Medium-chain triglycerides: an update.

A review of the literature on the medical and nutritional use of medium-chain triglycerides (MCTs) since 1970 is presented with additional discussions on the various modifications and applications of the MCTs in the synthesis of certain structured lipids. The metabolism of MCTs in the liver and extrahepatic tissues is discussed along with further documentation of the use of MCTs in malabsorption and hyperlipidemia cases. Recent applications of MCTs and modified MCTs in hyperalimentation, deficiency in the carnitine system, epilepsy, obesity, and other special areas of application are cited. The use of medium-chain monodiglycerides for dissolving cholesterol gallstones is presented. The contraindications for the use of MCTs in ketosis, acidosis, and cirrhosis are also discussed. Suggestions for use of MCTs in a variety of medical and nutritional applications are presented.

Studies in man of partially absorbed dietary fats.

A saturated long-chain triglyceride (SLCT) has been prepared by esterification with glycerol of the saturated long-chain fatty acid fraction of coconut oil, isolated by molecular distillation of the hydrolyzed oil. The resultant SLCT is comprised principally of stearate (89%) and palmitate (11%). The intestinal absorption of SLCT in man was compared with that of corn oil or a 1:1 mixture of SLCT and corn oil. Each fat or the mixture was incorporated in isocaloric amounts into a complete formula diet deriving 30% of its caloric content from fat, 55% from carbohydrate (dextrose), and 15% from protein (casein). The formula diets were administered in sequential feeding periods as the sole source of food. The results indicate that SLCT was poorly absorbed (31 to 39%) compared with virtually complete absorption of corn oil (98%). Fat absorption was improved when the dietary fat was an equal mixture of SLCT and corn oil. The poor absorption of SLCT was ascribed to its high melting point and related to impaired emulsification and micellar solubilization in the small intestine.

Effect of low and high amounts of a structured lipid containing fish oil on protein metabolism in enterally fed burned rats.

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.

Lipid emulsions and reticuloendothelial system function in healthy and burned guinea pigs.

The effect of total parenteral nutrition (TPN) regimens containing various quantities of long-chain triglyceride (LCT) and medium-chain triglyceride (MCT) emulsions on bacterial clearance and organ sequestration was evaluated in healthy and burned guinea pigs. In healthy guinea pigs, increasing the LCT component of TPN to 75% or greater of the nonprotein calories resulted in hepatomegaly, splenomegaly, a significant reduction in the sequestration of intravenously administered Pseudomonas aeruginosa by the liver and spleen, and a markedly increased clearance of bacteria into the lung. In burned guinea pigs, replacement of LCT with MCT emulsions at 75% of the nonprotein caloric intake reduced the sequestration of bacteria in the lung and restored to normal hepatic and splenic uptake. These results suggest that LCT emulsions at 75% of nonprotein calories result in reticuloendothelial system overload and increased bacterial sequestration in the lungs in normal and burned animals. In contrast, administration of MCT emulsions to the burned animal is less likely to result in increased pulmonary sequestration and decreased hepatic or splenic reticuloendothelial system function.

Enteral nutrition with structured lipid: effect on protein metabolism in thermal injury.

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.

Structured lipid made from fish oil and medium-chain triglycerides alters tumor and host metabolism in Yoshida-sarcoma-bearing rats.

The effects of structured lipid composed of fish oil and medium-chain triglycerides (Fish/MCT) on tumor and the host metabolism was compared with conventional long-chain triglycerides (LCTs) in Yoshida-sarcoma-bearing rats receiving TPN for 3 d. The two parenterally fed groups were divided into two treatments, saline or tumor necrosis factor (TNF), given intravenously at 20 micrograms/kg body wt. Changes in tumor volume, body weight, urinary nitrogen, whole-body and tissue protein kinetics, and fatty acid composition were measured. The study revealed that Fish/MCT feeding inhibited tumor growth, which could be attributed to decreased tumor protein synthesis. Body weight and nitrogen were better maintained by Fish/MCT feeding. In addition, the effects of Fish/MCT on tumor growth were synergistic with TNF treatment. The results demonstrate that dietary fat composition can influence fatty acid compositions of tumor tissue as well as tumor protein kinetics after a short period of TPN feeding.

Persistence of metabolic effects after long-term oral feeding of a structured triglyceride derived from medium-chain triglyceride and fish oil in burned and normal rats.

