Glucose and fatty acid kinetics in fasted rats: Effects of previous protein intake

Glucose and fatty acid turnover and oxidation were measured in 48 hr fasted adult rats who had been previously fed two isocaloric diets of 5% and 18% protein content for 29 days. Kinetics were investigated with the simultaneous primed constant infusions of 6-³H glucose, U-¹⁴C glucose and 1-¹³C palmitic acid. Plasma glucose concentration and glucose turnover were reduced while plasma fatty acid concentration and turnover were increased in the rats fed the 5% protein diet. The protein:carbohydrate ratio of a previous diet affects both glucose and lipid metabolism in adult rats who are fasted.     

Effect of aging on in vivo and in vitro ethanol metabolism and its toxicity in F344 rats

To investigate the effect of aging on ethanol metabolism, 24 male and female F344 rats aged 2 and 12 mo that were fed a laboratory diet received ethanol (1.2 and 2.5 g/kg body wt) intraperitoneally. In male rats, in vivo ethanol elimination significantly decreased according to age both at high (436 +/- 38 vs. 294 +/- 27 mg/kg.h; p less than 0.01) and low (365 +/- 19 vs. 261 +/- 8 mg/kg.h; p less than 0.01) blood ethanol concentrations. Age did not influence the specific activity of hepatic or gastric alcohol dehydrogenase, whereas the activity was significantly decreased with age in the liver (p less than 0.05) and in the stomach (p less than 0.001) when related to body weight. In addition, the activity of the hepatic microsomal ethanol oxidizing system decreased significantly according to age (8.7 +/- 0.5 vs. 6.00 +/- 0.3 nmol/min.mg micr. protein; p less than 0.001). To study the response of ethanol-metabolizing enzymes to chronic ethanol ingestion, 2- and 19-mo-old male F344 rats were pair-fed nutritionally adequate liquid diets containing 36% of total calories either as ethanol or isocaloric carbohydrate for 3 wk. In this experiment specific alcohol dehydrogenase activity was not significantly affected by age, whereas the hepatic microsomal function estimated by the determination of cytochrome P450, microsomal ethanol oxidizing system, and aniline hydroxylation as well as hepatic mitochondrial low Km-acetaldehyde dehydrogenase activity was found to be markedly depressed with age (p less than 0.01). Chronic ethanol consumption increased microsomal enzyme activities in older rats to levels comparable to those observed in young animals prior to ethanol administration. Chronic ethanol feeding also resulted in an increased hepatic fat accumulation, which was significantly enhanced in older rats. In contrast to male rats, in vivo ethanol metabolism was practically identical for 2- and 12-mo-old female rats. These data demonstrate an enhanced toxicity of alcohol in older compared to younger male but not female rats associated with a delay in alcohol elimination both at high and low ethanol blood concentrations and a decrease in ethanol- and acetaldehyde-metabolizing enzyme activities.     

Alcohol Causes a Fatty Pancreas. A Rat Model of Ethanol-Induced Pancreatic Steatosis

To develop an animal model of alcoholic pancreatic steatosis, female Wistar rats were pair fed liquid diets, containing ethanol as 36% of calories or an isocaloric amount of carbohydrate for 3 weeks. Electron microscopic examination showed lipid vesicles localized principally at the bases of pancreatic acinar cells in the ethanol-fed rats. Ethanol feeding significantly increased pancreatic content of cholesteryl ester without changing levels of other lipids. Ethanol feeding enhanced labeled acetate, palmitate, oleate, and lino-leate incorporation into cholesteryl ester. Therefore, increased esterification of cholesterol may, in part, explain the observed accumulation of cholesteryl ester.     

Evidence for enhanced secretory function of hepatic macrophages after long-term ethanol feeding in rats

ABSTRACT— Rats were pair-fed nutritionally adequate liquid diets, containing ethanol as 36% of energy or an isocaloric amount of carbohydrate for 4–6 weeks. Ruffle formation of hepatic macrophages in the periportal area observed with a transmission electron microscope (which reflects their extent in activation) was more remarkable in ethanol-fed rats than in control rats. The ability of hepatic macrophages to produce superoxide anions assessed in situ by formazan deposition after liver perfusion with nitro-blue tetrazolium and phorbol myristate acetate was enhanced after such ethanol feeding. A similar result was seen 24 h after withdrawal of ethanol feeding. These findings suggest that long-term ethanol consumption may activate hepatic macrophages in secretory function.     

