Thermic effect of medium-chain and long-chain triglycerides in man

The thermic effects of 400 kcal meals of medium-chain triglycerides (MCT) and long-chain triglycerides (LCT) were compared in seven healthy men. Metabolic rate was measured before the meals and for 6 h after the meals by indirect calorimetry. Mean postprandial oxygen consumption was 12% higher than basal oxygen consumption after the MCT meal but was only 4% higher than the basal oxygen consumption after the LCT meal. There was a 25-fold increase in plasma beta-hydroxybutyrate concentration and a slight increase in serum insulin concentration after MCT ingestion but not after LCT ingestion. Plasma triglyceride concentrations increased 68% after the LCT meal and did not change after the MCT meal. These data raise the possibility that long-term substitution of MCT for LCT would produce weight loss if energy intake remained constant.     

Effects of medium-chain triglycerides, long-chain triglycerides, or 2-monododecanoin on fatty acid composition in the portal vein, intestinal lymph, and systemic circulation in rats.

BACKGROUND: Fatty acid absorption patterns can have a major impact on the fatty acid composition in the portal, intestinal lymph, and systemic circulation. This study sought to determine the effects of long-chain triglycerides (LCT), medium-chain triglycerides (MCT), and 2-monododecanoin (2mono) on intestinal fatty acid composition during continuous feeding over a brief period. METHODS: The lipid sources were 100% LCT, 100% MCT, a 50:50 mixture of LCT and MCT (LCT/MCT), and a 50:50 mixture of LCT and 2mono (LCT/2mono). A total of 27 rats were randomly given 1 of the 4 diets at 200 kcal/kg/d, with 30% of total calories from lipids over 3 hours. RESULTS: MCT significantly increased each of the medium-chain fatty acids (C6:0, C8:0, and C10:0) as free fatty acids in the portal vein and about 10%/mol of C10:0 as triglycerides in the lymph compared with the other groups. There was significantly less C10:0 in lymphatic triglycerides with LCT/MCT than with MCT, but more than in the LCT and LCT/2mono diets. MCT also significantly increased the contents of C16:0, C18:0, C18:1, and C20:4 in the lymphatic triglycerides compared with all other groups including LCT/MCT. The amount of linoleic acid (C18:2) in lymphatic triglycerides followed the relative amounts of this fatty acid in the diet, with the greatest in LCT followed by LCT/MCT and LCT/2mono and least in MCT. A so-called structured lipid composed of the medium-chain fatty acid dodecanoic acid on the 2 position and long-chain fatty acids on the 1 and 3 positions appeared to be endogenously synthesized in response to the LCT/2mono diet. CONCLUSIONS: The original differences in MCT and LCT content in the diets were preserved in the fatty acid composition in the intestinal free fatty acids and triglycerides during feeding. In addition, the duration of lipid administration can play a role in altering fatty acid composition in the intestine.     

Hematological and biochemical effects of parenteral nutrition with medium-chain triglycerides: comparison with long-chain triglycerides

Twenty-four malnourished patients requiring total parenteral nutrition were randomly assigned to receive a daily infusion of either Lipofundin MCT-LCT [a new lipid preparation containing medium-chain triglycerides (MCTs)], or Lipofundin S [a long-chain triglyceride (LCT) preparation] for 6-28 d. No adverse clinical effects were apparent in patients receiving the new emulsion. Hematological indices were unchanged. Plasma urea concentrations rose less on MCT-LCT than on LCT and the plasma bilirubin concentration was lower. Plasma ketones were higher immediately after the MCT-LCT infusion but not at other times. Triglyceride and fatty acid concentrations were similar. Insulin concentrations were higher on MCT-LCT than on LCT. Daily nitrogen balance values were not significantly different between the two groups. Urinary carnitine excretion fell dramatically on both lipids.     

Role of carnitine in utilization of dietary medium-chain triglycerides by term infants

The role of carnitine in oxidation of dietary medium-chain fatty acids (as medium-chain triglycerides) was studied in term human infants. Infants were fed, alternately, formulas with fat content that was predominantly long-chain triglycerides, or 40% medium-chain triglycerides. Urinary acylcarnitine excretion was significantly higher and the ratio of free to total carnitine was significantly lower when infants were fed the formula with medium-chain triglycerides. Two groups of 10 infants were fed a commercial soy-protein-based formula modified to contain 40% of fat calories as medium-chain triglycerides and with or without added L-carnitine. By 56 d, infants fed the formula without added L-carnitine excreted significantly more medium-chain dicarboxylic acids than did the same infants at 28 d and significantly more than infants consuming the carnitine-supplemented formula at either 28 or 56 d. Results are consistent with a role for carnitine in metabolism of dietary medium-chain triglycerides in infants.     

