Antiketogenic effect of glucose per se in vivo in man and in vitro in isolated rat liver cells

Utilization of dietary glucose in the metabolic syndrome

This review is focused on the fate of dietary glucose under conditions of chronically high energy (largely fat) intake, evolving into the metabolic syndrome. We are adapted to carbohydrate-rich diets similar to those of our ancestors. Glucose is the main energy staple, but fats are our main energy reserves. Starvation drastically reduces glucose availability, forcing the body to shift to fatty acids as main energy substrate, sparing glucose and amino acids. We are not prepared for excess dietary energy, our main defenses being decreased food intake and increased energy expenditure, largely enhanced metabolic activity and thermogenesis. High lipid availability is a powerful factor decreasing glucose and amino acid oxidation. Present-day diets are often hyperenergetic, high on lipids, with abundant protein and limited amounts of starchy carbohydrates. Dietary lipids favor their metabolic processing, saving glucose, which additionally spares amino acids. The glucose excess elicits hyperinsulinemia, which may derive, in the end, into insulin resistance. The available systems of energy disposal could not cope with the excess of substrates, since they are geared for saving not for spendthrift, which results in an unbearable overload of the storage mechanisms. Adipose tissue is the last energy sink, it has to store the energy that cannot be used otherwise. However, adipose tissue growth also has limits, and the excess of energy induces inflammation, helped by the ineffective intervention of the immune system. However, even under this acute situation, the excess of glucose remains, favoring its final conversion to fat. The sum of inflammatory signals and deranged substrate handling induce most of the metabolic syndrome traits: insulin resistance, obesity, diabetes, liver steatosis, hyperlipidemia and their compounded combined effects. Thus, a maintained excess of energy in the diet may result in difficulties in the disposal of glucose, eliciting inflammation and the development of the metabolic syndrome.

Progressive adaptation of hepatic ketogenesis in mice fed a high-fat diet

Hepatic ketogenesis provides a vital systemic fuel during fasting because ketone bodies are oxidized by most peripheral tissues and, unlike glucose, can be synthesized from fatty acids via mitochondrial beta-oxidation. Since dysfunctional mitochondrial fat oxidation may be a cofactor in insulin-resistant tissue, the objective of this study was to determine whether diet-induced insulin resistance in mice results in impaired in vivo hepatic fat oxidation secondary to defects in ketogenesis. Ketone turnover (micromol/min) in the conscious and unrestrained mouse was responsive to induction and diminution of hepatic fat oxidation, as indicated by an eightfold rise during the fed (0.50+/-0.1)-to-fasted (3.8+/-0.2) transition and a dramatic blunting of fasting ketone turnover in PPARalpha(-/-) mice (1.0+/-0.1). C57BL/6 mice made obese and insulin resistant by high-fat feeding for 8 wk had normal expression of genes that regulate hepatic fat oxidation, whereas 16 wk on the diet induced expression of these genes and stimulated the function of hepatic mitochondrial fat oxidation, as indicated by a 40% induction of fasting ketogenesis and a twofold rise in short-chain acylcarnitines. Together, these findings indicate a progressive adaptation of hepatic ketogenesis during high-fat feeding, resulting in increased hepatic fat oxidation after 16 wk of a high-fat diet. We conclude that mitochondrial fat oxidation is stimulated rather than impaired during the initiation of hepatic insulin resistance in mice.