Rat hepatic microsomal glucose-6-phosphatase protein levels are increased in streptozotocin-induced diabetes

Summary Hepatic microsomal glucose-6-phosphatase activity levels and the hepatic output of glucose are increased in diabetes. We have used protein chemistry and immunological techniques to determine the mechanism by which the activity levels of the glucose-6-phosphatase system are increased in streptozotocin-induced diabetic rats. In the streptozotocin-induced diabetic rats, the activity of the glucose-6-phosphatase enzyme increased four-fold without appreciably altering the transport capacity of the glucose-6-phosphatase system. The solubilized diabetic rat liver glucose-6-phosphatase enzyme appeared to be very similar to the solubilized enzyme from control rat liver microsomes. They exhibit the same Km, are labile at 30 °C, are stabilized by sodium fluoride and they migrate to the same position during density gradient centrifugation. Immunological studies demonstrated that a greater amount of hepatic microsomal glucose-6-phosphatase enzyme protein is present in diabetic rats than in control rats. Thus, we have determined for the first time that increased levels of the glucose-6-phosphatase protein are present in streptozotocin-induced diabetes. The significance of this finding in relation to the regulation of the hepatic microsomal glucose-6-phosphatase system is discussed.     

Effect of chronic feeding of ethanol diet of “average” fat content on rat pancreas

The present study was done to determine the influence of dietary fat on the effect of ethanol on pancreatic macromolecular content and secretion. Weight-matched groups of Sprague-Dawley rats were divided into controls fed Rodent-Blox ad libitum; American Institute of Nutrition-76 (AIN-76) diet containing 12% calories as fat with 36% of carbohydrate calories replaced with 5% (weight/volume) concentration of ethanol fed ad libitum pair fed with animals given isocaloric amounts of AIN-76 diet for three to six months. Compared with Rodent-Blox fed controls, tissue content of trypsinogen, chymotrypsinogen, amylase, and lipase; specific activity and concentration of trypsinogen, chymotrypsinogen; and concentration of amylase were decreased at six months in AIN-76 fed controls. These changes did not result from diminished food intake, but were due to adaptation to the liquid diet. Animals fed AIN-76 diet plus ethanol did not show significant difference in the total content, specific activity, concentration, and secretion of digestive enzymes compared with those animals pair fed isocaloric amounts of AIN-76 diet. Activation of trypsinogen by exogenous trypsin was lower in rats fed AIN-76 diet and a similar change was observed in animals fed AIN-76 diet with ethanol for six months. These findings are in contrast to increased secretion of proteases and decreased trypsin inhibitor observed previously in animals fed ethanol in a diet containing "high" fat. These data indicate that ethanol effect on the pancreas is modified by dietary intake of fat and/or carbohydrates.     

In vivo insulin resistance in individual peripheral tissues of the high fat fed rat: assessment by euglycaemic clamp plus deoxyglucose administration

Summary We have examined peripheral insulin action in conscious rats chronically fed high fat (60% calories as fat) or high carbohydrate (lab chow) diets using the euglycaemic clamp plus 3 H-2-deoxyglucose technique. A response parameter of individual tissue glucose metabolic rate (the glucose metabolic index, based on tissue deoxyglucose phosphorylation) was used to assess diet effects in eight skeletal muscle types, heart, lung and white and brown adipose tissue. Comparing high fat with high carbohydrate fed rats, basal glucose metabolism was only mildly reduced in skeletal muscle (only diaphragm was significant,p<0.05), but was more substantially reduced in other tissues (e.g. white adipose tissue 61% and heart 33%). No evidence of basal hyperinsulinaemia was found. In contrast, widespread insulin resistance was found during the hyperinsulinaemic clamp (150 mU/l) in high fat fed animals; mean whole body net glucose utilization was 34% lower (p<0.01), and the glucose metabolic index was lower in skeletal muscle (14 to 56%,p< 0.05 in 6 out of 8 muscles), white adipose (27%,p<0.05) and brown adipose tissue (76%,p<0.01). The glucose metabolic index was also lower at maximal insulin levels in muscle and fat, suggesting the major effect of a high fat diet was a loss of insulin responsiveness. White adipose tissue differed from muscle in that incremental responses (maximal insulin minus basal) were not reduced by high fat feeding. The heart showed an effect opposite to other tissues, with an increase in insulin-stimulated glucose metabolism in high fat versus chow fed rats. We conclude that high fat feeding, without a major increase in body weight or basal hyperinsulinaemia, causes widespread but varying degrees of in vivo insulin resistance in peripheral tissues, with major effects in principally oxidative skeletal muscle.     

