Effects of fasting on insulin action and glucose kinetics in lean and obese men and women

Intramuscular lipid metabolism in the insulin resistance of smoking

OBJECTIVE

Smoking decreases insulin action and increases the risk of type 2 diabetes in humans. Mechanisms responsible for smoking-induced insulin resistance are unclear. We hypothesized smokers would have increased intramuscular triglyceride (IMTG) and diacylglycerol (DAG) concentration and decreased fractional synthesis rate (FSR) compared with nonsmokers.

RESEARCH DESIGN AND METHODS

Nonsmokers (n = 18, aged 20 +/- 0.5 years, BMI 22 +/- 0.4 kg/m(2), body fat 20 +/- 2%, 0 cigarettes per day) and smokers (n = 14, aged 21 +/- 0.7 years, BMI 23 +/- 0.4 kg/m(2), body fat 20 +/- 3%, 18 +/- 0.7 cigarettes per day) were studied in a fasted condition after a standardized diet. [U-(13)C]palmitate was infused during 4 h of rest followed by a skeletal muscle biopsy and intravenous glucose tolerance test.

RESULTS

Smokers were less insulin sensitive (S(i)) compared with nonsmokers (S(i) 5.28 +/- 0.5 nonsmokers vs. 3.74 +/- 0.3 smokers 10(-4) x microU(-1) x ml(-1), P = 0.03). There were no differences in IMTG or DAG concentration (IMTG 24.2 +/- 3.4 nonsmokers vs. 27.2 +/- 5.9 smokers microg/mg dry wt, DAG 0.34 +/- 0.02 nonsmokers vs. 0.35 +/- 0.02 smokers microg/mg dry wt) or IMTG FSR between groups (0.66 +/- 0.1 nonsmokers vs. 0.55 +/- 0.09 smokers %/hr). Intramuscular lipid composition was different, with increased percent saturation of IMTG (32.1 +/- 1.2 nonsmokers vs. 35.2 +/- 1.0 smokers %, P = 0.05) and DAG (52.8 +/- 1.7 nonsmokers vs. 58.8 +/- 2.2 smokers %, P = 0.04) in smokers. Smokers had significantly decreased peroxisome proliferator-activated receptor-gamma (1.76 +/- 0.1 nonsmokers vs. 1.42 +/- 0.11 smokers arbitrary units [AU], P = 0.03) and increased monocyte chemotactic protein-1 (3.11 +/- 0.41 nonsmokers vs. 4.83 +/- 0.54 smokers AU, P = 0.02) mRNA expression compared with nonsmokers. We also found increased insulin receptor substrate-1 Ser(636) phosphorylation in smokers compared with nonsmokers (0.73 +/- 0.08 nonsmokers vs. 1.14 +/- 0.09 smokers AU, P = 0.002).

CONCLUSIONS

These data suggest: 1) IMTG concentration and turnover are not related to alterations in insulin action in smokers compared to nonsmokers, 2) increased saturation of IMTG and DAG in skeletal muscle may be related to insulin action, and 3) basal inhibition of insulin receptor substrate-1 may decrease insulin action in smokers.

Gender differences in lipid metabolism and the effect of obesity

There are many differences between men and women, and between lean and obese subjects, in fatty acid and very low-density lipoprotein triglyceride and apolipoprotein B-100 metabolism. Currently, observations in this area are predominantly descriptive. The mechanisms responsible for sexual dimorphism in lipid metabolism are largely unknown.

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.

Dietary fat and carbohydrates differentially alter insulin sensitivity during caloric restriction

BACKGROUND & AIMS

We determined the effects of acute and chronic calorie restriction with either a low-fat, high-carbohydrate (HC) diet or a low-carbohydrate (LC) diet on hepatic and skeletal muscle insulin sensitivity.

