indomethacin decreased the plasma oxidation of L[1-14C]leucine and prevented the increased excretion of total nitrogen and urea in sham-operated animals treated with LEM. PCS animals showed a constant excretion of nitrogen and urea independent of treatment. alpha 1-Acid glycoprotein levels increased significantly in sham-operated animals treated with LEM but not in the PCS group until indomethacin was added. The coadministration of LEM and indomethacin in shams also enhanced the levels of alpha 2-macroglobulin and alpha 1-acid glycoprotein over values found with LEM alone. These findings confirm the catabolic effect of LEM in normal animals and identify the essential role of the liver in the acute phase response. The data also suggest that indomethacin may modify the acute phase response by reducing plasma amino acid oxidation as well as enhancing the levels of some specific acute phase proteins.

A prospective randomized crossover trial was conducted to determine the effect of a branched chain amino acid (BCAA)-enriched solution on whole body leucine kinetics and fractional rates of albumin synthesis in patients with intra-abdominal metastatic adenocarcinoma. Ten malnourished cancer patients were provided isonitrogenous amounts of both a conventional total parenteral nutrition (TPN) formula containing 19% BCAA and a BCAA-enriched TPN formula containing 50% of the amino acids as BCAA in a random order. Whole body protein turnover was determined by a 10 hour continuous infusion of leucine 14C. Increased whole body leucine flux (68 +/- 5 mumols/kg BW/hr versus 145 +/- 11; mean +/- SEM; P less than 0.001) and oxidation (13 +/- 2 mumols/kg BW/hr to 46 +/- 5; P less than 0.001) were determined on the BCAA-enriched TPN. Increased whole body protein synthesis (2.2 +/- 0.2 g protein/kg BW/day versus 3.9 +/- 0.3; P less than 0.005) and leucine balance (2.5 +/- 0.4 g leucine/d versus 6.5 +/- 0.6; P less than 0.001) were also observed in patients receiving the BCAA-enriched TPN solution. Leucine release from protein breakdown was not statistically elevated (1.65 +/- 0.18 g protein/kg BW/d versus 2.48 +/- 0.40; P greater than 0.05) but, incorporation of leucine 14C into plasma albumin was significantly elevated (2.37 +/- 0.23 mumols/g/hr to 4.21 +/- 0.33; P less than 0.001) when the patients received BCAA-enriched TPN. Despite the better leucine balance, the improvement in the 24-hour urinary nitrogen balance was not statistically significant (6.6 +/- 3.9 g protein/d versus 11.4 +/- 2.9; control versus BCAA-enriched; P = 0.15). BCAA-enriched formulas improve whole body leucine kinetics, fractional rates of albumin synthesis, and leucine balance, and thus may favorably influence protein metabolism in cancer cachexia.

The effect of increasing nitrogen intake with constant calorie supply was studied in protein-depleted rats. Animals were randomized into three paired groups to receive either total parenteral nutrition (TPN) or enteral (EF) isocaloric feedings (240 kcal X kg-1 X day-1) differing only in their amount of amino acids. The diets, composed of dextrose, a safflower oil emulsion and crystalline amino acids, were infused continuously for 5 days. Daily nitrogen intakes were for controls (EF, n = 7; TPN, n = 6) 0.25 g/kg; for groups 1 (EF, n = 7; TPN, n = 8) 1.4 g/kg and for groups 2 (EF, n = 7; TPN, n = 7) 2.5 g/kg. The rats in groups 1 and 2 obtained significant protein repletion in terms of body weight, nitrogen retention, liver protein and serum albumin regardless of the route of nutrient delivery. However, rats in groups 2 (higher nitrogen intake) presented a more rapid and greater recovery despite a reduction in nitrogen utilization. Whole-body leucine kinetics studied at the end of controlled feeding periods were similar for EF and TPN rats. However, increasing nitrogen intake (1 and 2) increased whole-body leucine flux and incorporation into protein resulting in a better balance, since leucine release from protein breakdown was comparable to controls. Liver protein fractional synthetic rates were reduced in EF while remaining at a high rate in TPN group 2 and still more elevated in TPN group 1, suggesting a preferential support of liver protein by enteral feeding or the effect of TPN as a non-physiologic route.

