Low-carbohydrate diet alters intracellular glucose metabolism but not overall glucose disposal in exercise-trained subjects

Tuning fatty acid oxidation in skeletal muscle with dietary fat and exercise

Both the consumption of a diet rich in fatty acids and exercise training result in similar adaptations in several skeletal muscle proteins. These adaptations are involved in fatty acid uptake and activation within the myocyte, the mitochondrial import of fatty acids and further metabolism of fatty acids by β-oxidation. Fatty acid availability is repeatedly increased postprandially during the day, particularly during high dietary fat intake and also increases during, and after, aerobic exercise. As such, fatty acids are possible signalling candidates that regulate transcription of target genes encoding proteins involved in muscle lipid metabolism. The mechanism of signalling might be direct or indirect targeting of peroxisome proliferator-activated receptors by fatty acid ligands, by fatty acid-induced NAD⁺-stimulated activation of sirtuin 1 and/or fatty acid-mediated activation of AMP-activated protein kinase. Lactate might also have a role in lipid metabolic adaptations. Obesity is characterized by impairments in fatty acid oxidation capacity, and individuals with obesity show some rigidity in increasing fatty acid oxidation in response to high fat intake. However, individuals with obesity retain improvements in fatty acid oxidation capacity in response to exercise training, thereby highlighting exercise training as a potential method to improve lipid metabolic flexibility in obesity.

Postprandial skeletal muscle metabolism following a high-fat diet in sedentary and endurance-trained males

Our objective was to determine the influence of a high-fat diet (HFD) on fasting and postprandial skeletal muscle substrate metabolism in endurance-trained (ET) compared with sedentary (SED) humans. SED (n = 17) and ET (n = 7) males were control-fed a 10-day moderate-fat diet followed by a 5-day isocaloric HFD (55% fat, 30% carbohydrate). Skeletal muscle biopsies were taken in the fasted condition and 4 h after a high-fat meal (820 kcals; 63% fat and 25% carbohydrate). Palmitate-induced suppression of pyruvate oxidation, an indication of substrate preference, and oxidation of fat and glucose were measured in homogenized skeletal muscle in fasted and fed states. Postprandial responses were calculated as percent changes from fasting to fed states. Postprandial suppression of pyruvate oxidation was maintained after the HFD in ET, but not SED skeletal muscle, suggesting greater adaptability to dietary intake changes in the former. Fasting total fat oxidation increased due to the HFD in ET skeletal muscle (P = 0.006), which was driven by incomplete fat oxidation (P = 0.008). Fasting fat oxidation remained unchanged in skeletal muscle of SED individuals. Yet, postprandial fat oxidation was similar between groups. Fasting glucose oxidation was elevated after the HFD in ET (P = 0.036), but not SED, skeletal muscle. Postprandial glucose oxidation was reduced due to the HFD in SED (P = 0.002), but not ET, skeletal muscle. These findings provide insight into differing substrate metabolism responses between SED and ET individuals and highlight the role that the prevailing diet may play in modulating fasting and postprandial metabolic responses in skeletal muscle.NEW & NOTEWORTHY The relationship between high dietary fat intake and physical activity level and their combined effect on skeletal muscle substrate metabolism remains unclear. We assessed the influence of the prevailing diet in modulating substrate oxidation in skeletal muscle of endurance-trained compared with sedentary humans during a high-fat challenge meal. Collectively, our findings demonstrate the adaptability of skeletal muscle in endurance-trained individuals to high dietary fat intake.

Effect of acute and short-term dietary fat ingestion on postprandial skeletal muscle protein synthesis rates in middle-aged, overweight, and obese men

Muscle anabolic resistance to dietary protein is associated with obesity and insulin resistance. However, the contribution of excess consumption of fat to anabolic resistance is not well studied. The aim of these studies was to test the hypothesis that acute and short-term dietary fat overload will impair the skeletal muscle protein synthetic response to dietary protein ingestion. Eight overweight/obese men [46.4 ± 1.4 yr, body mass index (BMI) 32.3 ± 5.4 kg/m²] participated in the acute feeding study, which consisted of two randomized crossover trials. On each occasion, subjects ingested an oral meal (with and without fat emulsion), 4 h before the coingestion of milk protein, intrinsically labeled with [1-¹³C]phenylalanine, and dextrose. Nine overweight/obese men (44.0 ± 1.7 yr, BMI 30.1 ± 1.1 kg/m²) participated in the chronic study, which consisted of a baseline, 1-wk isocaloric diet, followed by a 2-wk high-fat diet (+25% energy excess). Acutely, incorporation of dietary amino acids into the skeletal muscle was twofold higher (P < 0.05) in the lipid trial compared with control. There was no effect of prior lipid ingestion on indices of insulin sensitivity (muscle glucose uptake, pyruvate dehydrogenase complex activity, and Akt phosphorylation) in response to the protein/dextrose drink. Fat overfeeding had no effect on muscle protein synthesis or glucose disposal in response to whey protein ingestion, despite increased muscle diacylglycerol C16:0 (P = 0.06) and ceramide C16:0 (P < 0.01) levels. Neither acute nor short-term dietary fat overload has a detrimental effect on the skeletal muscle protein synthetic response to dietary protein ingestion in overweight/obese men, suggesting that dietary-induced accumulation of intramuscular lipids per se is not associated with anabolic resistance.

Mechanisms Preserving Insulin Action during High Dietary Fat Intake

Prolonged intervention studies investigating molecular metabolism are necessary for a deeper understanding of dietary effects on health. Here we provide mechanistic information about metabolic adaptation to fat-rich diets. Healthy, slightly overweight men ingested saturated or polyunsaturated fat-rich diets for 6 weeks during weight maintenance. Hyperinsulinemic clamps combined with leg balance technique revealed unchanged peripheral insulin sensitivity, independent of fatty acid type. Both diets increased fat oxidation potential in muscle. Hepatic insulin clearance increased, while glucose production, de novo lipogenesis, and plasma triacylglycerol decreased. High fat intake changed the plasma proteome in the immune-supporting direction and the gut microbiome displayed changes at taxonomical and functional level with polyunsaturated fatty acid (PUFA). In mice, eucaloric feeding of human PUFA and saturated fatty acid diets lowered hepatic triacylglycerol content compared with low-fat-fed control mice, and induced adaptations in the liver supportive of decreased gluconeogenesis and lipogenesis. Intake of fat-rich diets thus induces extensive metabolic adaptations enabling disposition of dietary fat without metabolic complications.