The persistence of metabolic effects following long-term oral feeding of a structured triglyceride rich in omega-3 fatty acids was studied in burned and normal rats, and compared with controls fed safflower oil, a long-chain triglyceride high in omega-6 fatty acid content. Male Sprague-Dawley rats were pair-fed a high fat diet as either structured triglyceride or safflower oil for 42 days. On day 43, a jugular catheter was placed, and rats received either a dorsal surface scald or sham injury. Following a 48-hour fast, body weight, nitrogen loss, energy metabolism, and liver weight were measured, and whole-body and tissue-specific protein kinetics were studied by constant intravenous infusion of [1-14C]leucine. Plasma albumin, free fatty acids, glucose, insulin, and triglyceride fatty acid composition were determined. Urinary nitrogen loss, energy expenditure, and plasma leucine concentration were elevated in burned rats, confirming the presence of an injury response. Rats previously fed structured triglyceride had greater liver weight, total liver protein, and percentage of leucine flux oxidized, and plasma levels of glucose and insulin were increased. Plasma leucine concentration was decreased in rats previously fed structured triglyceride. Plasma triglyceride and phospholipid fatty acid analysis showed a reduction in arachidonic acid and an increase in omega-3 fatty acids in rats previously fed structured triglyceride. Long-term feeding of structured triglyceride induced major systemic metabolic changes related to the dietary fatty acid composition that persist after the diet is discontinued.

Enhanced skeletal muscle and liver protein synthesis with structured lipid in enterally fed burned rats.

We assessed the effects of total enteral nutrition with long-chain triacylglycerides (LCT), medium-chain triacylglycerides (MCT), or two structured lipids, modified dairy fat (MDF) and modified MCT (Captex 810B, Capital City Products, Columbus, OH), on protein and energy metabolism in hypermetabolic burned rats (25% to 30% body surface area). Male Sprague-Dawley rats (200 +/- 10 g) were continuously gastrostomy-fed isovolemic diets that provided 50 kcal/d, 2 g amino acids/d and 40% nonprotein calories as lipid for three days. Changes in body weight, nitrogen balance, serum albumin, indirect calorimetry, whole body leucine kinetics, and rectus muscle and liver protein kinetics were determined. Whole body leucine kinetics and tissue fractional protein synthetic rates (FSR, percent per day) were estimated using a four-hour constant intravenous infusion of L-[1-14C]leucine on day 3. The group of rats enterally fed MDF lost less body weight than the other groups (P less than or equal to .05). MDF and Captex 810B produced a positive and significantly greater (P less than or equal to .05) daily and cumulative nitrogen balance than either LCT or MCT. Oxygen consumption (P less than or equal to .05) and total energy expenditure (P less than or equal to .05) were elevated approximately 22% with MDF as compared with LCT or MCT. Rectus muscle FSR and absolute rate of protein synthesis were increased 19% with MDF (P less than or equal to .05) as compared with LCT or MCT.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term feeding with structured lipid composed of medium-chain and N-3 fatty acids ameliorates endotoxic shock in guinea pigs.

The metabolic and physiologic responses to 7-hour endotoxin infusion (5.0 mg/kg h) were evaluated in guinea pigs following 6 weeks of dietary enrichment with diets containing either chemically structured lipid (SL) composed of medium-chain triglycerides (MCT) and long-chain triglycerides (LCT) in the form of N-3 polyunsaturated fatty acids (PUFAs), or safflower oil (SO), which is high in N-6 fatty acids. Plasma phospholipid fatty acid profiles, arterial blood pH, PCO2, PO2, HCO3, lactate, blood pressure, oxygen consumption, and energy expenditure were examined. Plasma phospholipid fatty acids profiles reflected dietary intake with SL-fed animals demonstrating a significantly higher N-3 to N-6 fatty acid ratio compared with SO-fed animals. SL-fed animals responded to endotoxemia with a mild metabolic acidosis with respiratory compensation, which was associated with moderate lactatemia (3 mmol/L). SO-fed animals developed a severe metabolic acidosis with acidemia and respiratory compensation, which was associated with hyperlactatemia (8 mmol/L, P less than .05 v SL). No differences were observed in blood pressure, oxygen consumption, energy expenditure, or respiratory quotient during endotoxemia between dietary groups compared with controls. We conclude that diets enriched with structured lipid composed of medium-chain and N-3 fatty acids can attenuate the sequelae of endotoxemia.