Branched chain amino acids were administered intragastrically in a septic-fractured rat model to determine the degree and mechanism of their protein-sparing ability. The septic injury model was first shown to produce a metabolic response characterized by hyperglycemia, reduced ketonemia and increased nitrogen loss. Branched chain amino acids were then administered either alone or as 25% or 50% (w/w) of a complete crystalline amino acid solution. L-(U-14C)-tyrosine was added to the diet to estimate protein synthesis in individual tissues. Branched chain amino acids, when given alone, spared total body nitrogen as compared with fasting by increasing the fractional synthesis of both mixed liver and muscle protein. Although the two complete amino acid mixtures produced similar nitrogen preservation and muscle synthesis in the septic animals, the crystalline amino acid diet containing 50% branched chain amino acids resulted in the greatest preservation of total liver nitrogen and the highest fractional synthetic rate. The effect of branched chain amino acids would not appear to be explained by their nitrogen content alone, and in starvation with injury and infection, increased intakes may have potential benefit. Clinical trials in starved, injured man appear to be indicated.

The in vivo rates of protein synthesis were assessed in tumor tissue, skeletal muscle, and whole body of rats bearing the Walker 256 carcinosarcoma. Estimates of protein synthesis in the nontumorous tissues were compared to tumor-free controls. Changes in size of the whole animal and tumor (i.e., growth) were measured, and fractional rates of growth, synthesis, and breakdown were estimated. Muscle protein synthesis and whole-body growth were significantly reduced in rats bearing larger tumors, and both were negatively correlated with tumor size (r = -0.723 and -0.825, respectively; P less than 0.01). Furthermore, whole-body and muscle protein synthesis were positively correlated with body growth (r = 0.380 and 0.563, respectively; P less than 0.05). Tumor growth followed first-order kinetics between days 7 and 13 following implantation, with a mean rate constant of 34.3%/day for the larger tumors and 27.7%/day for the small tumors. The difference in tumor growth became statistically significant over the final 3 days of tumor volume measurements. Fractional protein synthesis was significantly lower in the larger compared to the smaller tumors (48.6 versus 84.8%/day; P less than 0.05) as measured on day 14. This finding indicates a lower protein breakdown rate for the larger tumors (14.3 versus 59.0%/day; P less than 0.01) and suggests that the process of protein breakdown could play a significant role in determining tumor size, leading support to the theory of tumors acting as nitrogen traps.

Leukocyte endogenous mediator (LEM), a low-molecular-weight peptide synthesized by monocytic cells during phagocytosis, has been implicated as the host's initiator of the protein metabolic response to infection and inflammation. To determine whether administration of LEM would alter protein kinetics, appearance and oxidation of plasma tyrosine as well as the rates of protein synthesis in liver and skeletal muscle were determined in fasted rats that received a 30-hour continuous infusion of either physiologic saline, LEM, or heat-inactivated LEM. The LEM was obtained from rabbit peritoneal exudate and the treatment solutions supplied 2.8 X 10(8) cell equivalents/100 g of body weight (BW) per day. Endogenous tyrosine oxidation increased from 4.0 +/- 0.4 mumol/100 g BW/h to 5.4 +/- 0.7 mumol/100 g BW/h in animals infused with heat-inactivated LEM and to 7.5 +/- 1.5 mumol/100 g BW/h in rats receiving LEM (P less than 0.01). Nonsecretory protein synthesis in the liver was greatest in rats administered LEM (2239 +/- 325 mg/d) when compared with control groups receiving physiologic saline (1122 +/- 195 mg/d) or heat-inactivated LEM (1374 +/- 62 mg/d; P less than 0.01), whereas skeletal protein synthetic rates were unchanged. Rates of muscle and collagen protein breakdown were estimated from the urinary excretion rate of Nt-methylhistidine and hydroxyproline, respectively, and their excretion rose by 30% (P less than 0.05) and 42% (P less than 0.05) with LEM administration. These results suggest that administration of LEM stimulates a mobilization of amino acids from peripheral tissues to support increased visceral protein anabolism while whole body amino acid oxidation is also enhanced. Since similar effects follow fever and infection, these results suggest that LEM may play an underlying role in the protein metabolic response to infection and inflammation.