Effects of ethanol feeding and withdrawal on plasma glutathione elimination in the rat

Chronic ethanol feeding increases hepatic turnover and sinusoidal efflux of glutathione in rats. The present study was performed to determine whether the observed increase in glutathione efflux was due to increased extrahepatic requirements for glutathione. The concentration and disposition of plasma glutathione were determined in rats fed liquid diets containing 36% of calories as ethanol or pair-fed an isocaloric mixture with carbohydrate replacing ethanol calories for 5 to 8 weeks. The half-life and plasma clearance of [35S]glutathione were found to be similar in ethanol-fed and control rats and in rats withdrawn 24 hr from ethanol. Uptakes of the sulfur moiety of [35S]glutathione by kidney, jejunal mucosa, liver, lung, spleen, muscle and heart were also unchanged by ethanol feeding. The plasma glutathione concentration was significantly higher in ethanol-withdrawn rats 22.30 +/- 3.06 nmoles per ml (p less than 0.05) compared to pair-fed controls (13.51 +/- 2.04), while rats continuing to drink ethanol had intermediate levels (16.96 +/- 2.22). Plasma cysteine levels were slightly, but not significantly, higher in ethanol-fed rats. These findings suggest that increased sinusoidal efflux of glutathione in ethanol-fed rats is due to a direct effect of ethanol on hepatic glutathione transport and not due to an alteration in extrahepatic disposition of glutathione. In order to characterize further the effects of ethanol feeding on glutathione-dependent detoxification, activities of glutathione S-transferase, glutathione reductase and gamma-glutamyltransferase were determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic ethanol consumption alters rat liver plasma membranes and potentiates release of alkaline phosphatase

The aim of this study was to investigate possible mechanisms involved in the elevation of serum alkaline phosphatase activity in alcoholics. Male Sprague-Dawley rats were pair-fed nutritionally adequate liquid diets containing ethanol as 36% of energy or an isocaloric amount of carbohydrate for 4-5 wk. Serum alkaline phosphatase activity was increased moderately but significantly. Hepatocytes isolated from ethanol-fed animals exhibited pronounced morphologic alterations of their plasma membranes by scanning electron microscopy and a reduced content of alkaline phosphatase despite an increase in total liver alkaline phosphatase content. Chronic ethanol feeding also potentiated the release of alkaline phosphatase from the cells during incubation with 50 mM ethanol. Furthermore, chronic ethanol feeding resulted in reduced recovery of alkaline phosphatase in hepatic plasma membranes isolated by sucrose gradient centrifugation but did not affect the recoveries of other plasma membrane markers (5'-nucleotidase and Na+,K+-adenosine triphosphatase) nor the subcellular distribution of alkaline phosphatase in the nuclear, mitochondrial, microsomal, and cytosolic fractions. These findings suggest that the increased serum alkaline phosphatase levels observed in response to chronic ethanol feeding may be due, at least in part, to increased lability of this plasma membrane enzyme.     

Subcellular responses of hepatocytes to diabetes in control-fed rats

The biochemistry and ultrastructure of hepatocytes from streptozotocin-diabetic rats adapted to a controlled feeding schedule are described. The microsomal enzyme glucose-6-phosphatase (G-6-Pase), required for glucose release from the hepatocyte was monitored in homogenate preparations at times after the initiation of feeding in rats trained to a 6 h feeding, 18 h fasting cycle. G-6-Pase specific activity which is increased in ad lib fed diabetic rats was not further increased with time after the initiation of feeding in the feeding trained animals. However, the known elevation in G-6-Pase latent activity of the diabetic rat was reduced during the feeding cycle of times of minimum and maximum plasma glucose. Enzyme latency is a reflection of the multicomponent nature of G-6-Pase activity; therefore, plasma glucose levels may influence elements of that multicomponent system. Hepatic rough and smooth endoplasmic reticulum (RER + SER) fractions from the diabetic animals exhibited high and equivalent G-6-Pase specific activities independent of feeding or fasting. Ultrastructural observations of periportal hepatocytes showed a high content of SER correlated with the high G-6-Pase specific activity and closely associated with dispersed particles of glycogen at all times after the initiation of feeding. Also, an increase in SER was observed in the fasted normal animals although particulate glycogen was nearly absent. These findings support earlier work indicating that diabetes stimulates the proliferation of hepatic SER and that the membranes of this organelle are altered from those of the normal animal.     