Protein metabolism during total parenteral nutrition (TPN) in injured rats using medium-chain triglycerides.

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)

Structured medium-chain and long-chain triglyceride emulsions are superior to physical mixtures in sparing body protein in the burned rat.

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.

Retained biliary tract stones. Nonsurgical treatment with capmul 8210, a new cholesterol gallstone dissolution agent.

The ability of Capmul 8210, a commercial solvent that predominantly consists of glyceryl 1-mono-octanoate, to dissolve retained common duct stones by direct infusion into the T-tube was tested in 20 patients with a total of 43 stones. Of 19 patients who completed their infusion, stone disappearance was observed in 15, giving a success rate of 79%. The dissolution time for a single stone averaged four days. A slight rise in serum alkaline phosphatase and amylase levels occurred in some patients, and rapidly returned to normal when treatment was concluded. Other side effects, such as nausea and vomiting epigastric discomfort, or diarrhea, occurred occasionally but were easily controlled medically. We believe that this agent is a useful adjunct in the management of postoperative choledocholithiasis in the patient with an indwelling T-tube.

Effect of structured lipid-enriched total parenteral nutrition in rats bearing yoshida sarcoma.

The efficacy of structured lipid, a triacylglycerol of medium and long chain fatty acids, as an element of nutritional support therapies in cancer cachexia was investigated. Using the Yoshida sarcoma to induce cachexia, male Sprague Dawley rats (90 g) were injected subcutaneously with tumor cells (n = 17) or sterile saline (n = 16). Seven days later, rats were randomized to two intravenous diets for 3 days at 220 kcal/kg body weight/d, including 2 g nitrogen/kg body weight/d and 39% of total calories as either structured lipid or long chain triglyceride. Nitrogen balance, tumor growth rate, energy metabolism, and plasma albumin and free fatty acid levels were measured, and whole-body protein kinetics and liver, muscle, and tumor fractional protein synthetic rates were evaluated by adding (14)C-leucine to the diet during the last 4 hours of feeding. Nitrogen balance improved (P < .05) in both tumor and control rats receiving structured lipid-enriched total parenteral nutrition, and was also greater in tumor rats compared with controls. There were no differences in tumor growth or protein kinetics between diet groups. Albumin was lower (P < .05) in tumor rats, but significantly higher in both tumor and control rats given structured lipid-enriched total parenteral nutrition. Free fatty acid was significantly higher in tumor rats versus controls. Whole-body protein kinetics were similar among the four groups. Liver weight, liver weight to body weight ratio, and liver protein synthetic rate were higher in tumor rats. Also, liver weight to body weight ratio was lower in tumor and control animals given structured lipid-enriched total parenteral nutrition. Muscle protein synthetic rate was significantly lower in tumor rats, but higher in tumor and control rats given long chain triglyceride-enriched total parenteral nutrition. The nutritional benefits of structured lipid-enriched total parenteral nutrition favor support of host tissue without promoting tumor growth.

Metabolic effects of medium chain triglyceride-enriched total parenteral nutrition in rats bearing Yoshida sarcoma.

The ability of medium chain triglyceride-enriched total parenteral nutrition to support host tissue in a model of cancer cachexia was assessed by measuring tumor growth, body weight, nitrogen balance, energy expenditure, leucine kinetics, fractional protein synthetic rate of tumor, liver, and abdominis rectus muscle, and plasma levels of glucose and albumin. Male Sprague-Dawley rats (85-90 gm) received 10(7) cells of viable Yoshida sarcoma subcutaneously on day 0. Control rats received injections of sterile saline. On day 10 rats underwent central venous cannulation and were randomized to one of three isocaloric diets. One group received amino acids and dextrose, while the other two groups were infused with amino acids, dextrose, and fat as either long chain triglyceride or a physical mixture of medium chain triglyceride: long chain triglyceride (3:1). On day 14 L-1-(14)C-leucine was added to the diet to study protein kinetics, and energy metabolism was measured by indirect calorimetry. Both tumor-bearing and nontumor-bearing rats demonstrated improved nitrogen balance when given medium chain triglyceride-enriched total parenteral nutrition. Tumor-bearing rats had reduced resting energy expenditure vs. nontumor-bearing, while rats receiving total parenteral nutrition without fat had significantly greater respiratory quotients. Tumor-bearing rats had lower total body weight vs. nontumor-bearing on day 10, but body weight of tumor-bearing and nontumor-bearing did not differ on day 14. Whole body protein breakdown decreased and leucine balance increased in tumor-bearing rats as compared to nontumor-bearing. Total liver mass was greater in tumor-bearing rats, but liver protein fractional protein synthetic rate decreased in tumor-bearing rats vs. nontumor-bearing. Tumor growth rate and fractional protein synthetic rate were not altered by the parenteral diet. The data confirm an altered metabolism in the tumor-bearing host, and suggest that medium chain triglyceride can better support host tissue.