Interorgan Metabolic Crosstalk in Human Insulin Resistance

Excessive energy intake and reduced energy expenditure drive the development of insulin resistance and metabolic diseases such as obesity and type 2 diabetes mellitus. Metabolic signals derived from dietary intake or secreted from adipose tissue, gut, and liver contribute to energy homeostasis. Recent metabolomic studies identified novel metabolites and enlarged our knowledge on classic metabolites. This review summarizes the evidence of their roles as mediators of interorgan crosstalk and regulators of insulin sensitivity and energy metabolism. Circulating lipids such as free fatty acids, acetate, and palmitoleate from adipose tissue and short-chain fatty acids from the gut effectively act on liver and skeletal muscle. Intracellular lipids such as diacylglycerols and sphingolipids can serve as lipotoxins by directly inhibiting insulin action in muscle and liver. In contrast, fatty acid esters of hydroxy fatty acids have been recently shown to exert a series of beneficial effects. Also, ketoacids are gaining interest as potent modulators of insulin action and mitochondrial function. Finally, branched-chain amino acids not only predict metabolic diseases, but also inhibit insulin signaling. Here, we focus on the metabolic crosstalk in humans, which regulates insulin sensitivity and energy homeostasis in the main insulin-sensitive tissues, skeletal muscle, liver, and adipose tissue.

Poly(delta-gluconolactone) and Poly(delta-gluconolactone-ε-caprolactone) from delta-Gluconolactone and ε-Caprolactone by Ring-Opening Polymerization

Poly(delta-gluconolactone) (PGL) and poly(delta-gluconolactone-ε-caprolactone) (P(GL-CL)) were synthesized through ring-opening polymerization (ROP) and characterized by FT-IR, NMR, XRD, intrinsic viscosity, GPC, DSC, and TGA. The crystallinity of P(GL-CL) with various d-GL/CL ratios (d-GL/CL = 5 : 5, 4 : 6, 3 : 7, 2 : 8, and 1 : 9) was 12.09 to 59.78% while PGL was amorphous. Melting temperature (Tm) of these polymers was 49.8 to 62.0°C and decomposition temperature was 282 to 489°C depending on the d-GL/CL ratios. In addition, all these polymers were degradable and the degradation rates could be controlled by adjusting d-GL/CL ratios. These results indicated that PGL and P(GL-CL) might be promising novel absorbable materials.

Lipid droplet dynamics and insulin sensitivity upon a 5-day high-fat diet in Caucasians and South Asians

A 5-day High-Fat High-Calorie diet (HFHC-diet) reduces insulin-stimulated glucose disposal (Rd) in South Asian, but not Caucasian healthy lean males. We aimed to investigate if differences in myocellular lipid handling are underlying this differential response. A two-step hyperinsulinemic-euglycemic clamp and muscle biopsies were performed in 12 healthy lean Caucasian and South Asian males (BMI < 25 kg/m², 19-25 years) before and after a 5-day HFHC-diet (regular diet + 375 mL cream/day; 1275 kcal/day; 94% fat). Triglyceride extractions and Western Blots for lipid droplet and mitochondrial proteins were performed. Intramyocellular lipid content and HFHC-diet response were similar between ethnicities (group effect: P = 0.094; diet effect: +~30%, P = 0.044). PLIN5 protein content increased upon the HFHC-diet (P = 0.031) and tended to be higher in South Asians (0.87 ± 0.42 AU vs. 1.35 ± 0.58 AU, P = 0.07). 4-HNE tended to increase in South Asians upon the HFHC-diet (interaction effect: P = 0.057). In Caucasians ΔPLIN5 content correlated with ΔR_d (Caucasians: r = 0.756, P = 0.011; South Asians: r = -0.085, P = 0.816), while in South Asians Δ4-HNE associated with ΔPLIN5 content (Caucasians: r = 0.312, P = 0.380; South Asians: r = 0.771, P = 0.003). These data indicate that in Caucasians, PLIN5 may be protective against HFHC-diet induced insulin resistance, which for reasons not yet understood is not observed in South Asians, who possess increased lipid peroxidation levels.

Gluconeogenesis during endurance exercise in cyclists habituated to a long-term low carbohydrate high-fat diet

KEY POINTS

Blood glucose is an important fuel for endurance exercise. It can be derived from ingested carbohydrate, stored liver glycogen and newly synthesized glucose (gluconeogenesis). We hypothesized that athletes habitually following a low carbohydrate high fat (LCHF) diet would have higher rates of gluconeogenesis during exercise compared to those who follow a mixed macronutrient diet. We used stable isotope tracers to study glucose production kinetics during a 2 h ride in cyclists habituated to either a LCHF or mixed macronutrient diet. The LCHF cyclists had lower rates of total glucose production and hepatic glycogenolysis but similar rates of gluconeogenesis compared to those on the mixed diet. The LCHF cyclists did not compensate for reduced dietary carbohydrate availability by increasing glucose synthesis during exercise but rather adapted by altering whole body substrate utilization.