METHODS

Twenty-two obese subjects (body mass index, 36.5 +/- 0.8 kg/m2) were randomized to an HC (>180 g/day) or LC (<50 g/day) energy-deficit diet. A euglycemic-hyperinsulinemic clamp, muscle biopsy specimens, and magnetic resonance spectroscopy were used to determine insulin action, cellular insulin signaling, and intrahepatic triglyceride (IHTG) content before, after 48 hours, and after approximately 11 weeks (7% weight loss) of diet therapy.

RESULTS

At 48 hours, IHTG content decreased more in the LC than the HC diet group (29.6% +/- 4.8% vs 8.9% +/- 1.4%; P < .05) but was similar in both groups after 7% weight loss (LC diet, 38.0% +/- 4.5%; HC diet, 44.5% +/- 13.5%). Basal glucose production rate decreased more in the LC than the HC diet group at 48 hours (23.4% +/- 2.2% vs 7.2% +/- 1.4%; P < .05) and after 7% weight loss (20.0% +/- 2.4% vs 7.9% +/- 1.2%; P < .05). Insulin-mediated glucose uptake did not change at 48 hours but increased similarly in both groups after 7% weight loss (48.4% +/- 14.3%; P < .05). In both groups, insulin-stimulated phosphorylation of c-Jun-N-terminal kinase decreased by 29% +/- 13% and phosphorylation of Akt and insulin receptor substrate 1 increased by 35% +/- 9% and 36% +/- 9%, respectively, after 7% weight loss (all P < .05).

CONCLUSIONS

Moderate calorie restriction causes temporal changes in liver and skeletal muscle metabolism; 48 hours of calorie restriction affects the liver (IHTG content, hepatic insulin sensitivity, and glucose production), whereas moderate weight loss affects muscle (insulin-mediated glucose uptake and insulin signaling).

Muscle acylcarnitines during short-term fasting in lean healthy men

The transition from the fed to the fasted resting state is characterized by, among other things, changes in lipid metabolism and peripheral insulin resistance. Acylcarnitines have been suggested to play a role in insulin resistance, as well as other long-chain fatty acid metabolites. Plasma levels of long-chain acylcarnitines increase during fasting, but this is unknown for muscle long-chain acylcarnitines. In the present study we investigated whether muscle long-chain acylcarnitines increase during fasting and we investigated their relationship with glucose/fat oxidation and insulin sensitivity in lean healthy humans. After 14 h and 62 h of fasting, glucose fluxes, substrate oxidation, and plasma and muscle acylcarnitines were measured before and during a hyperinsulinaemic–euglycaemic clamp. Hyperinsulinaemia decreased long-chain muscle acylcarnitines after 14 h of fasting, but not after 62 h of fasting. In both the basal state and during the clamp, glucose oxidation was lower and fatty acid oxidation was higher after 62 h compared with 14 h of fasting. Absolute changes in glucose and fatty acid oxidation in the basal compared with hyperinsulinaemic state were not different. Muscle long-chain acylcarnitines did not correlate with glucose oxidation, fatty acid oxidation or insulin-mediated peripheral glucose uptake. After 62 h of fasting, the suppression of muscle long-chain acylcarnitines by insulin was attenuated compared with 14 h of fasting. Muscle long-chain acylcarnitines do not unconditionally reflect fatty acid oxidation. The higher fatty acid oxidation during hyperinsulinaemia after 62 h compared with 14 h of fasting, although the absolute decrease in fatty acid oxidation was not different, suggests a different set point.