Modification by sex of diet and ethanol effect on rat pancreatic acinar cell metabolism

The present study was done to determine the role of sex of the animal on the effect of diet and ethanol on pancreatic acinar cell function. Weight-matched groups of Sprague-Dawley rats of either sex were divided into groups of three each and fed Wayne Lab-Blox ad libitum, Lieber-DeCarli diet with 36% of carbohydrate calories replaced with ethanol ad libitum, and Lieber-DeCarli diet in an amount isocaloric to ethanol-fed animals for a period of 3 months. Despite similar amounts of protein, fat, and carbohydrates fed to male and female rats, the female rats had lower amylase content in the tissue when fed Lieber-DeCarli diet and a higher specific activity of trypsinogen in the tissue of animals fed Lab-Blox. Specific activity of chymotrypsinogen increased in males fed Lieber-DeCarli diet and decreased in females fed the same diet when compared with animals of the same sex fed Lab-Blox. Secretion of various digestive enzymes was also different in male and female rats, whereas trypsin inhibitor secretion was similar. These data indicate different adaptive responses in male and female rats to diets with similar proportions of nutrients. When ethanol-fed male rats were compared with ethanol-fed female rats, there was a significant increase in secretion of trypsinogen and amylase (and a proportional but statistically nonsignificant increase in lipase) in female rats. These data indicate that chronic feeding of ethanol results in a nonparallel secretion of digestive enzymes in both sexes with a greater discordance between the trypsinogen secretion and trypsin inhibitor in female rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of heat and detergent on glucose-6-phosphatase activity in microsomes isolated from rats of different ages

Hepatic microsomal glucose-6-phosphatase activity in male Fischer rats declines with age. It is unknown whether the phospholipid composition of the microsomal membrane contributes to this age-related decline in enzyme activity. This report compares the effect of two lipid modifiers, heat and detergent, on the activity of glucose-6-phosphatase in microsomes isolated from male Fischer 344 rats of different ages. The heat lability of this enzyme in intact and detergent-disrupted microsomes was not age dependent. These results suggest that the decline in glucose-6-phosphatase activity during liver senescence is not due to lipid changes in the microenvironment of this enzyme that are detectable by thermolability and detergent sensitivity.     

Effect of amphotericin B on hepatic cytochrome P-450 and glucose-6-phosphatase in the rat

The effect of amphotericin B on hepatic microsomal cytochrome P-450 (P-450) concentration was measured in vitro, in vivo and ex vivo in the rat. In vitro, both amphotericin B (0–500 μg/ml) and its vehicle, sodium deoxycholate (0–410 μg/ml), caused similar dose-dependent decreases of P-450 concentrations and glucose-6-phosphatase activity. Intravenous amphotericin B (3 mg/kg) given daily for 3 days decreased antipyrine clearance from control values of 1.24 ± 0.24 ml/min to 0.67 ± 0.12 ml/min (p < 0.001); whereas antipyrine clearance was unchanged by sodium deoxycholate. The P-450 concentration on the third day was reduced from 0.74 ± 0.14 nmol/mg protein in control rats to 0.33 ± 0.09 nmol/mg protein in rats treated with amphotericin B (p < 0.001). Sodium deoxycholate had no effect on P-450 concentration. In contrast, amphotericin B had no effect on either antipyrine clearance or P-450 concentration following enzyme induction by phenobarbital. Amphotericin B had no effect on microsomal glucose-6-phosphatase activity in vivo. Neither amphotericin B nor sodium deoxycholate induced lipid peroxidation, measured as malondialdehyde production. These results show that amphotericin B decreases hepatic cytochrome P-450 content and function in the rat. These effects can not be observed in the enzyme induced state. Amphotericin B has no effect on glucose-6-phosphatase in vivo, the key enzyme of the gluconeogenesis, indicating selective effects on hepatic microsomal function.     

Effects of glucocorticoids on microsomal membrane synthesis in hepatocytes from adrenalectomized rats.