ABSTRACT

Endogenous glucose production (EGP) occurs via hepatic glycogenolysis (GLY) and gluconeogenesis (GNG) and plays an important role in maintaining euglycaemia. Rates of GLY and GNG increase during exercise in athletes following a mixed macronutrient diet; however, these processes have not been investigated in athletes following a low carbohydrate high fat (LCHF) diet. Therefore, we studied seven well-trained male cyclists that were habituated to either a LCHF (7% carbohydrate, 72% fat, 21% protein) or a mixed diet (51% carbohydrate, 33% fat, 16% protein) for longer than 8 months. After an overnight fast, participants performed a 2 h laboratory ride at 72% of maximal oxygen consumption. Glucose kinetics were measured at rest and during the final 30 min of exercise by infusion of [6,6-(2) H2 ]-glucose and the ingestion of (2) H2 O tracers. Rates of EGP and GLY both at rest and during exercise were significantly lower in the LCHF group than the mixed diet group (Exercise EGP: LCHF, 6.0 ± 0.9 mg kg(-1)  min(-1) , Mixed, 7.8 ± 1.1 mg kg(-1)  min(-1) , P < 0.01; Exercise GLY: LCHF, 3.2 ± 0.7 mg kg(-1)  min(-1) , Mixed, 5.3 ± 0.9 mg kg(-1)  min(-1) , P < 0.01). Conversely, no difference was detected in rates of GNG between groups at rest or during exercise (Exercise: LCHF, 2.8 ± 0.4 mg kg(-1)  min(-1) , Mixed, 2.5 ± 0.3 mg kg(-1)  min(-1) , P = 0.15). We conclude that athletes on a LCHF diet do not compensate for reduced glucose availability via higher rates of glucose synthesis compared to athletes on a mixed diet. Instead, GNG remains relatively stable, whereas glucose oxidation and GLY are influenced by dietary factors.

Effect of a 1-week, eucaloric, moderately high-fat diet on peripheral insulin sensitivity in healthy premenopausal women

OBJECTIVES

To determine whether a weight-maintaining, moderate (50%) high-fat diet is deleterious to insulin sensitivity in healthy premenopausal women.

DESIGN/SETTING/PARTICIPANTS

23 African-American and non-Hispanic white, healthy, overweight, and obese premenopausal women recruited in New York City, USA, fed either a eucaloric, 1-week long high-fat (50% of total Kcal from fat) diet or a eucaloric, 1-week long low-fat (30% of total Kcal from fat) diet, assigned in a randomized crossover design.

MAIN OUTCOME MEASURES

Peripheral insulin sensitivity and metabolic flexibility during a euglycemic hyperinsulinemic (80 mU/m(2)/min) clamp measured during the follicular phase of the menstrual cycle, at the end of each diet period.

RESULTS

Peripheral insulin sensitivity (mg kg/fat-free mass/min (µU/mL)×10(-1)) was not decreased after the high-fat diet vs the low-fat diet (0.09±0.01 vs 0.08±0.01, p=0.09, respectively) in the combined group of African-American and white women, with no significant diet by race interaction (p=0.6). Metabolic flexibility (change in substrate utilization, ΔNPRQ, in response to insulin during the clamp) was similarly unaltered by the diet (0.12±0.01 vs 0.11, p=0.48, for the high-fat diet vs the low-fat diet, respectively) in the combined group of women, with no significant diet by race interaction (p=0.9). African-American women had a lower insulin clearance compared with the white women, regardless of the diet (p<0.05).

CONCLUSIONS

We conclude that a short term (1 week), moderate (50%), eucaloric high-fat diet does not lower peripheral insulin sensitivity in healthy, overweight and obese premenopausal women.

A 5-day high-fat, high-calorie diet impairs insulin sensitivity in healthy, young South Asian men but not in Caucasian men

South Asians (SAs) develop type 2 diabetes at a younger age and lower BMI compared with Caucasians (Cs). The underlying cause is still poorly understood but might result from an innate inability to adapt to the Westernized diet. This study aimed to compare the metabolic adaptation to a high-fat, high-calorie (HFHC) diet between both ethnicities. Twelve healthy, young lean male SAs and 12 matched Cs underwent a two-step hyperinsulinemic-euglycemic clamp with skeletal muscle biopsies and indirect calorimetry before and after a 5-day HFHC diet. Hepatic triglyceride content (HTG) and abdominal fat distribution were assessed using magnetic resonance imaging and spectroscopy. At baseline, SAs had higher insulin clamp levels than Cs, indicating reduced insulin clearance rate. Despite the higher insulin levels, endogenous glucose production was comparable between groups, suggesting lower hepatic insulin sensitivity in SAs. Furthermore, a 5-day HFHC diet decreased the insulin-stimulated (nonoxidative) glucose disposal rate only in SA. In skeletal muscle, no significant differences were found between groups in insulin/mammalian target of rapamycin signaling, metabolic gene expression, and mitochondrial respiratory chain content. Furthermore, no differences in (mobilization of) HTG and abdominal fat were detected. We conclude that HFHC feeding rapidly induces insulin resistance only in SAs. Thus, distinct adaptation to Western food may partly explain their propensity to develop type 2 diabetes.

Cardiovascular and Hormonal Aspects of Very-Low-Carbohydrate Ketogenic Diets

In recent years, restriction of carbohydrate intake for weight loss has become widespread. Our research group began studying physiological responses to very-low-carbohydrate ketogenic diets (VLCKDs) in the late 1990s because we felt there was a significant void in the literature and limited understanding of metabolic responses to VLCKDs. This launched us into a line of research examining the physiological effects of VLCKDs. In this paper, we briefly overview nine studies we have published on isoenergetic and hypoenergetic VLCKDs in men and women. These studies have focused on blood lipid responses to VLCKDs, but we have also addressed changes in body weight, body composition, and hormones. Compared with low-fat diets, short-term VLCKDs consistently result in improvements in fat loss, fasting and postprandial triacylglycerols, high-density lipoprotein-cholesterol, the distribution of low-density lipoprotein-cholesterol subclasses, and insulin resistance. These are the key metabolic abnormalities of metabolic syndrome, a problem of epidemic proportions in the United States. There is substantial variability in total cholesterol and low-density lipoprotein-cholesterol responses to VLCKD. The factors responsible for this variability are not known, and studies designed to identify methods to predict blood lipid responses to VLCKD and other dietary approaches represent critical areas for nutrition researchers. Further research is warranted to validate the physiological effects of VLCKD over longer periods of time, including studies that modify the quality of macronutrients (i.e., the type of fat and protein) and the interaction with other interventions (e.g., exercise, dietary supplements, drugs).