Influence of acute exercise with and without carbohydrate replacement on postprandial lipid metabolism

Acute exercise, undertaken on the day before an oral fat tolerance test (OFTT), typically reduces postprandial triglycerides (TG) and increases high-density lipoprotein-cholesterol (HDL-C). However, the benefits of acute exercise may be overstated when studies do not account for compensatory changes in dietary intake. The objective of this study was to determine the influence of acute exercise, with and without carbohydrate (CHO) replacement, on postprandial lipid metabolism. Eight recreationally active young men underwent an OFTT on the morning after three experimental conditions: no exercise [control (Con)], prolonged exercise without CHO replacement (Ex-Def) and prolonged exercise with CHO replacement to restore CHO and energy balance (Ex-Bal). The exercise session in Ex-Def and Ex-Bal consisted of 90 min cycle ergometry at 70% peak oxygen uptake (V̇o2peak) followed by 10 maximal 1-min sprints. CHO replacement was achieved using glucose solutions consumed at 0, 2, and 4 h postexercise. Muscle glycogen was 40 ± 4% ( P < 0.05) and 94 ± 3% ( P = 0.24) of Con values on the morning of the Ex-Def and Ex-Bal OFTT, respectively. Postprandial TG were 40 ± 14% lower and postprandial HDL-C, free fatty acids, and 3-hydroxybutyrate were higher in Ex-Def compared with Con ( P < 0.05). Most importantly, these exercise effects were not evident in Ex-Bal. Postprandial insulin and glucose and the homeostatic model assessment of insulin resistance (HOMAIR) were not significantly different across trials. There was no relation between the changes in postprandial TG and muscle glycogen across trials. In conclusion, the influence of acute exhaustive exercise on postprandial lipid metabolism is largely dependent on the associated CHO and energy deficit.

Abdominal and gynoid fat mass are associated with cardiovascular risk factors in men and women

CONTEXT

Abdominal obesity is an established risk factor for cardiovascular disease (CVD). However, the correlation of dual-energy x-ray absorptiometry (DEXA) measurements of regional fat mass with CVD risk factors has not been completely investigated.

OBJECTIVE

The aim of this study was to investigate the association of estimated regional fat mass, measured with DEXA and CVD risk factors.

DESIGN, SETTING, AND PARTICIPANTS

This was a cross-sectional study of 175 men and 417 women. DEXA measurements of regional fat mass were performed on all subjects, who subsequently participated in a community intervention program.

MAIN OUTCOME MEASURES

Outcome measures included impaired glucose tolerance, hypercholesterolemia, hypertriglyceridemia, and hypertension.

RESULTS

We began by assessing the associations of the adipose measures with the cardiovascular outcomes. After adjustment for confounders, a sd unit increase in abdominal fat mass was the strongest predictor of most cardiovascular variables in men [odds ratio (OR)=2.63-3.37; P<0.05], whereas the ratio of abdominal to gynoid fat mass was the strongest predictor in women (OR=1.48-2.19; P<0.05). Gynoid fat mass was positively associated with impaired glucose tolerance, hypertriglyceridemia, and hypertension in men (OR=2.07-2.15; P<0.05), whereas the ratio of gynoid to total fat mass showed a negative association with hypertriglyceridemia and hypertension (OR=0.42-0.62; P<0.005).

CONCLUSIONS

Abdominal fat mass is strongly independently associated with CVD risk factors in the present study. In contrast, gynoid fat mass was positively associated, whereas the ratio of gynoid to total fat mass was negatively associated with risk factors for CVD.

Skeletal muscle munc18c and syntaxin 4 in human obesity

Background

Animal and cell culture data suggest a critical role for Munc18c and Syntaxin 4 proteins in insulin mediated glucose transport in skeletal muscle, but no studies have been published in humans.

Methods

We investigated the effect of a 12 vs. 48 hr fast on insulin action and skeletal muscle Munc18c and Syntaxin 4 protein in lean and obese subjects. Healthy lean (n = 14; age = 28.0 +/- 1.4 yr; BMI = 22.8 +/- 0.42 kg/m²) and obese subjects (n = 11; age = 34.6 +/- 2.3 yr; BMI = 36.1 +/- 1.5 kg/m²) were studied twice following a 12 and 48 hr fast. Skeletal muscle biopsies were obtained before a 3 hr 40 mU/m²/min hyperinsulinemic-euglycemic clamp with [6,6-²H₂]glucose infusion.