Leukocyte endogenous mediator is a low molecular-weight protein synthesized by circulating monocytes and fixed macrophages of the reticuloendothelial system. Exogenous administration of leukocyte endogenous mediator to a well-nourished animal stimulates both specific and nonspecific immune function and replicates the protein metabolic response to infection, characterized by fever and increased amino acid oxidation, skeletal protein degradation and synthesis of "acute-phase" proteins. Leukocyte endogenous mediator administration also affords protection against semi-lethal doses of bacteremia in the well-nourished animal. In the protein-depleted host, synthesis or release of leukocyte endogenous mediator in response to infection appears to be reduced and the attenuated metabolic response may be attributed, in part, to a deficit in its production. However, nutritional repletion of the malnourished patient results in restoration of the capacity to produce leukocyte endogenous mediator usually within three to seven days, if adequate dietary protein is provided. Since protein malnutrition is associated with increased incidence and severity of bacterial infections, we postulate that the reduced synthesis and/or release of leukocyte endogenous mediator in protein malnutrition is detrimental. In those critically-ill, malnourished patients who cannot endogenously synthesize leukocyte endogenous mediator, and for clinical reasons cannot be repleted rapidly or are already infected and/or undergoing operative stress, exogenous administration of leukocyte endogenous mediator should be considered along with nutritional support. Administration of this protein to a seriously-ill malnourished individual should produce a metabolic profile of fever, increased urinary nitrogen excretion and falls in serum albumin concentrations that are generally considered pathologic. However, administration of leukocyte endogenous mediator over short periods of time should also provide the anabolic impetus for the augmented synthesis of proteins beneficial to recovery.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of enteral feeding with safflower oil or a structured lipid (SL) derived from 60% medium-chain triglyceride (MCT) and 40% fish oil (MCT/fish oil) on protein and energy metabolism were compared in gastrostomy-fed burned rats (30% body surface area) by measuring oxygen consumption, carbon dioxide production, nitrogen balance, total liver protein, whole-body leucine kinetics, and rectus muscle and liver protein fractional synthetic rates (FSR, %/day). Male Sprague-Dawley rats (195 +/- 5g) received 50 ml/day of an enteral regimen containing 50 kcal, 2 g amino acids, and 40% nonprotein calories as lipid for three days. Protein kinetics were estimated by using a continuous L-[1-14C] leucine infusion technique on day 2. Thermally injured rats enterally fed MCT/fish oil yielded significantly higher daily and cumulative nitrogen balances (p less than or equal to 0.025) and rectus muscle (39%) FSR (p less than or equal to 0.05) when compared with safflower oil. MCT/fish oil showed a 22% decrease (p less than or equal to 0.005) in per cent flux oxidized and a 7% (p less than or equal to 0.05) decrease in total energy expenditure (TEE) versus safflower oil. A 15% increase in liver FSR was accompanied by a significant elevation (p less than or equal to 0.025) in total liver protein with MCT/fish oil. This novel SL shares the properties of other structured lipids in that it reduces the net protein catabolic effects of burn injury, in part, by influencing tissue protein synthetic rates. The reduction in TEE is unique to MCT/fish oil and may relate to the ability of fish oil to diminish the injury response.

Seventeen patients undergoing 19 major urological operations for bladder cancer or rectal leiomyosarcoma (1 case) were studied after randomization for 3 hypocaloric dietary regimens, preoperative keto-adaptation by a carbohydrate-free, oral protein diet continued in the postoperative period by isotonic amino acid infusions, postoperative amino acid infusions only and 5 per cent dextrose infusions. In these normally nourished patients serum transferrin (plus 13 mg./dl. minus 30 mg./dl., minus 69 mg./dl., p less than 0.05 for the first and combined amino acid groups against the third group) and 2 other short half-life hepatic secretory proteins, prealbumin and retinol-binding protein, represented sensitive indexes of visceral protein and nutritional support, superior to nitrogen balance, anthropometric assays, delayed hypersensitivity skin test reactivity and serum albumin. Near isotonic amino acid infusions were more effective in preserving visceral protein status than 5 per cent dextrose but preoperative keto-adaptation was not shown to have any increased benefit over protein-sparing therapy given only after an operation.

The present study was designed to determine whether alterations in host metabolism associated with progressive tumor growth were a result of the anorexia frequently observed with cancer or could be attributed to other direct tumor effects. Rates of tyrosine flux, oxidation, and incorporation into protein, as well as fractional protein-synthetic rates in nonsecretory liver, muscle, and tumor, were determined in overnight-fasted rats, 5 to 6 (Stage I), 10 to 11 (Stage II), and 15 to 16 (Stage III) days following s.c. implantation of RNC-254 fibrosarcoma. Tumor-bearing rats were allowed to consume a purified diet containing 20% protein ad libitum, and results were compared to non-tumor-bearing rats pair fed quantities of food equivalent to tumor-bearing animals or allowed to consume the diet ad libitum. Results demonstrate that during later stages of tumor growth (Stage III) calorie intake and nontumor body weight gain were reduced in tumor-bearing rats (p less than 0.05). Fifteen and 16 days following implantation, there were significant changes in amino acid kinetics that were not observed after earlier periods of tumor growth and that could not be explained by any reduction in dietary intake. Rates of tyrosine appearance in the plasma and subsequent incorporation into whole-body protein were increased 33 and 34%, respectively (p less than 0.05), when compared to non-tumor-bearing rats fed equivalent quantities of food. Whole-body tyrosine oxidation rates were unchanged. Skeletal protein synthesis, as reflected by gastrocnemius or rectus abdominus muscle, was reduced from 10.5 and 10.1%/day to 7.4 and 6.0%/day, respectively (p less than 0.05), in tumor-bearing compared to pair-fed animals. The findings suggest that significant alterations in protein metabolism occur in advanced stages of experimental neoplastic disease which cannot be explained by reductions in dietary intake and are aimed at providing adequate quantities of endogenous amino acids for net tumor growth.