Effect of low-calorie versus low-carbohydrate ketogenic diet in type 2 diabetes

OBJECTIVE

Effective diabetic management requires reasonable weight control. Previous studies from our laboratory have shown the beneficial effects of a low-carbohydrate ketogenic diet (LCKD) in patients with type 2 diabetes after its long term administration. Furthermore, it favorably alters the cardiac risk factors even in hyperlipidemic obese subjects. These studies have indicated that, in addition to decreasing body weight and improving glycemia, LCKD can be effective in decreasing antidiabetic medication dosage. Similar to the LCKD, the conventional low-calorie, high nutritional value diet is also used for weight loss. The purpose of this study was to understand the beneficial effects of LCKD compared with the low-calorie diet (LCD) in improving glycemia.

METHODS

Three hundred and sixty-three overweight and obese participants were recruited from the Al-Shaab Clinic for a 24-wk diet intervention trial; 102 of them had type 2 diabetes. The participants were advised to choose LCD or LDKD, depending on their preference. Body weight, body mass index, changes in waist circumference, blood glucose level, changes in hemoglobin and glycosylated hemoglobin, total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, triglycerides, uric acid, urea and creatinine were determined before and at 4, 8, 12, 16, 20, and 24 wk after the administration of the LCD or LCKD. The initial dose of some antidiabetic medications was decreased to half and some were discontinued at the beginning of the dietary program in the LCKD group. Dietary counseling and further medication adjustment were done on a biweekly basis.

RESULTS

The LCD and LCKD had beneficial effects on all the parameters examined. Interestingly, these changes were more significant in subjects who were on the LCKD as compared with those on the LCD. Changes in the level of creatinine were not statistically significant.

CONCLUSION

This study shows the beneficial effects of a ketogenic diet over the conventional LCD in obese diabetic subjects. The ketogenic diet appears to improve glycemic control. Therefore, diabetic patients on a ketogenic diet should be under strict medical supervision because the LCKD can significantly lower blood glucose levels.

High-fat feeding inhibits exercise-induced increase in mitochondrial respiratory flux in skeletal muscle

Twenty one healthy untrained male subjects were randomized to follow a high-fat diet (HFD; 55-60E% fat, 25-30E% carbohydrate, and 15E% protein) or a normal diet (ND; 25-35E% fat, 55-60E% carbohydrate, and 10-15E% protein) for 2(1/2) wk. Diets were isocaloric and tailored individually to match energy expenditure. At 2(1/2) wk of diet, one 60-min bout of bicycle exercise (70% of maximal oxygen uptake) was performed. Muscle biopsies were obtained before and after the diet, immediately after exercise, and after 3-h recovery. Insulin sensitivity (hyperinsulinemic-euglycemic clamp) and intramyocellular triacylglycerol content did not change with the intervention in either group. Indexes of mitochondrial density were similar across the groups and intervention. Mitochondrial respiratory rates, measured in permeabilized muscle fibers, showed a 31 ± 11 and 26 ± 9% exercise-induced increase (P < 0.05) in state 3 (glycolytic substrates) and uncoupled respiration, respectively. However, in HFD this increase was abolished. At recovery, no change from resting respiration was seen in either group. With a lipid substrate (octanoyl-carnitine with or without ADP), similar exercise-induced increases (31-62%) were seen in HFD and ND, but only in HFD was an elevated (P < 0.05) respiratory rate seen at recovery. With HFD complex I and IV protein expression decreased (P < 0.05 and P = 0.06, respectively). A fat-rich diet induces marked changes in the mitochondrial electron transport system protein content and in exercise-induced mitochondrial substrate oxidation rates, with the effects being present hours after the exercise. The effect of HFD is present even without effects on insulin sensitivity and intramyocellular lipid accumulation. An isocaloric high-fat diet does not cause insulin resistance.

Reduced malonyl-CoA content in recovery from exercise correlates with improved insulin-stimulated glucose uptake in human skeletal muscle

This study evaluated whether improved insulin-stimulated glucose uptake in recovery from acute exercise coincides with reduced malonyl-CoA (MCoA) content in human muscle. Furthermore, we investigated whether a high-fat diet [65 energy-% (Fat)] would alter the content of MCoA and insulin action compared with a high-carbohydrate diet [65 energy-% (CHO)]. After 4 days of isocaloric diet on two occasions (Fat/CHO), 12 male subjects performed 1 h of one-legged knee extensor exercise (approximately 80% peak workload). Four hours after exercise, insulin-stimulated glucose uptake was determined in both legs during a euglycemic-hyperinsulinemic clamp. Muscle biopsies were obtained in both legs before and after the clamp. Four hours after exercise, insulin-stimulated glucose uptake was improved (approximately 70%, P<0.001) independent of diet composition and despite normal insulin-stimulated regulation of insulin receptor substrate-1-associated phosphatidylinositol 3-kinase, Akt, GSK-3, and glycogen synthase. Interestingly, exercise resulted in a sustained reduction (approximately 20%, P<0.05) in MCoA content 4 h after exercise that correlated (r=0.65, P<0.001) with improved insulin-stimulated glucose uptake. Four days of Fat diet resulted in an increased content of intramyocellular triacylglycerol (P<0.01) but did not influence muscle MCoA content or whole body insulin-stimulated glucose uptake. However, at the muscular level proximal insulin signaling and insulin-stimulated glucose uptake appeared to be compromised, although to a minor extent, by the Fat diet. Collectively, this study indicates that reduced muscle MCoA content in recovery from exercise may be part of the adaptive response leading to improved insulin action on glucose uptake after exercise in human muscle.