Lipid infusion with different triglyceride cores (long-chain vs medium-chain/long-chain triglycerides): effect on plasma lipids and bilirubin binding in premature infants.

The possible beneficial effects of infusing a lipid emulsion containing 50% by weight of medium-chain triglycerides (MCT) compared with a standard long-chain triglyceride (LCT) emulsion were studied in 18 premature neonates (gestational age less than 34 weeks) requiring parenteral nutrition. The infants were assigned in a double-blind manner to receive one of the two lipid emulsions over 17 hours a day as a supplemental regimen for total parenteral nutrition. A lipid load of 1 g/kg per day was initiated on the third day of life and was increased at the rate of 1 g/kg per day until a maximal dose of 3 g/kg per day was obtained on the fifth day of life and maintained thereafter. Both bound and unbound bilirubin decreased with both infusion regimens during the study period. Despite a marked increase in plasma free fatty acid levels (260% in the MCT/LCT group compared with 210% in the LCT group), the fraction of unbound (free) bilirubin was significantly lower in the MCT/LCT group (34% vs 13%). Free fatty acid levels, corrected to albumin, were positively correlated to the percentage of free bilirubin only for the LCT lipid infusion. The finding of a significant elevation of plasma cholesterol levels only in the MCT/LCT group is now under investigation. Use of the MCT-containing emulsion was not associated with a higher frequency of adverse effects than the commonly used LCT-containing emulsion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of excess xylitol on nitrogen and glucose metabolism in parenterally fed rats.

The objectives of this study were to investigate the effects of an excess of xylitol on nitrogen balance and glucose metabolism in parenterally fed rats. Female Sprague-Dawley rats (200-250 g, n = 17) were catheterized for total parenteral nutrition and then randomized into two groups based on subsequent diet. The two diets used were isonitrogenous (1.5 g of nitrogen per kilogram per day) and isocaloric, with half the calories (125 kcal/kg per day) being derived from lipid (125 kcal/kg per day) and the other half from either glucose or xylitol (125 kcal/kg per day). The rats were fed a half-strength total parenteral nutrition diet for the day after surgery and a full-strength total parenteral nutrition diet for the following 4 days. Urines were collected daily for the determination of nitrogen balance. On day 5, the rats were given a 7- to 8-hour infusion of 6.6-d2 glucose (6 mg/h and 2-d1 glucose (12 mg/h). At the conclusion of the isotope infusion period, the rats were killed and blood was collected. Urine output was increased by 22% per day in the xylitol-treated rats, and they excreted 46.5 mmol of xylitol per liter per kilogram per day (7.1 g/kg per day, approximately 22.7% of dose). The xylitol group lost weight, had poorer nitrogen balance (341 +/- 31 vs 83 +/- 29 mg/kg per day [mean +/- standard error of the mean], p < .05), and developed fatty livers. Analysis of the liver fat distribution pattern indicated that the source of the excess hepatic lipid was dietary fat.(ABSTRACT TRUNCATED AT 250 WORDS)     

Different effects of long-chain and medium-chain triglycerides on glucose oxidation during total parenteral nutrition.

This study was undertaken to clarify differences in the effects of lipid emulsions containing either long-chain or medium-chain triglycerides (MCT) on glucose metabolism during total parenteral nutrition (TPN). Glucose kinetics were assessed in beagle dogs using primed, constant infusions of [14C]- and [6-3H]glucose. The rate of appearance of glucose, the percent of VCO2 derived from the oxidation of glucose, the rate of glucose oxidation, and the percent of glucose uptake oxidized were measured at the end of 72 hours of each of the two TPN regimens, ie, TPN in which soybean oil served as long-chain triglyceride comprising 40% of nonprotein calories (L-TPN), and TPN in which tricaprylin emulsion served as MCT (M-TPN). Glucose intake was 5.9 +/- 0.5 mg/kg per minute in L-TPN and 5.8 +/- 0.2 mg/kg per minute in M-TPN. There was no significant difference in the rate of glucose appearance between L-TPN and M-TPN. The rate of glucose oxidation was higher with M-TPN than with L-TPN (p < .05). Not only the percent of VCO2 derived from the oxidation of glucose but also the percent of glucose uptake oxidized tended to be higher during M-TPN than during L-TPN. These findings suggest that the glucose metabolism of dogs receiving L-TPN is different from that of dogs receiving M-TPN.     