Effects of insulin on ketogenesis following fasting in lean and obese men

The ketone bodies (KBs) D-3-hydroxybutyrate (D-3HB) and acetoacetate (AcAc) play a role in starvation and have been associated with insulin resistance. The dose-response relationship between insulin and KBs was demonstrated to be shifted to the right in type 2 diabetes patients. However, KB levels have also been reported to be decreased in obesity. We investigated the metabolic adaptation to fasting with respect to glucose and KB metabolism in lean and obese men without type 2 diabetes using stable glucose and D-3HB isotopes in a two-step pancreatic clamp after 38 h of fasting. We found that D-3HB fluxes in the basal state were higher in lean compared to obese men: 15.2 (10.7-27.1) vs. 7.0 (3.5-15.1) micromol/kg lean body mass (LBM) x min, respectively, P < 0.01. No differences were found in KB fluxes between lean and obese volunteers during the pancreatic clamp (step 1: 6.9 (1.8-12.0) vs. 7.4 (4.2-17.8) micromol/kg LBM x min, respectively; and step 2: 2.9 (0-7.2) vs. 3.4 (0.85-18.7) micromol/kg LBM x min, respectively), despite similar plasma insulin levels. Meanwhile, peripheral glucose uptake was higher in lean compared to obese men (step 1: 15.2 (12.3-25.6) vs. 14.7 (11.9-22.7) micromol/kg LBM x min, respectively, P < or = 0.05; and step 2: 12.5 (7.0-17.3) vs. 10.8 (5.2-15.0) micromol/kg LBM x min, respectively, P < or = 0.01). These data show that obese subjects who display insulin resistance on insulin-mediated peripheral glucose uptake have the same sensitivity for the insulin-mediated suppression of ketogenesis. This implies differential insulin sensitivity of intermediary metabolism in obesity.

Calorie restriction of a high-carbohydrate diet elevates the threshold of PTZ-induced seizures to values equal to those seen with a ketogenic diet

The purpose of this study was to evaluate the contributions of ketonemia, caloric restriction, and carbohydrates to seizure protection in rats fed selected diets. Male Sprague-Dawley rats were fed experimental diets of two basic types, one high in carbohydrates and restricted to 90, 65, or 50% of the normal daily caloric requirement and the other a normal rodent chow diet restricted to 90 or 65% of the daily caloric requirement. After consuming their respective diets for 20 days, animals were subjected to tail-vein infusion of pentylenetetrazole (PTZ) to determine seizure threshold, taken as the dose required to evoke the first clonic reaction. Seizure thresholds were compared to those of rats fed control diets of either normal rodent chow fed ad libitum or a standard high-fat (ketogenic) diet calorie-restricted to 90% of daily caloric requirement, all animals age- and weight-matched at the time of diet onset. All diets were balanced for vitamins and minerals and contained at least 10% protein (by weight). Seizure threshold and ketonemia were elevated in both experimental diets in approximate proportion to the degree of calorie restriction. Animals fed the most severely restricted high-carbohydrate diet (50%) had seizure thresholds equal to those fed the ketogenic diet but had significantly lower ketonemia.

A ketogenic diet increases the resistance to pentylenetetrazole-induced seizures in the rat

PURPOSE

The purpose of this study was to test the hypothesis that a ketogenic diet would increase the resistance of rats to pentylenetetrazole (PTZ)-induced seizures and to understand the relation of ketonemia to seizure resistance.

METHODS

A freely consumed, high-fat (ketogenic) diet was administered to male Sprague-Dawley rats for 5-10 weeks, while control animals were fed either rodent chow or a high-carbohydrate diet. Ketonemia was measured as plasma levels of beta-hydroxybutyric acid (beta-OHB). Seizures were induced by tail-vein infusion of pentylenetetrazole.

RESULTS

The ketogenic diet produced a highly significant (p<0.01) increase in beta-OHB levels within 5 days. Induction of seizures by PTZ 35 days after animals were placed on their respective diets showed that ketogenic animals had a significantly (p<0.01) increased threshold for seizure induction compared with those fed an isocaloric diet of either high-carbohydrate or normal rodent chow. Ketogenic animals did not exhibit increased seizure severity relative to controls, despite receiving consistently higher doses of PTZ.

CONCLUSIONS

The ketogenic diet resulted in an increased seizure threshold, confirming the hypothesis, and seizure threshold was found to be a direct function of the level of ketonemia.

High blood ketone body concentration in type 2 non-insulin dependent diabetic patients