Biochemical and morphological studies were performed on livers from normal, adrenalectomized (ADX), and ADX and dexamethasone (DEX)-treated rats to investigate the effects of glucocorticoids on microsomal membrane synthesis. Overnight fasted normal, ADX and ADX rats treated 2 or 4 h with DEX received [3H]leucine and [14C]glycerol. Livers were removed, and tissue specimens were prepared for electron microscopy and tissue fractionation. Liver microsomal subfractions were prepared and subsequently washed to produce rough and smooth microsomal membranes. Radioactivity and membrane composition were determined, and glucose-6-phosphatase activity was measured in washed microsomal membranes. Adrenalectomy caused decreased microsomal membrane synthesis. Two and 4 h of DEX administration restored microsomal membrane synthesis to normal levels. ADX also caused an alteration in composition of the microsomal membranes (reflected in decreased phospholipid-protein ratios), which was restored to normal levels by 4 h after DEX administration. The earliest effects of the hormone on membrane synthesis were observed in smooth microsomes as part of a smooth endoplasmic reticulum (SER) proliferation. These findings were supported by observations made with the electron microscope. The proliferating SER was enriched in at least one component: glucose-6-phosphatase. Although the specific relationship of SER to glucocorticoid action remains unclear, the interpretation is offered that SER proliferation and alteration in glucose-6-phosphatase distribution are component parts of the total response of the hepatocyte to glucocorticoids.     

Evidence for enhanced secretory function of hepatic macrophages after long-term ethanol feeding in rats.

Rats were pair-fed nutritionally adequate liquid diets, containing ethanol as 36% of energy or an isocaloric amount of carbohydrate for 4-6 weeks. Ruffle formation of hepatic macrophages in the periportal area observed with a transmission electron microscope (which reflects their extent in activation) was more remarkable in ethanol-fed rats than in control rats. The ability of hepatic macrophages to produce superoxide anions assessed in situ by formazan deposition after liver perfusion with nitro-blue tetrazolium and phorbol myristate acetate was enhanced after such ethanol feeding. A similar result was seen 24 h after withdrawal of ethanol feeding. These findings suggest that long-term ethanol consumption may activate hepatic macrophages in secretory function.     

Reduced thermic effect of feeding in obesity: Role of norepinephrine

Several studies have suggested that obese subjects have a reduced thermic effect of feeding when compared to normal weight controls. The present study was undertaken to further define this apparent abnormality, and evaluate the role of norepinephrine in the thermic response to food. A test formula meal of 800 calories (85% carbohydrate, 15% protein) was taken by 7 control and 6 moderately obese subjects whose obesity was adult in onset. The rise in resting oxygen consumption following the test meal was greater in the control than in the obese group (p < 0.01), and there was a significant inverse correlation between the relative degree of obesity and this response to feeding (r = ?0.59, p < 0.05). Norepinephrine concentrations were greater in the obese than in the control group both before (p < 0.05) and after (p < 0.05) feeding. No correlations were found between the plasma norepinephrine concentrations and the rise in oxygen consumption after feeding. Four of the 6 obese subjects were restudied after weight reduction. The reduced-obese group showed a trend toward normalization of basal measurements and responses to feeding. It is concluded that the reduced thermic response to feeding seen in the obese subjects studied cannot be directly accounted for by diminished sympathetic nervous system activity as reflected by plasma levels of norepinephrine.     

The action of hydrocortisone, insulin, and glucagon on chicken liver hexokinase and glucose-6-phosphatase and on the plasma glucose and free fatty acid concentrations.

Chicken liver hexokinase and glucose-6-phosphatase activities were measured i vitro following the administration of glucagon, insulin, and hydrocortisone in vivo. Insulin lowered hexokinase activity but did not affect glucose-6-phosphatase activity. Neither enzyme was altered by glucagon. Hydrocortisone suppressed hexokinase activity 1, 4, and 24 hr after injection. White the glucose-6-phosphatase activity per unit weight of liver was unaltered by hydrocortisone, the total liver weight was increased and the ratio of enzyme activity to wet weight remained constant. Hydrocortisone caused hyperglycaemic at all three time intervals. Hydrocortisone elevated the plasma FFA concentration up to 4 hr but was increased threefold over the fed control values after 24 hr. The effect of these three hormones on the regulation of glucose flux into and out of liver may be partly explained by actions on hexokinase and glucose-6-phosphatase. It is likely that other liver enzyme activities which alter the flux through glucose-6-phosphatase are also changed. Hydrocortisone greatly increases the flux through gluconeogenesis such that hyperglycaemia and a large increase in glycogen content are achieved.     