Postprandial thermogenesis in lean and obese subjects after meals supplemented with medium-chain and long-chain triglycerides

The thermic effect of medium-chain triglycerides (MCTs) was studied in six lean and six obese young males by evaluating postprandial thermogenesis (PPT) after the ingestion of mixed meals containing either 38 g long-chain triglycerides (LCTs) or 30 g MCTs plus 8 g LCTs. Postabsorptive resting metabolic rate (RMR) was higher (P less than 0.05) in the obese individuals than in the lean ones. PPT, evaluated as 6-h incremental areas above RMR, was greater (P less than 0.05) in both groups after meals containing MCTs. The thermic effect of MCTs was 119.7 +/- 33.9 and 144.7 +/- 48.8 kJ/6 h in the lean and the obese subjects, respectively. The postprandial response of glucose, insulin, and free fatty acids did not depend on the type of oil contained in the meal. Our study shows that PPT is enhanced in both lean and obese subjects when LCTs in a mixed meal are replaced with MCTs.     

Effects of medium-chain and long-chain triacylglycerols in pediatric surgical patients

Medium-chain triacylglycerols (MCTs) have been shown to provide better nutritional support than long-chain triacylglycerols (LCTs). This study compares the efficacy of MCT combined with LCT with LCT alone in pediatric patients with surgical stress. Two groups of patients (n = 19 in each) received equivalent amounts of glucose (12 g. kg. d) and amino acids (2 g. kg. d), but one group received 10% Lipofundin MCT/LCT and the other received 10% LCT (1.5 g. kg. d) in a randomized study. Total parenteral nutrition (TPN) was given for 14 d. Blood and urine samples were collected before and after TPN administration for determination of various biochemical parameters. Indirect calorimetry was also performed to determine respiratory quotients and fuel utilization. After 14 d of TPN in the MCT/LCT group, there was a significantly higher blood lymphocyte percentage, a decreasing tendency of serum asparate aminotransferase and of total and direct bilirubin (P < 0.05). These changes were not observed in the LCT group. A significantly better nitrogen balance and a higher ketogenesis from day 3 were observed in the MCT/LCT group. The MCT/LCT group showed a more marked increased utilization of fat than the LCT group, whereas carbohydrate oxidation was less in the MCT/LCT group than in the LCT group (P < 0.05). In children after surgery, MCT/LCT is more protein sparing and induces a better immune response when compared with LCT-containing lipid emulsion. A TPN regimen containing MCT/LCT is likely to result in rapid oxidation of fats for energy without compromising the respiratory system.     

Use of a mixture of medium-chain triglycerides and longchain triglycerides versus long-chain triglycerides in critically ill surgical patients: a randomized prospective double-blind study

Twenty critically-ill surgical patients who needed total parenteral nutrition were randomly enrolled in a double-blind study comparing two intravenous fat emulsions: one containing a mixture of 50% medium-chain triglycerides and 50% long-chain triglycerides and another containing 100% longchain triglycerides. The purpose of this study was to investigate metabolic and biochemical differences between both emulsions with special reference to liver enzymes. After a baseline period of 24 h with only glucose and NaCl infusion, the lipid emulsion was added continuously during 24 h over 5 days. The parenteral nutrition was administered in mixture bags containing amino-acids, glucose and lipids together. Two-thirds of the non-protein calories were administered as glucose 40% and one third as either long-chain triglycerides or a mixture of medium-chain triglycerides and long-chain triglycerides. The total amount of non-protein calories received was the measured energy expenditure during the baseline period plus 10% and was fixed during the study. Plasma substrate concentrations, energy expenditure, and nitrogen balance were determined and arterial blood samples were taken. No toxic effects or complications attributable to one of the two emulsions were observed. There was no significant difference in energy expenditure, nitrogen balance, liver function tests, carnitine, transferrin, pre-albumin, albumin, cholesterol, triglycerides and free fatty acids. The only parameter that showed a different pattern of reaction between the two emulsions was serum bilirubin concentration. In this study no evidence of any advantageous effect of a mixture of medium-chain triglycerides and long-chain triglycerides was seen.     