The General Clinical Research Center facilities have been largely responsible for expansion of knowledge in the field of hospital nutrition. Expansion of this knowledge base has led to major medical advances in this century. Without the meticulous attention necessary for metabolic balance studies many if not most of these advances would have been seriously delayed. The role that General Clinical Research Centers have played and will continue to play cannot be overestimated.

Accelerated maternal catabolism ensures adequate fetal growth in the postabsorptive pregnant mammal during late gestation. However, the relative importance of maternal stores and diet in supplying nutrient requirements to the fetus is unknown. We have measured rates of protein synthesis and breakdown in maternal and fetal tissues during continuous intravenous feeding on days 17 and 20 in the rat, using leucine tracer infusions. Nutrient solutions supplying 180 and 250 kcal.kg-1.day-1 were tested. Changes in fetal size were measured in these rats and in rats maintained on ad libitum chow feeding. In comparison to previous results in the postabsorptive rat, leucine flux from endogenous sources into plasma was significantly suppressed, with only 12-20% of circulating leucine derived from maternal tissues. Muscle protein catabolism (previously described on day 20 of gestation) was abolished with continuous feeding. Fetal growth rate averaged 46%/day on day 17 of gestation in all groups but was reduced to 12%/day on day 20 in the rats intravenously fed 180 kcal.kg-1.day-1 and 22%/day in the rats fed 250 kcal.kg-1.day-1. Fetal growth on day 20 in the rats fed rat chow ad libitum was unaffected. We conclude that by decreasing the maternal contribution to circulating nutrients, continuous intravenous feeding may adversely affect fetal growth in late pregnancy. The findings in the current study underline the physiological importance of maternal-fetal nutrient exchange.

Rates of protein synthesis were measured in whole fetuses and maternal tissues at 17 and 20 days of gestation in postabsorptive rats using continuous infusion of L-[1-14C]leucine. Fetal protein degradation rates were derived from the fractional rates of synthesis and growth. Whole-body (plasma) leucine kinetics in the mother showed a significant reduction of the fraction of plasma leucine oxidized in the mothers bearing older fetuses, a slight increase in the plasma flux, with total leucine oxidation and incorporation into protein remaining similar at the two gestational ages. Estimates of fractional protein synthesis in maternal tissues revealed an increase in placental and hepatic rates at 20 days of gestation, whereas the fractional synthetic rate in muscle remained unchanged. A model for estimation of the redistribution of leucine between plasma and tissues is described in detail. This model revealed a more efficient utilization of leucine in fetal protein synthesis in comparison with other maternal tissues, a greater dependency of the fetus on plasma supply of leucine, and a significant increase (2-fold) in the release of leucine from maternal muscle as the fetal requirements increased proportionately with its size. The latter conclusion, supported by nitrogen analysis and the ratio of bound-to-free leucine in maternal tissues, confirms the importance of maternal stores in maintaining the homeostasis of essential amino acids during late pregnancy.

Previous studies have demonstrated that protein malnutrition adversely affects the synthesis and release of interleukin 1 by monocytic cells. The purpose of this study was to investigate the protein and trace metal metabolism of severely protein-malnourished guinea pigs given exogenous interleukin 1 in the postabsorptive state or while concomitantly receiving total parenteral nutrition (TPN). Protein-depleted guinea pigs in the postabsorptive state failed to exhibit fever, granulocytosis, accelerated amino acid oxidation, or an acute-phase protein response after administration of interleukin 1 or endotoxin. However, a reduction in the serum concentration of zinc (P less than 0.05) and iron (P less than 0.05) was observed in the guinea pigs given interleukin 1, but not in those given endotoxin. The short-term (1 day) administration of TPN restored positive leucine balance (P less than 0.001) and reduced leucine release from protein breakdown (P less than 0.001). No other differences in the protein metabolism or trace metal response to interleukin 1 were observed as a result of short-term TPN feeding. Body temperatures of these malnourished guinea pigs given interleukin 1 and TPN became febrile or hypothermic depending on initial body temperatures. It is concluded that an attenuated capacity to synthesize interleukin 1 and a failure to mount an appropriate acute-phase protein response to exogenously administered interleukin 1 exist in this protein-malnourished animal model. Furthermore, this diminished protein response appears to be independent of short-term substrate provision because TPN for 1 day was unable to restore the full effects of the acute-phase response as mediated by interleukin 1.