Results

Glucose rate of disappearance (Rd) during the clamp was lower in obese vs. lean subjects after the 12 hr fast (obese: 6.25 +/- 0.67 vs. lean: 9.42 +/- 1.1 mg/kgFFM/min, p = 0.007), and decreased significantly in both groups after the 48 hr fast (obese 3.49 +/- 0.31 vs. lean: 3.91 +/- 0.42 mg/kgFFM/min, p = 0.002). Munc18c content was not significantly different between lean and obese subjects after the 12 hour fast, and decreased after the 48 hr fast in both groups (p = 0.013). Syntaxin 4 content was not altered by obesity or fasting duration. There was a strong positive relationship between plasma glucose concentration and Munc18c content in lean and obese subjects during both 12 and 48 hr fasts (R² = 0.447, p = 0.0015). Significant negative relationships were also found between Munc18c and FFA (p = 0.041), beta-hydroxybutyrate (p = 0.039), and skeletal muscle AKT content (p = 0.035) in lean and obese subjects.

Conclusion

These data indicate Munc18c and Syntaxin 4 are present in human skeletal muscle. Munc18c content was not significantly different between lean and obese subjects, and is therefore unlikely to explain obesity-induced insulin resistance. Munc18c content decreased after prolonged fasting in lean and obese subjects concurrently with reduced insulin action. These data suggest changes in Munc18c content in skeletal muscle are associated with short-term changes in insulin action in humans.

Muscle adaptation to short-term fasting in healthy lean humans

CONTEXT

It has been demonstrated repeatedly that short-term fasting induces insulin resistance, although the exact mechanism in humans is unknown to date. Intramyocellular sphingolipids (i.e. ceramide) have been suggested to induce insulin resistance by interfering with the insulin signaling cascade in obesity.

OBJECTIVE

Our objective was to study peripheral insulin sensitivity together with muscle ceramide concentrations and protein kinase B/AKT phosphorylation after short-term fasting. MAIN OUTCOME MEASURES AND DESIGN: After 14- and 62-h fasting, glucose fluxes were measured before and after a hyperinsulinemic euglycemic clamp. Muscle biopsies were performed in the basal state and during the clamp to assess muscle ceramide and protein kinase B/AKT.

RESULTS

Insulin-mediated peripheral glucose uptake was significantly lower after 62-h fasting compared with 14-h fasting. Intramuscular ceramide concentrations tended to increase during fasting. During the clamp the phosphorylation of protein kinase B/AKT at serine(473) in proportion to the total amount of protein kinase B/AKT was significantly lower. Muscle ceramide did not correlate with plasma free fatty acids.

CONCLUSIONS

Fasting for 62 h decreases insulin-mediated peripheral glucose uptake with lower phosphorylation of AKT at serine(473). AKT may play a regulatory role in fasting-induced insulin resistance. Whether the decrease in AKT can be attributed to the trend to higher muscle ceramide remains unanswered.

Genetic and dietary salt contributors to insulin resistance in Dahl salt-sensitive (S) rats

Insulin resistance has been extensively investigated during the past decade because of its proposed role in initiating a cluster of cardiovascular risk factors including hypertension. Insulin resistance is an inherited genetic trait that precedes hypertension in Dahl salt-sensitive (S) rats, and is not present in Dahl salt-resistant (R) rats. Owing to the co-existence of insulin resistance and salt sensitivity of blood pressure in Dahl S, but not R rats, Dahl S rats are used to elucidate the role of dietary salt as a potential link in exacerbating both phenotypes (insulin resistance and salt sensitivity). In light of available data, examining the impact of dietary salt on insulin resistance in Dahl S rats in terms of salt concentration and duration of exposure helps answer the following question: What percentage of dietary salt and for what duration of exposure would we expect an enhanced insulin resistance in Dahl S rats? This commentary gathers all available research done on insulin resistance in Dahl S rats in an attempt to unravel dietary salt contribution to insulin resistance in Dahl S rats.