Total parenteral nutrition with short- and long-chain triglycerides: triacetin improves nitrogen balance in rats.

Little is known about the long-term metabolic effects of parenteral administration of short-chain triglycerides. These studies were undertaken to investigate triacetin, the water-soluble triglyceride of acetate when it is incorporated into nutritionally balanced total parenteral nutrition formulas. Male Sprague-Dawley rats (n = 22) were fed an isovolemic, isocaloric and isonitrogenous diet for 7 d. The lipid energy represented 30% of the nonprotein energy with short-chain triglycerides representing 0, 50 or 90% of the lipid energy. Plasma acetate concentration was determined as well as indicators of protein metabolism: daily and cumulative nitrogen balance, whole body leucine kinetics and rectus muscle and liver fractional protein synthetic rates. No overt toxic effects were observed at any point during the study. As the proportion of short-chain triglycerides in the diet increased from 0 to 50 or 90% of the lipid energy, cumulative nitrogen balance increased 50 or 120%, respectively (P less than 0.05). Whole-body and tissue leucine kinetics (determined during the last 2.5 h of the 7-d study) were unaffected by the lipid composition of the diet. Plasma acetate concentration was not significantly different among groups. These results indicate that incorporation of the short-chain triglyceride, triacetin, in nutritionally balanced total parenteral nutrition formulas improves nitrogen balance with no overt toxic effects. These data indicate that triacetin may have a future role as a parenteral nutrient, and that further studies of its use are warranted.     

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.     

The influence of intravenous medium- and long-chain triglycerides and carnitine on the excretion of dicarboxylic acids

Four groups of male Wistar rats were alimented parenterally for 3 days. Groups 1 and 2 received medium-chain triglycerides (MCT) and groups 3 and 4 long-chain triglycerides (LCT). Groups 2 and 4 were supplemented with 100 mg L-carnitine/kg/day. The MCT-alimented rats presented with a distinct excretion of the dicarboxylic acids: adipic acid (C6), suberic acid (C8), and sebacic acid (C10). The acids excreted corresponded to the infused pattern of monocarboxylic acids: caproic acid (C6), caprylic acid (C8), and capric acid (C10). Dicarboxylic acid excretion after MCT administration may reflect an insufficient capacity of beta-oxidation on one hand or a preferential omega-oxidation of medium-chain fatty acids on the other. Carnitine supplementation lead to a further increase of the dicarboxylic acids in the MCT-group. beta-OH-butyric acid excretion decreased after carnitine in the MCT as well as in the LCT group. An increased transport of fatty acid-carnitine compounds out of the mitochondria is discussed as an important effect of carnitine supplementation. Hereby medium-chain fatty acids may be more accessible for omega-oxidation.     

Plasma lipids and lipoproteins during 6 d of maintenance feeding with long-chain, medium-chain, and mixed-chain triglycerides.

This study assessed fasting plasma lipids and lipoproteins and postprandial plasma lipids in healthy male subjects fed liquid-formula diets containing 40% of total energy as long-chain (LCT, primarily C18:1 and C18:2), medium-chain (MCT, C8:0-C10:0), or mixed-chain (structured lipid, STL, mostly C8:0, C10:0, and C22:0) triglycerides for 6 d. None of the diets altered plasma cholesterol concentrations. HDL cholesterol was decreased 14% by the STL diet (P < 0.044) and 15% by the MCT diet (P < 0.004) but was unchanged by the LCT diet. Plasma triglycerides were elevated 42% by the MCT diet (P < 0.006), but were unaltered by either the STL or LCT diets. Neither the STL nor the MCT diets produced changes in fasting lipoprotein lipid composition; however, during the LCT diet, VLDLs became enriched in triglyceride and LDLs became enriched in cholesterol. Postprandial triglyceridemia was significantly greater after subjects consumed the LCT diet than it was after they consumed either the STL or MCT diets, which were similar. Short-term feeding of MCT and STL diets produces significant changes in lipid metabolism. An understanding of the long-term effects of these diets awaits further study.     