Alternative lipid sources for enteral and parenteral nutrition: long- and medium-chain triglycerides, structured triglycerides, and fish oils.

This article reviews current investigations of the use of alternative lipid sources to enhance the metabolic and immune functions of hospitalized patients. Conventional lipids have been implicated as the cause of a variety of iatrogenic side effects in critically ill hospitalized patients, and long-chain triglycerides of the omega-6 family have been shown to be potentially detrimental to immune function. Alternative lipids (fish oils, medium-chain triglycerides, and structured triglycerides) have been proposed as substitutes for conventional long-chain, polyunsaturated omega-6 fatty acids. Unlike long-chain triglycerides, medium-chain triglycerides are more rapidly cleared from the blood and are completely oxidized for energy. However, medium-chain triglycerides contain no essential fatty acids. On the other hand, structured triglycerides offer the advantages of long-chain triglycerides (essential fatty acids) and of medium-chain triglycerides (rapid clearance and oxidation). Fish oils, which contain long-chain polyunsaturated omega-3 fatty acids, appear to be anti-inflammatory and to affect immune function differently from the omega-6 long-chain triglycerides.

Dietary fat level as determinant of protein-sparing actions of structured triglycerides.

This study assessed the effects of total parenteral nutrition (TPN) containing long-chain triglycerides (LCTs), an equimolar physical mixture of LCT and medium-chain triglycerides (MCTs), and a structured triglyceride synthesized from equimolar amounts of MCT and LCT on energy and protein metabolism after thermal injury (25% body surface area full-thickness scald burn). Male Sprague-Dawley rats (245-271 g) received isovolemic diets intravenously that supplied 250 kcal.kg-1.day-1, 2 g amino acid nitrogen.kg-1.day-1, and 50% of nonprotein calories as lipid and 50% as dextrose for 3 days. Whole-body and tissue leucine kinetics were estimated by a 4-h continuous infusion of L-[1-14C]leucine on day 3. Nitrogen balance, plasma albumin, plasma glucose, energy expenditure, and whole-body and liver and rectus muscle protein kinetic parameters were determined. No significant differences were noted in any of the parameters measured. This study suggests that the unique protein-sparing actions usually associated with structured triglyceride administration are not seen when they are provided as 50% of nonprotein calories. In addition, the ratio of MCT to LCT in the starting mixture from which the structured triglycerides are synthesized may be an important determinant of the protein-sparing actions attributed to these lipids.

Invited comment: lipids and the development of immune dysfunction and infection.

Excessive W-6 PUFA metabolism due to high levels of dietary fat intake can encourage infection via prolonged inflammation, enhanced Gram negative survival, reticuloendothelial blockage, immunosuppression, and monokine depression. Lipids can influence host immunity by altering eicosanoid metabolism and membrane structure and function. Further investigations are essential to answer questions regarding the levels and properties of various essential fatty acids in TPN lipid emulsions. Combining the features of LCT in the form of W-3 PUFA (fish oil) and MCT in the form of medium-chain triglyceride in a "structured lipid" may decrease infection and may improve survival rates by producing fewer inflammatory eicosanoids of the two- and four-series, and serving as a more "efficient fuel." The introduction of W-3 polyunsaturated fatty acids into the TPN emulsions as well as into normal diets may provide an important therapeutic advance in the pathogenesis of disease. Such unique antiinflammatory properties of W-3 PUFA require intensive research.

Novel triglycerides for special medical purposes.