The present study was undertaken to compare the effectiveness of a physical mixture of long-chain and medium-chain triglycerides with an emulsion consisting of chemically synthesized triglycerides composed of medium-chain and long-chain fatty acids in similar proportions. Sprague-Dawley rats received a 25% body surface area full-thickness scald burn on the dorsum. For the next three days, all rats received 300 kcal/kg/day as 160 kcal/kg/day glucose, 50 kcal/kg/day amino acid, and an additional 90 kcal/kg/day lipid emulsion as either long-chain triglyceride, medium-chain triglyceride, a 1:1 physical mix of medium-chain and long-chain triglycerides or a chemically structured triglyceride made up of 60% medium-chain fatty acid and a 40% safflower oil. Rats receiving the chemically structured lipid emulsion showed the greatest gain in body weight, the greatest positive nitrogen balance, and the highest serum albumin concentration, outstripping rats receiving the long-chain triglyceride, medium-chain triglyceride, and even the physical mixture long-chain and medium-chain triglycerides (P less than 0.01). A 30% increase in oxygen consumption and 35% increase in energy expenditure in rats given the medium-chain triglyceride emulsion alone (P less than 0.01) was observed. This study confirms that the metabolism of chemically structured triglycerides composed of medium-chain and long-chain fatty acids markedly differs from similar physical mixtures. For these reasons, the new structured lipid emulsions may prove advantageous in feeding the severely injured patient.

The effect of total enteral nutrition with structured and conventional lipids on protein and energy metabolism was assessed in gastrostomy-fed burned rats (30% body surface area) by measuring nitrogen balance, serum albumin, energy expenditure, and rectus muscle and liver fractional synthetic rates of protein (FSRs). Male Sprague-Dawley rats weighing 200 +/- 10 g received isovolemic diets that provided 50 kcal/d, 2 g/d amino acids, and 40% nonprotein calories as lipid for 3 d. The lipid source was either long-chain triglycerides (LCTs), medium-chain triglycerides (MCTs), structured lipid (SL), or a physical mix (PM) of the oils used in SL. Burned rats enterally fed either SL (p less than 0.01) or PM (p less than 0.05) yielded significantly higher daily and cumulative nitrogen balances and rectus muscle and liver FSRs than those fed either LCTs or MCTs. Rats fed SL or MCTs maintained higher serum albumin concentrations than rats fed either PM or LCTs. This study shows that the enteral administration of a mixed fuel system containing SL or its PM improves protein anabolism and attenuates net protein catabolism after thermal injury.

The effect of intravenous infusion of monoacetoacetin (glycerol monoacetoacetate) as a non-protein energy source was evaluated in burned rats. During 3 days of parenteral nutrition, in which animals received 14 g of amino acids/kg body wt. per day exclusively (group I) or with the addition of isoenergetic amounts (523 kJ/kg per day) of dextrose (group II), a 1:1 mixture of dextrose and monoacetoacetin (group III) or monoacetoacetin (group IV), significant decreases in urinary nitrogen excretion and whole-body leucine oxidation were observed in the three groups given additional non-protein energy as compared with group I. Serum ketone bodies (acetoacetate and 3-hydroxybutyrate) were decreased in rats given dextrose, whereas glucose and insulin increased significantly. Monoacetoacetin-infused animals (group IV) had high concentrations of ketone bodies without changes in glucose and insulin, whereas animals infused with both monoacetoacetin and glucose (group III) showed intermediate values. On day 4 of nutritional support, whole-body L-leucine kinetics were measured by using a constant infusion of L-[1-14C]leucine. In comparison with group I, the addition of dextrose or monoacetoacetin produced a significant decrease in plasma leucine appearance and release from whole-body protein breakdown. Gastrocnemius-muscle protein-synthesis rates were also higher in the three groups receiving additional non-protein energy. These findings suggest that monoacetoacetin can effectively replace dextrose as an intravenous energy source in stressed rats. Both fuels are similar in decreasing weight loss, nitrogen excretion, leucine release from whole-body protein breakdown and oxidation, in spite of differences in energy substrate and insulin concentrations.