Alterations in the Regulatory Properties of Hepatic Fatty Acid Oxidation and Carnitine Palmitoyltransferase I Activity after Ethanol Feeding and Withdrawal

The effects of prolonged ethanol feeding on the regulatory properties of both hepatic fatty acid oxidation and carnitine palmitoyltransferase I activity (CPT-I) were studied in rats fed a high-fat diet containing 36% of total calories as ethanol (ethanol group) or an isocaloric amount of carbohydrate (control group). Prolonged ethanol feeding progressively decreased CPT-I activity and increased enzyme sensitivity and sensitization to inhibition by malonyl-CoA in liver mitochondria. Similarly, long-term ethanol feeding progressively increased the sensitivity of CPT-I, as well as that of fatty acid oxidation, to inhibition by 4-hydroxyphenylglyoxylate. Short-term addition of ethanol or acetaldehyde to the incubations markedly increased the sensitivity of CPT-I to inhibition by malonyl-CoA in a subsequent assay in hepatocytes isolated from ethanol-treated rats, but not in cells from control animals. This effect may be mediated by the ethanol- or acetaldehyde-induced increase of intracellular malonyl-CoA levels. The present results show that ethanol feeding to rats leads to profound alterations in the regulatory properties of hepatic CPT-I, which seem to be determinant for the decreased capacity of fatty acid oxidation by the liver in this state. Nevertheless, all the above-mentioned alterations of the fatty acid oxidative system were reversible, disappearing after 2 to 4 days of ethanol withdrawal.     

Short-term energy balance in patients with infections: carbohydrate-based versus fat-based diets.

The effects of a carbohydrate-based diet (50% carbohydrate calories, 30% fat calories, 20% protein calories) versus a fat-based diet (28% carbohydrate calories, 55% fat calories, 17% protein calories) on oxidation rates of carbohydrate, fat, and protein were assessed in 12 patients with infections by indirect calorimetry and estimation of urea nitrogen production rate. The diets were given continuously for 18 hours in a randomized cross-over study on 2 consecutive days. Energy supply (kcal/d) was adjusted individually to meet the energy expenditure measured on the preceding day after an overnight fast and was 1,647 +/- 129 (SEM) for the carbohydrate-based diet and 1,655 +/- 131 for the fat-based diet. Oxidation rates (kcal/d) for carbohydrate (carbohydrate-based diet, 525 +/- 70; fat-based diet, 363 +/- 84) were different between the diets (P less than .05), whereas no difference could be found for fat (carbohydrate-based diet, 820 +/- 117; fat-based diet, 968 +/- 136) and protein (carbohydrate-based diet, 252 +/- 29; fat-based diet, 236 +/- 23). However, during carbohydrate-based feeding, carbohydrate balance (288 +/- 93 kcal/d) and fat balance (-327 +/- 107 kcal/d) were significantly different from zero (P less than .05), indicating continuous oxidation of endogenous fat and storage of administered glucose. During the fat-based diet, carbohydrate and fat balances were not different from zero. A correlation between energy and substrate balances was not seen during either diet.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of dietary fat in peripheral thyroid hormone metabolism

Short term changes in serum 3,3′,5-triiodothyronine (T₃) and 3,3′,5′-triiodothyronine (reverse T₃, rT₃) were studied in four healthy nonobese male subjects under varying but isocaloric and weight maintaining conditions. The four 1500 kcal diets tested during 72 hr, consisted of: 1, 100% fat; II, 50% fat, 50% protein; III, 50% fat, 50% carbohydrate (CHO), and IV, a mixed control diet. The decrease of T₃ (50%) and increase of rT₃ (123%) in the all-fat diet equalled changes noted in total starvation. In diet III (750 kcal fat, 750 kcal CHO) serum T₃ decreased 24% (NS) and serum rT₃ rose significantly 34% (p < 0.01). This change occurred in spite of the 750 kcal CHO. This amount of CHO by itself does not introduce changes in thyroid hormone levels and completely restores in refeeding models the alterations of T₃ and rT₃ after total starvation. The conclusion is drawn that under isocaloric conditions in man fat in high concentration itself may play an active role in inducing changes in peripheral thyroid hormone metabolism.