To assess the metabolic disturbances, and, in particular, the occurrence of high blood ketone body concentration in post-absorptive Type 2 (non-insulin-dependent) diabetic patients as compared to a matched normal population, a study was carried out in a group of 78 Type 2 diabetic outpatients matched for age and sex and in 78 normal individuals. In all subjects we measured HbA1c, and fasting levels of glucose, FFA, lactate, pyruvate, glycerol, alanine, 3-hydroxybutyrate, acetoacetate, uric acid, total cholesterol, triglycerides, creatinine, growth hormone, cortisol, glucagon, free insulin, and C-peptide. Multistix strips were used for urine ketone determination. As expected HbA1c, and plasma glucose were higher in Type 2 diabetics. This was associated with multiple metabolic disturbances as shown by higher circulating concentrations of FFA, glycerol and gluconeogenic precursors. Similarly, blood levels of ketones (351 +/- 29 vs 159 +/- 15 umol/l; P < 0.0001) were increased, in spite of higher plasma free-insulin (77 +/- 7 vs. 49 +/- 14 pmol/l; p < 0.0001) and C-peptide concentration (0.63 +/- 0.03 vs. 0.46 +/- 0.07 nmol/l; P < 0.05) and no differences in plasma levels of cortisol, and growth hormone. Plasma glucagon levels were higher in Type 2 diabetics. Blood ketone body levels were directly correlated with both plasma glucose and FFA concentrations. These observations clearly show that Type 2 diabetes is a pathologic condition characterised by multiple metabolic disturbances which are fully apparent in the basal state. Furthermore, we emphasise that Type 2 diabetic patients, though not insulin deficient, may present a significant increase in their fasting levels of ketone bodies.

Comparison of the effects of various amino acids on glycogen synthesis, lipogenesis and ketogenesis in isolated rat hepatocytes

Several amino acids were found to stimulate glycogen synthesis and lipogenesis, and to inhibit ketogenesis in isolated rat hepatocytes. When hepatocytes were incubated in the presence of 20 mM-glucose, the amino acids could be classified in decreasing order of efficiency as follows: glutamine and proline, alanine, aminoisobutyric acid, asparagine and histidine for stimulation of glycogen synthesis; glutamine, proline and alanine for stimulation of lipogenesis; proline and glutamine for inhibition of ketogenesis. The study of the time course revealed that the rates were not linear and were preceded by a lag period. In all conditions studied, glutamine and proline were found to have similar quantitative effects on glycogen synthesis and lipid metabolism. However, their effects differ qualitatively. Indeed, the effects of proline on glycogen synthesis, lipogenesis and glutamate and aspartate content were faster. Moreover, proline increased the hydroxybutyrate/acetoacetate ratio, whereas glutamine did not change it. Incubation of hepatocytes with aminoisobutyric acid or under hypo-osmotic conditions, which increased cell volume and mimicked the amino acid-induced stimulation of glycogen synthesis, had little effect on lipogenesis. In hepatocytes incubated without glucose, ketogenesis was inhibited, in decreasing order of efficiency, by alanine, asparagine, glutamine and proline. Under these conditions, glutamine increased, alanine decreased and asparagine did not affect the concentration of malonyl-CoA. This indicates that the latter cannot be responsible for the inhibition of ketogenesis by alanine and asparagine.

Hyperglycemia per se can reduce plasma free fatty acid and glycerol levels in the acutely insulin-deficient dog

To evaluate the in vivo effect of hyperglycemia per se on plasma free fatty acid (FFA) and glycerol concentrations, euglycemic and hyperglycemic clamp studies were performed in six overnight fasted dogs in the state of insulin deficiency produced by somatostatin (SRIF) infusion. The mean blood glucose concentrations during the steady-state (the second hour of each study) averaged 4.65 +/- 0.10 mmol/L in euglycemic clamp and 14.11 +/- 0.10 mmol/L in hyperglycemic clamp. During the SRIF infusion, plasma FFA concentrations increased from 0.32 +/- 0.05 mumol/mL at the basal state to 0.76 +/- 0.04 mumol/mL at the steady-state in euglycemic clamp and from 0.26 +/- 0.04 mumol/mL to 0.43 +/- 0.02 mumol/mL in hyperglycemic clamp. Plasma glycerol concentrations increased from the basal value of 0.07 +/- 0.01 mumol/mL to 0.15 +/- 0.01 mumol/mL during the steady-state in euglycemic clamp and from 0.06 +/- 0.01 mumol/mL to 0.08 +/- 0.01 mumol/mL in hyperglycemic clamp. The steady-state concentrations of plasma FFA and glycerol in hyperglycemic clamp were significantly lower than those in euglycemic clamp (P less than .001; respectively). These results suggest that hyperglycemia per se might decrease plasma FFA and glycerol concentrations at least in part by decreasing lipolysis in the acutely insulin-deficient dog.