The clinical use of intravenous lipid emulsions have been routine for over 25 years. For most of that time period the use of the vegetable oils, soybean and safflower, were the exclusive lipid source for these emulsions. Recently intravenous medium-chain triglycerides have been commercially available. This review will discuss several important new research developments coming from the laboratory which should prove to enhance the nutritional effectiveness as well as minimize the adverse effects of lipid emulsions. The use of medium-chain triglycerides either enterally or parenterally has shown them to be superior energy sources when compared to long-chain triglycerides. Under experimental conditions of burn injury, their support of certain aspects of protein metabolism is superior to that of the current emulsions. This may be due to their rapid and preferential oxidation and poor storage into adipose tissue, and increased thermogenesis which has been observed from either enteral or parenteral administration in humans. This increased metabolic rate is not accompanied by an increase in temperature. Lipid emulsions have been described as having many different effects on variable aspects of the immunologic system. Some of these could be considered to be beneficial or without harm, and others are considered potentially deleterious. We have focused on the effects of parenteral lipid emulsions, in animals as well as in humans, on the function of the reticuloendothelial system.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of different lipid sources in total parenteral nutrition on whole body protein kinetics and tumor growth.

This study examined the short-term effects of three total parenteral nutrition solutions, each containing a different lipid source, on host and tumor protein metabolism in a rat cancer model. Each diet contained 220 kcal/kg per day, including 2 g of nitrogen/kg per day and 50% of nonprotein calories as either a structured lipid of medium-chain triglycerides and fish oil, a physical mix of medium-chain triglycerides and fish oil, or Liposyn II, a long-chain triglyceride. A 3-day intravenous feeding infusion began on day 7 after tumor implantation. Tumor growth rate, nitrogen balance, energy expenditure, and plasma albumin, glucose, and free fatty acids were measured, and whole body protein kinetics and fractional synthetic rates in liver, muscle, and tumor tissues were assessed using a constant infusion of 14C-leucine. The results revealed that tumor growth rate was slowed in structured lipid-fed animals (p = .06, one-way analysis of variance) with significant increases in rates of tumor protein synthesis and tumor protein breakdown (p < .001, one-way analysis of variance). Although muscle fractional synthetic rates were significantly decreased in tumor-bearing animals (p < .05, two-way analysis of variance), the rates in structured lipid-fed animals were restored. Nitrogen balance improved significantly in structured lipid-fed animals. The results demonstrate that the source of lipid in total parenteral nutrition solutions can influence tumor and host protein metabolism, and that a structured lipid composed of medium-chain triglycerides and fish oil seems to improve protein metabolism in host tissue without stimulating tumor growth.

The effect of increasing levels of fish oil-containing structured triglycerides on protein metabolism in parenterally fed rats stressed by burn plus endotoxin.

This report investigates the effect of various levels of medium-chain/fish oil structured triglycerides on protein and energy metabolism in hypermetabolic rats. Male Sprague-Dawley rats (192 to 226 g) were continuously infused with isovolemic diets that provided 200 kcal/kg per day and 2 g of amino acid nitrogen per kilogram per day. The percentage of nonnitrogen calories as structured triglyceride was varied: no fat, 5%, 15%, or 30%. A 30% long-chain triglyceride diet was also provided as a control to compare the protein-sparing abilities of these two types of fat. Nitrogen excretion, plasma albumin, plasma triglycerides, and whole-body and liver and muscle protein kinetics were determined after 3 days of feeding. Whole-body protein breakdown, flux, and oxidation were similar in all groups. The 15% structured triglyceride diet maximized whole-body protein synthesis (p < .05). Liver fractional synthetic rate was significantly greater in animals receiving 5% of nonprotein calories as structured triglyceride (p < .05). Muscle fractional synthetic rate was unchanged. Plasma triglycerides were markedly elevated in the 30% structured triglyceride-fed rats. The 30% structured triglyceride diet maintained plasma albumin levels better than those diets containing no fat, 5% medium-chain triglyceride/fish oil structured triglyceride, or 30% long-chain triglycerides. Nitrogen excretion was lower in animals receiving 30% of nonnitrogen calories as a structured triglyceride than in those receiving 30% as long-chain triglycerides, but this difference did not reach statistical significance (p = .1). These data suggest that protein metabolism is optimized when structured triglyceride is provided at relatively low dietary fat intakes.