Determining the relationship between dietary carbohydrate intake and insulin resistance

Insulin resistance underlies type 2 diabetes, CVD and the metabolic syndrome, driven by changes in diet, lifestyle, energy over–consumption and obesity. Nutritional recommendations for insulin resistance remain an area of controversy, particularly the quantity and types of dietary carbohydrate. The present review gives an overview of insulin resistance, its relationship to impaired insulin secretion and the metabolic syndrome, research methodologies used to measure insulin action and the epidemiological and intervention studies on the relationship between dietary carbohydrate and insulin resistance. Epidemiological studies provide little evidence to suggest that total dietary carbohydrate predicts risk of type 2 diabetes, and high–carbohydrate, high–fibre diets with low–glycaemic index (GI) may even contribute to diabetes prevention. Despite inherent limitations associated with techniques used to measure insulin resistance and dietary assessment, most intervention studies reveal an increase in glucose tolerance or insulin sensitivity with high–carbohydrate, low–fat diets in non–diabetic and diabetic individuals. When energy is restricted the source or reduced content of carbohydrate does not appear to be as important as fat for body weight. Thus, low energy intake is key to weight loss and augmentation of insulin sensitivity. Given this, widespread adoption of popular low–carbohydrate high–fat diets highlights the necessity to evaluate dietary interventions regarding safety and metabolic effects. While current evidence supports FAO/WHO recommendations to maintain a high–carbohydrate diet with low–GI foods, the relationships between carbohydrate and insulin sensitivity remains an important research area. Emerging technologies should further enhance understanding of gene–diet interactions in insulin resistance, providing useful information for future nutrition policy decisions.

Beneficial effects of ketogenic diet in obese diabetic subjects

OBJECTIVE

Obesity is closely linked to the incidence of type II diabetes. It is found that effective management of body weight and changes to nutritional habits especially with regard to the carbohydrate content and glycemic index of the diet have beneficial effects in obese subjects with glucose intolerance. Previously we have shown that ketogenic diet is quite effective in reducing body weight. Furthermore, it favorably alters the cardiac risk factors even in hyperlipidemic obese subjects. In this study the effect of ketogenic diet in obese subjects with high blood glucose level is compared to those with normal blood glucose level for a period of 56 weeks.

MATERIALS AND METHODS

A total of 64 healthy obese subjects with body mass index (BMI) greater than 30, having high blood glucose level and those subjects with normal blood glucose level were selected in this study. The body weight, body mass index, blood glucose level, total cholesterol, LDL-cholesterol, HDL-cholesterol, triglycerides, urea and creatinine were determined before and at 8, 16, 24, 48, and 56 weeks after the administration of the ketogenic diet.

RESULTS

The body weight, body mass index, the level of blood glucose, total cholesterol, LDL-cholesterol, triglycerides, and urea showed a significant decrease from week 1 to week 56 (P < 0.0001), whereas the level of HDL-cholesterol increased significantly (P < 0.0001). Interestingly these changes were more significant in subjects with high blood glucose level as compared to those with normal blood glucose level. The changes in the level of creatinine were not statistically significant.

CONCLUSION

This study shows the beneficial effects of ketogenic diet in obese diabetic subjects following its long-term administration. Furthermore, it demonstrates that in addition to its therapeutic value, low carbohydrate diet is safe to use for a longer period of time in obese diabetic subjects.

High-fat/low-carbohydrate diet reduces insulin-stimulated carbohydrate oxidation but stimulates nonoxidative glucose disposal in humans: An important role for skeletal muscle pyruvate dehydrogenase kinase 4

AIM

The aim of this report was to study the effect of high-fat (HF)/low-carbohydrate (CHO) diet on regulation of substrate metabolism in humans.

METHODS

Ten healthy men consumed either a HF (75% energy as fat) or control (35%) diet for 6 d in random order. On d 7, blood glucose disappearance rate (Rd) was determined before and during a hyperinsulinemic euglycemic clamp. Substrate oxidation was determined by indirect calorimetry. Muscle biopsies were obtained prediet, postdiet, and postclamps.

RESULTS

Rd was similar under basal conditions but slightly elevated (approximately 10%, P < 0.05) during the last 30 min of the clamp after the HF diet. HF diet reduced CHO oxidation under basal (by approximately 40%, P < 0.05) and clamp conditions (by approximately 20%, P < 0.05), increased insulin-mediated whole-body nonoxidative glucose disposal (by 30%, P < 0.05) and muscle glycogen storage (by approximately 25%, P < 0.05). Muscle pyruvate dehydrogenase complex activity was blunted under basal and clamp conditions after HF compared with control (P < 0.05) and was accompanied by an approximately 2-fold increase (P < 0.05) in pyruvate dehydrogenase kinase 4 (PDK4) mRNA and protein expression.

CONCLUSION

Short-term HF/low-CHO dietary intake did not induce whole-body insulin resistance, but caused a shift in im glucose metabolism from oxidation to glycogen storage. Insulin-stimulated CHO oxidation and muscle pyruvate dehydrogenase complex activity were blunted after the HF diet. Up-regulation of muscle PDK4 expression was an early molecular adaptation to these changes, and we showed for the first time in healthy humans, unlike insulin-resistant individuals, that insulin can suppress PDK4 but not PDK2 gene expression in skeletal muscle.