Effects of xylitol versus glucose on urea synthesis and alanine metabolism in rats

The relation between xylitol concentration (1.0 and 5.5 mmol/1), the Capacity of Urea-N Synthesis, and the rate of Alanine Metabolism was investigated in nephrectomized rats of 200 g and compared with the effect of glucose at concentrations between 5.5 and 15.5 mmol/1. The xylitol and glucose concentrations were controlled by "clamp" techniques and the endogenous hormonal effects by somatostatin. The Capacity of Urea-N Synthesis was determined during alanine infusion to constant amino acid concentrations within the interval 7.3-11.6 mmol/1. The rate of alanine metabolism was assessed as alanine infusion rate corrected for changes in alanine concentration. At normal hormonal response, xylitol at 1.0 mmol/1 and 5.5 mmol/1 reduced urea synthesis from 10.3 +/- 1.1 mumol/(min.100 g) in controls to on average 6.2 +/- 0.9 mumol/(min.100 g) (mean +/- SD, n = 2 x 10, p < 1.01). Alanine metabolism was reduced to the same extent. Glucose concentration increased from 5.4 +/- 1.0 mmol/1 in controls to 8.1 +/- 1.4 mmol/1 at both xylitol concentrations. Xylitol reduced plasma glucagon concentration to one third and tripled plasma insulin concentration. During somatostatin and blood glucose maintained above 8 mmol/1, the Capacity of Urea-N Synthesis fell to 6.1 +/- 1.0 mumol/(min.100 g). In that situation, xylitol at 1.0 mmol/1 reduced neither urea synthesis nor alanine metabolism, whereas xylitol at 5.5 mmol/1 further reduced urea synthesis to 3.4 +/- mumol/(min.100 g) (n = 10, p < 0.05) and almost stopped alanine metabolism. Thus xylitol, independently of glucose and hormonal responses, inhibited urea synthesis and alanine metabolism. This may have therapeutic implications at catabolic conditions.     

中链脂肪酸对营养性肥胖大鼠体重及脂质代谢的作用

目的研究中链脂肪酸(MCFA)对营养性肥胖大鼠体重、体脂和血脂水平的影响。方法采用预防肥胖模型法,每天给予SD大鼠高脂营养饲料,并灌胃含不同剂量MCFA的食用油脂,饲养38天,观察体重、摄食量、体脂重量、血脂、胰岛素水平在不同剂量组动物间的差异。结果高、中、低三个剂量组大鼠分别摄入0·975、0·325和0·163g/kg bw MCFA38天,高剂量组与模型对照组相比,体重明显较低[分别(295·2±29·7)g和(324·6±9·1)g](P<0·05),脂肪垫重量明显较轻[分别(5·30±1·57)g和(7·20±1·74)g](P<0·05),脂肪细胞明显较小;各剂量组总的摄食量不显著低于模型对照组;血清甘油三酯(TG)、总胆固醇(TC)、胰岛素水平与模型对照组比较差异无显著性,中、高剂量组高密度脂蛋白胆固醇(HDL-C)水平高于模型对照组[分别(0·71±0·11)mmol/L、(0·73±0·12)mmol/L、(0·60±0·06)mmol/L](P<0·05),高剂量组大鼠的瘦素水平显著低于模型对照组[分别(1·61±0·39)ng/ml和(2·04±0·46)ng/ml](P<0·05)。结论在饲料摄入量没有差别的前提下,摄入0·975g/kg bw MCFA对营养性肥胖大鼠的体重增长有一定抑制作用,对血脂水平没有不良影响。     

Effects of carnitine coingested caffeine on carnitine metabolism and endurance capacity in athletes

The purpose of this study was to examine whether caffeine (CAF), carnitine (CAR), or CAF+CAR mixture administration affects exercise endurance time via carnitine metabolism. Water (CON), CAF, CAR, or CAF+CAR mixture was administered to five male rugby athletes participating in this study by a randomized double-blind fashion who were made to ride a cycle ergometer for exercise. The CAF effect on exercise endurance time was small, but the CAR trial significantly increased the exercise endurance time compared with CON trial; a further CAF+CAR mixture trial had greater effects on the exercise endurance time than those of a CON, CAF, or CAR trial. A CAR or CAF+CAR mixed trial increased urinary nonesterified carnitine (NEC) and total carnitine (TCAR), but no changes were observed in acid-soluble acylcarnitine (ASAC) and acid-insoluble acylcarnitine (AIAC) excretion. A CAR or CAF+CAR mixed trial resulted in higher levels of plasma NEC, ASAC, and TCAR fractions than the CON and CAF trials did on exhaustion time. Total cholesterol, triglyceride, and free fatty acid in blood were significantly increased at exhaustion time, but they were not affected in the CAF or the CAR trial. These results suggest that carnitine ingestion could promote fat oxidation, resulting in higher endurance performance in athletes, and especially these ergogenic effects of carnitine coingested with caffeine may be greater than those of carnitine alone.     