Fat adaptation followed by carbohydrate loading compromises high-intensity sprint performance

The aim of this study was to investigate the effect of a high-fat diet (HFD) followed by 1 day of carbohydrate (CHO) loading on substrate utilization, heart rate variability (HRV), effort perception [rating or perceived exertion (RPE)], muscle recruitment [electromyograph (EMG)], and performance during a 100-km cycling time trial. In this randomized single-blind crossover study, eight well-trained cyclists completed two trials, ingesting either a high-CHO diet (HCD) (68% CHO energy) or an isoenergetic HFD (68% fat energy) for 6 days, followed by 1 day of CHO loading (8-10 g CHO/kg). Subjects completed a 100-km time trial on day 1 and a 1-h cycle at 70% of peak oxygen consumption on days 3, 5, and 7, during which resting HRV and resting and exercising respiratory exchange ratio (RER) were measured. On day 8, subjects completed a 100-km performance time trial, during which blood samples were drawn and EMG was recorded. Ingestion of the HFD reduced RER at rest (P < 0.005) and during exercise (P < 0.01) and increased plasma free fatty acid levels (P < 0.01), indicating increased fat utilization. There was a tendency for the low-frequency power component of HRV to be greater for HFD-CHO (P = 0.056), suggestive of increased sympathetic activation. Overall 100-km time-trial performance was not different between diets; however, 1-km sprint power output after HFD-CHO was lower (P < 0.05) compared with HCD-CHO. Despite a reduced power output with HFD-CHO, RPE, heart rate, and EMG were not different between trials. In conclusion, the HFD-CHO dietary strategy increased fat oxidation, but compromised high intensity sprint performance, possibly by increased sympathetic activation or altered contractile function.

Comparison of a very low-carbohydrate and low-fat diet on fasting lipids, LDL subclasses, insulin resistance, and postprandial lipemic responses in overweight women

OBJECTIVE

Very low-carbohydrate diets are widely used for weight loss yet few controlled studies have determined how these diets impact cardiovascular risk factors compared to more traditional low-fat weight loss diets. The primary purpose of this study was to compare a very low-carbohydrate and a low-fat diet on fasting blood lipids, LDL subclasses, postprandial lipemia, and insulin resistance in overweight and obese women.

METHODS

Thirteen normolipidemic, moderately overweight (body fat >30%) women were prescribed two hypocaloric (-500 kcal/day) diets for 4 week periods, a very low-carbohydrate (<10% carbohydrate) and a low-fat (<30% fat) diet. The diets were consumed in a balanced and randomized fashion. Two fasting blood draws were performed on separate days and an oral fat tolerance test was performed at baseline, after the very low-carbohydrate diet, and after the low-fat diet.

RESULTS

Compared to corresponding values after the very low-carbohydrate diet, fasting total cholesterol, LDL-C, and HDL-C were significantly (p < or = 0.05) lower, whereas fasting glucose, insulin, and insulin resistance (calculated using the homeostatic model assessment) were significantly higher after the low-fat diet. Both diets significantly decreased postprandial lipemia and resulted in similar nonsignificant changes in the total cholesterol/HDL-C ratio, fasting triacylglycerols, oxidized LDL, and LDL subclass distribution.

CONCLUSIONS

Compared to a low-fat weight loss diet, a short-term very low-carbohydrate diet did not lower LDL-C but did prevent the decline in HDL-C and resulted in improved insulin sensitivity in overweight and obese, but otherwise healthy women. Small decreases in body mass improved postprandial lipemia, and therefore cardiovascular risk, independent of diet composition.

Muscle triglyceride and glycogen in endurance exercise: implications for performance

The importance of muscle glycogen as a metabolic substrate in sustaining prolonged exercise is well acknowledged. Being stored in proximity to the site of contraction and able to sustain high rates of adenosine diphosphate (ADP) phosphorylation, glycogen is viewed as the primary fuel for the maintenance of exercise of a moderate to intense nature. As such, to ensure optimal exercise performance, endurance athletes are encouraged to maximise the availability of muscle glycogen through the ingestion of a high carbohydrate (CHO) diet prior to competition. The skeletal muscle cell also contains significant quantities of triglyceride. Recent improvements in the ability to measure these intramyocellular triglyceride (IMTG) stores have confirmed that IMTG acts as a significant fuel substrate during prolonged exercise. While early research of the role of muscle glycogen in endurance exercise provided clear prescriptive information for the endurance-trained athlete, no such direction for optimising exercise performance is yet apparent from research concerning IMTG. In this article, we review the processes of muscle glycogen and triglyceride storage and metabolism. Attention is given to the effects of short-term alterations in diet on muscle substrate, particularly IMTG storage, and the implications of this to endurance exercise performance and competition preparation. We demonstrate that like glycogen, IMTG formation may be relatively rapid, and its storage predominates under conditions that promote minimal glycogen formation. This observation suggests that the role of IMTG is to maintain a readily available substrate to ensure that physical activity of a moderate nature can be performed when glycogen availability is not optimal. Under these conditions, IMTG may offer a similar availability of energy as glycogen in the endurance-trained athlete. Given the potential value of this substrate, the possibility of maximising IMTG storage without compromising glycogen availability prior to competition is considered.

Nutrition and prevention of type 2 diabetes

In recent years, the prevalence of type 2 diabetes has increased alarmingly worldwide, giving diabetes the dimension of an epidemic. Striking parallel increases in the prevalence of obesity reflect the importance of body fatness as a contributing factor to diabetes incidence. Moreover, it has been estimated that up to 75% of the risk of type 2 diabetes is attributable to obesity. Recent clinical trials and observational epidemiologic studies demonstrate the efficacy of lifestyle changes, including decreased energy intake, decreased fat intake, and weight loss, as well as regular participation in physical activity, in improving insulin sensitivity (SI) and reducing the risk of diabetes. This review evaluates evidence of the effect of diet on insulin resistance, insulin secretion, and glucose tolerance, and reflects on directions for future work toward primary prevention of type 2 diabetes.

Prolonged adaptation to fat-rich diet and training; effects on body fat stores and insulin resistance in man

OBJECTIVE

To investigate the effect of prolonged adaptation to training and fat- or carbohydrate-rich diet on body composition and insulin resistance.

DESIGN

Longitudinal study. Of three groups two consumed a fat-rich diet, of which one performed regular training (FAT-Train, n=17) and the other maintained normal habitual activity (Fat-Control, n=8). The third group trained and consumed a carbohydrate-rich diet (CHO-Train, n=16).

SUBJECTS

Forty-one untrained, healthy male subjects.

MEASUREMENTS

Before and after 7 weeks body composition was estimated from skinfold measurements. At rest the respiratory exchange ratio (RER) was determined by the Douglas bag technique. Glycogen was determined in m vastus lateralis and concentrations of insulin and triacylglycerol in serum and glucose, fatty acid and beta-hydroxy-butyrate in plasma was measured. The insulin resistance index was calculated from fasting plasma insulin and glucose values.

RESULTS

Across the 7 weeks body weight was reduced (1.3+/-0.3%) in all three groups, however body fat mass was decreased only in the CHO-Train (13%) and maintained in the two FAT-groups. RER at rest was similarly decreased (5%) in the three groups. Plasma insulin tended to decrease (16%) in CHO-Train (P=0.065) and remained unchanged in the two FAT-groups. In contrast plasma glucose (4.6+/-0.1 mmol/l) and plasma FA (453+/-27 micromol/l) remained unchanged across the 7 weeks. The calculated insulin resistance index HOMA-R(mod) was significantly decreased by 19% in CHO-train but remained unchanged in both of the FAT-groups, whereas the calculated insulin secretion index HOMA-beta(mod) was unchanged in all three groups.

CONCLUSION

In the present study we demonstrate that despite of a mild energy deficit body fat mass was maintained after prolonged adaptation to fat-rich diet both when normal physical activity was maintained and when training was performed. In contrast a significant decrease in fat mass was observed when carbohydrate-rich diet and training was combined. Furthermore we observed that the insulin resistance index was significantly decreased only when training was combined with a carbohydrate-rich diet.

A ketogenic diet favorably affects serum biomarkers for cardiovascular disease in normal-weight men

Very low-carbohydrate (ketogenic) diets are popular yet little is known regarding the effects on serum biomarkers for cardiovascular disease (CVD). This study examined the effects of a 6-wk ketogenic diet on fasting and postprandial serum biomarkers in 20 normal-weight, normolipidemic men. Twelve men switched from their habitual diet (17% protein, 47% carbohydrate and 32% fat) to a ketogenic diet (30% protein, 8% carbohydrate and 61% fat) and eight control subjects consumed their habitual diet for 6 wk. Fasting blood lipids, insulin, LDL particle size, oxidized LDL and postprandial triacylglycerol (TAG) and insulin responses to a fat-rich meal were determined before and after treatment. There were significant decreases in fasting serum TAG (-33%), postprandial lipemia after a fat-rich meal (-29%), and fasting serum insulin concentrations (-34%) after men consumed the ketogenic diet. Fasting serum total and LDL cholesterol and oxidized LDL were unaffected and HDL cholesterol tended to increase with the ketogenic diet (+11.5%; P = 0.066). In subjects with a predominance of small LDL particles pattern B, there were significant increases in mean and peak LDL particle diameter and the percentage of LDL-1 after the ketogenic diet. There were no significant changes in blood lipids in the control group. To our knowledge this is the first study to document the effects of a ketogenic diet on fasting and postprandial CVD biomarkers independent of weight loss. The results suggest that a short-term ketogenic diet does not have a deleterious effect on CVD risk profile and may improve the lipid disorders characteristic of atherogenic dyslipidemia.

Dietary fat content alters insulin-mediated glucose metabolism in healthy men

BACKGROUND

A high dietary fat intake is involved in the pathogenesis of insulin resistance.

OBJECTIVE

The aim was to compare the effect of different amounts of dietary fat on hepatic and peripheral insulin sensitivity.

DESIGN

Six healthy men were studied on 3 occasions after consuming for 11 d diets with identical energy and protein contents but different percentages of energy as fat and carbohydrate as follows: 0% and 85% [low-fat, high-carbohydrate (LFHC) diet], 41% and 44% [intermediate-fat, intermediate-carbohydrate (IFIC) diet], and 83% and 2% [high-fat, low-carbohydrate (HFLC) diet]. Insulin sensitivity was quantified by using a hyperinsulinemic euglycemic clamp (plasma insulin concentration: approximately 190 pmol/L).

RESULTS

During hyperinsulinemia, endogenous glucose production was higher after the HFLC diet (2.5 +/- 0.3 micromol x kg(-1) x min(-1); P < 0.05) than after the IFIC and LFHC diets (1.7 +/- 0.3 and 1.2 +/- 0.4 micromol x kg(-1) x min(-1), respectively). The ratio of dietary fat to carbohydrate had no unequivocal effects on insulin-stimulated glucose uptake. In contrast, insulin-stimulated, nonoxidative glucose disposal tended to increase in relation to an increase in the ratio of fat to carbohydrate, from 14.8 +/- 5.1 to 20.6 +/- 1.9 to 26.2 +/- 2.9 micromol x kg(-1) x min(-1) (P < 0.074 between the 3 diets). Insulin-stimulated glucose oxidation was significantly lower after the HFLC diet than after the IFIC and LFHC diets: 1.7 +/- 0.8 compared with 13.4 +/- 2.1 and 19.0 +/- 2.1 micromol x kg(-1) x min(-1), respectively (P < 0.05). During the clamp study, plasma fatty acid concentrations were higher after the HFLC diet than after the IFIC and LFHC diets: 0.22 +/- 0.02 compared with 0.07 +/- 0.01 and 0.05 +/- 0.01 mmol/L, respectively (P < 0.05).

CONCLUSION

A high-fat, low-carbohydrate intake reduces the ability of insulin to suppress endogenous glucose production and alters the relation between oxidative and nonoxidative glucose disposal in a way that favors storage of glucose.