Effects of parenteral infusion with medium-chain triglycerides and safflower oil emulsions on hepatic lipids, plasma amino acids and inflammatory mediators in septic rats

This study was designed to investigate the effects of preinfusion with total parenteral nutrition (TPN) using medium-chain triglycerides (MCT) versus safflower oil (SO) emulsion as fat sources on hepatic lipids, plasma amino acid profiles, and inflammatory-related mediators in septic rats. Normal rats, with internal jugular catheters, were divided into two groups and received TPN. TPN provided 300kcal/kg/day with 40% of the non-protein energy provided as fat. All TPN solutions were isonitrogenous and identical in nutrient composition except for the fat emulsion, which was made of SO or a mixture of MCT and soybean oil (9:1) (MO). After receiving TPN for 6 days, each group of rats was further divided into control and sepsis subgroups. Sepsis was induced by cecal ligation and puncture, whereas control rats received sham operation. All rats were classified into four groups as follows: MCT control group (MOC, n= 8), MCT sepsis group (MOS, n= 8), safflower oil control group (SOC, n= 8), and safflower oil sepsis group (SOS, n= 11). The results of the study demonstrated that the MOS group had lower hepatic lipids than did the SOS group. Plasma leucine and isoleucine levels were significantly lower in the SOS than in the SOC group, but no differences in these two amino acids were observed between the MOC and MOS groups. Plasma arginine levels were significantly lower in septic groups than in those without sepsis despite whether MCT or safflower oil was infused. Plasma glutamine and alanine levels, however, did not differ between septic and non-septic groups either in the SO or MO groups. No differences in interleukin-1b, interleukin-6, tumor necrosis factor-alpha, and leukotriene B(4)concentrations in peritoneal lavage fluid were observed between the two septic groups. These results suggest that catabolic reaction is septic rats preinfused MCT is not as obvious as those preinfused safflower oil. Compared with safflower oil, TPN with MCT administration has better effects on reducing sepsis-induced liver fat deposition. Preinfusion with MCT before sepsis, however, had no effect on inflammatory-related cytokines or leukotriene in peritoneal lavage fluid. In addition, plasma arginine appears to be a more sensitive indicator than glutamine for septic insult.     

Effects of medium-chain triglycerides on brush border membrane-bound enzyme activity in rat small intestine

The effects of various types of dietary fat on brush border membrane-bound enzymes in rat intestinal mucosa were examined. Four groups of five rats were pair-fed defined diets for 10 d. The control group was fed a diet containing 57% sucrose and 2% corn oil as a fixed carbohydrate reference; the three experimental groups received diets containing 57% sucrose and 2% corn oil plus 13% fat in the form of medium-chain triglycerides (MCT) or long-chain triglycerides (LCT) (either lard as a highly saturated fat or corn oil as a highly unsaturated fat). Feeding LCT compared to the control diet, decreased sucrase activity in mucosal brush borders of the duodenum and jejunum. In these segments of MCT-fed rats, sucrase activity was similar to that in the control animals. In another experiment, measuring immunoreactive sucrase-isomaltase in jejunal brush border membranes revealed that feeding a high corn oil diet, but not a high MCT diet, led to a reduction in the sucrase catalytic activity per unit weight of enzyme protein, suggesting that the degradation status of sucrase-isomaltase might be altered by the different types of dietary fats. With MCT feeding, jejunal alkaline phosphatase activity was enhanced to a large extent compared to the activity in other groups. Feeding MCT, compared to lard or corn oil, also increased microvillus phospholipids of the jejunal mucosa. These results suggest that MCT, unlike LCT, do not suppress the activity of mucosal microvillus membrane enzymes in rat small intestine.