Adaptation to a fat-rich diet: effects on endurance performance in humans

The focus of this review is on studies where dietary fat content was manipulated to investigate the potential ergogenic effect of fat loading on endurance exercise performance. Adaptation to a fat-rich diet is influenced by several factors, of which the duration of the adaptation period, the exercise intensity of the performance test and the content of fat and carbohydrate in the experimental diet are the most important. Evidence is presented that short term adaptation, < 6 days, to a fat-rich diet is detrimental to exercise performance. When adaptation to a fat-rich diet was performed over longer periods, studies where performance was tested at moderate intensity, 60 to 80% of maximal oxygen uptake, demonstrate either no difference or an attenuated performance after consumption of a fat-rich compared with a carbohydrate-rich diet. When performance was measured at high intensity after a longer period of adaptation, it was at best maintained, but in most cases attenuated, compared with consuming a carbohydrate-rich diet. Furthermore, evidence is presented that adaptation to a fat-rich diet leads to an increased capacity of the fat oxidative system and an enhancement of the fat supply and subsequently the amount of fat oxidised during exercise. However, in most cases muscle glycogen storage is compromised, and although muscle glycogen breakdown is diminished to a certain extent, this is probably part of the explanation for the lack of performance enhancement after adaptation to a fat-rich diet.

Dietary carbohydrates and insulin action in humans

The metabolic syndrome represents a vicious cycle whereby insulin resistance leads to compensatory hyperinsulinaemia, which maintains normal plasma glucose but may exacerbate insulin resistance. Excess insulin secretion may eventually reduce beta-cell function due to amyloid deposition, leading to raised blood glucose and further deterioration of beta-cell function and insulin sensitivity via glucose toxicity. Reducing postprandial glucose and insulin responses may be a way to interrupt this process, but there is disagreement about the dietary approach to achieve this. Glucose and insulin responses are determined primarily by the amount of carbohydrate consumed and its rate of absorption. Slowly absorbed, low glycaemic-index (GI) foods are associated with increased HDL cholesterol and reduced risk of type 2 diabetes. There is some evidence that low-GI foods improve insulin sensitivity in humans, although studies using established techniques (glucose clamp or frequently sampled intravenous glucose tolerance test) have not been done. Low carbohydrate diets have been suggested to be beneficial in the treatment of the metabolic syndrome because of reduced postprandial insulin. However, they may increase fasting glucose and impair oral glucose tolerance--effects which define carbohydrate intolerance. The effects of low carbohydrate diets on insulin sensitivity depend on what is used to replace the dietary carbohydrate, and the nature of the subjects studied. Dietary carbohydrates may affect insulin action, at least in part, via alterations in plasma free fatty acids. In normal subjects a high-carbohydrate/low-GI breakfast meal reduced free fatty acids by reducing the undershoot of plasma glucose, whereas low-carbohydrate breakfasts increased postprandial free fatty acids. It is unknown if these effects occur in insulin-resistant or diabetic subjects. Thus further work needs to be done before a firm conclusion can be drawn as to the optimal amount and type of dietary carbohydrate for the treatment of the metabolic syndrome.

Metabolic adaptations to a high-fat diet in endurance cyclists

We examined the time course of metabolic adaptations to 15 days of a high-fat diet (HFD). Sixteen endurance-trained cyclists were assigned randomly to a control (CON) group, who consumed their habitual diet (30% +/- 8% mJ fat), or a HFD group, who consumed a high-fat isocaloric diet (69% +/- 1% mJ fat). At 5-day intervals, the subjects underwent an oral glucose tolerance test (OGTT); on the next day, they performed a 2.5-hour constant-load ride at 70% peak oxygen consumption (VO2peak), followed by a simulated 40-km cycling time-trial while ingesting a 10% 14C-glucose + 3.44% medium-chain triglyceride (MCT) emulsion at a rate of 600 mL/h. In the OGTT, plasma glucose concentrations at 30 minutes increased significantly after 5 days of the HFD and remained elevated at days 10 and 15 versus the levels measured prior to the HFD (P < .05). The activity of carnitine acyltransferase (CAT) in biopsies of the vastus lateralis muscle also increased from 0.45 to 0.54 micromol/g/min over days 0 to 10 of the HFD (P < .01) without any change in citrate synthase (CS) or 3-hydroxyacyl-coenzyme A dehydrogenase (3-HAD) activities. Changes in glucose tolerance and CAT activity were associated with a shift from carbohydrate (CHO) to fat oxidation during exercise (P < .001), which occurred within 5 to 10 days of the HFD. During the constant-load ride, the calculated oxidation of muscle glycogen was reduced from 1.5 to 1.0 g/min (P < .001) after 15 days of the HFD. Ingestion of a HFD for as little as 5 to 10 days significantly altered substrate utilization during submaximal exercise but did not attenuate the 40-km time-trial performance.

Influence of diet on the metabolic responses to exercise

The relationship between dietary intake and skeletal-muscle exercise metabolism is central to the interests of exercise physiologists. This area has been examined experimentally for over 100 years. Classic studies with male subjects demonstrated the importance of dietary CHO in maximizing muscle and liver glycogen stores in an attempt to optimize exercise performance. CHO becomes the predominant fuel for exercise at power outputs above 50-60% Vo2max and its availability limits prolonged aerobic exercise at intensities corresponding to 65-85% VO2max. Recent information suggests that female subjects are less able to maximize muscle glycogen stores through dietary means. Contemporary studies have documented in more detail the greater reliance on CHO metabolism following a high-CHO-low-fat and -protein diet and the greater reliance on fat metabolism following a low-CHO-high-fat and protein diet. More emphasis on documenting key enzymic changes in the energy-producing pathways and transport proteins has appeared. However, very little is known regarding the mechanisms that induce these changes over the short or long term in human skeletal muscle. For example, the central role of PDH activity in the selection of intramuscular fuel during exercise and the role of carnitine palmitoyltransferase 1 in the entry of NEFA into the mitochondria, and the effects of diet on these enzymes has received little attention to date. Many research studies have examined extreme diet variations (% total energy; > 85% CHO v. < 5-10% CHO) for short periods of time in an attempt to maximize diet-induced alterations and study the mechanisms responsible for the changes. However, future studies will need to examine less-severe diet alterations for longer periods of time that more accurately reflect what the normal population might experience, such as a diet containing (% total energy) 60 fat, 20 CHO, 20 protein or the recently popular diet with (% total energy) 30 fat, 40 CHO, 30 protein.