The effects of prolonged fasting on plasma triglyceride kinetics in man

Effects of Different Types of Intermittent Fasting Interventions on Metabolic Health in Healthy Individuals (EDIF): A Randomised Trial with a Controlled-Run in Phase

The effects of intermittent fasting (IF) on health promotion in the healthy population remain controversial. Therefore, our study aimed to analyse the efficacy and feasibility of different IF protocols and evaluated the effects within a cohort with a controlled-run in phase on the body mass index (BMI) as the primary outcome, the body composition, and metabolic and haematological markers in healthy participants. A total of 25 individuals were randomised into three fasting groups: 16/8 fasting (n = 11), 20/4 fasting (n = 6), and alternate-day fasting (ADF, n = 8). Assessments were conducted at baseline (visit 1), after a four-week controlled-run in phase (visit 2), and after eight weeks of fasting (visit 3). Both the BMI (p = 0.01) and bodyweight (p = 0.01) were significantly reduced in the ADF group, which was not seen in the 16/8 and 20/4 groups (p > 0.05). Adherence was different but not statistically among the groups (16/8: 84.5 ± 23.0%; 20/4: 92.7 ± 9.5%; and ADF: 78.1 ± 33.5%, p = 0.57). Based on our obtained results, the data suggest that some fasting interventions might be promising for metabolic health. However, adherence to the specific fasting protocols remains challenging even for the healthy population.

The proteasome regulates body weight and systemic nutrient metabolism during fasting

The ubiquitin-proteasome system (UPS) and the autophagy-lysosome pathway are the primary means of degradation in mammalian tissues. We sought to determine the individual contribution of the UPS and autophagy to tissue catabolism during fasting. Mice were overnight fasted for 15 h before regaining food access ("Fed" group, n = 6) or continuing to fast ("Fast" group, n = 7) for 3 h. In addition, to investigate the effects of autophagy on systemic metabolism and tissue degradation, one group of mice was fasted for 18 h and treated with chloroquine ("Fast + CLQ" group, n = 7) and a fourth group of mice was treated with bortezomib ("Fast + Bort" group, n = 7) to assess the contribution of the UPS. Body weight, tissue weight, circulating hormones and metabolites, intracellular signaling pathways, and protein synthesis were investigated. Fasting induced the loss of body weight, liver mass, and white adipose tissue in the Fast and the Fast + CLQ group, whereas the Fast + Bort group maintained tissue and body weight. Fasting reduced glucose and increased β hydroxybutyrate in the circulation of all mice. Both changes were most profound in the Fast + Bort group compared with the other fasting conditions. Molecular signaling indicated a successful inhibition of hepatic UPS with bortezomib and an upregulation of the PI3K/AKT/mTOR pathway. The latter was further supported by an increase in hepatic protein synthesis with bortezomib. Inhibition of the UPS through bortezomib blocks body weight loss and tissue catabolism during an acute overnight fast in mice. The effects were likely mediated through a combined effect of the drug on biomolecule degradation and synthesis.NEW & NOTEWORTHY Bortezomib treatment prevents tissue and body weight loss during fasting. The loss of proteasome activity with bortezomib exacerbates fasting-induced ketogenesis. During fasting, bortezomib increases AMPK and PI3K/AKT signaling in the liver, which promotes protein synthesis.

Effects of Different Fasting Interventions on Cardiac Autonomic Modulation in Healthy Individuals: A Secondary Outcome Analysis of the EDIF Trial

The impact of a fasting intervention on electrocardiographic (ECG) time intervals and heart rate variability (HRV) is a focus that is scarcely analyzed. The main focus of these secondary outcome data was to describe the impact of a different fasting intervention on ECG and HRV analyses. Twenty-seven healthy individuals participated in this study (11 females, aged 26.3 ± 3.8 years, BMI 24.7 ± 3.4 kg/m2), including a pre-intervention controlled run-in period. Participants were randomized to one of the three fasting cohorts: (I) alternate day fasting (ADF, n = 8), (II) 16/8 fasting (16/8 h of fasting/feasting, n = 11) and (III) 20/4 fasting (20/4 h of fasting/feasting, n = 8). An analysis of baseline ECG parameters and HRV parameters following different fasting interventions demonstrated the safety of these interventions without impacting on heart rate variability parameters during Schellong-1 testing, and revealed comparable preserved autonomic cardiac modulation (ACM) independently of the fasting intervention. In conclusion, different short-term fasting interventions demonstrated no safety ECG-based concerns and showed comparable ACM based on ECG and HRV assessments. Finally, our research topic might strengthen the scientific knowledge of intermittent fasting strategies and indicate potential clinically preventive approaches with respect to occurring metabolic disease and obesity in healthy young subjects.

Metabolic effects of alternate-day fasting in males with obesity with or without type 2 diabetes

Alternate-day fasting induces oscillations in energy stores. We hypothesized that repeated oscillations increases insulin secretion and sensitivity, and improve metabolic health in patients with obesity with or without type 2 diabetes (T2DM). Twenty-three male patients fasted every other day for 30 h for 6 weeks. Experiments included resting energy expenditure, continuous glucose monitoring, intravenous glucose tolerance test, euglycemic hyperinsulinemic clamp, body composition, hepatic triglyceride content, muscle biopsies which were performed at baseline, during 3 weeks without allowed weight loss, and after additional 3 weeks with weight loss. Bodyweight decreased ∼1% and further ∼3% during weeks one to three and four to six, respectively (p < 0.05). Only minor changes in fat mass occurred in weeks 1-3. With weight loss, visceral fat content decreased by 13 ± 3% and 12 ± 2% from baseline in patients with and without T2DM, respectively (p < 0.05). Hepatic triglyceride content decreased by 17 ± 9% and 36 ± 9% (with diabetes) and 27 ± 8% and 40 ± 8% (without diabetes) from baseline to week 3 and week 6, respectively (all p < 0.05). Muscle lipid and glycogen content oscillated with the intervention. Glucose homeostasis, insulin secretion and sensitivity was impaired in patients with T2DM and did not change without weight loss, but improved (p < 0.05) when alternate day fasting was combined with weight loss. In conclusion, alternate-day fasting is feasible in patients with obesity and T2DM, and decreases visceral fat and liver fat deposits. Energy store oscillations by alternate-day fasting do not improve insulin secretion or sensitivity per se. Clinical Trial registration: (ClinicalTrials.gov), (ID NCT02420054).

Hepatokines-a novel group of exercise factors

Regular physical activity not only improves the exercise capacity of the skeletal muscle performing the contractions, but it is beneficial for the whole body. An extensive search for "exercise factors" mediating these beneficial effects has been going on for decades. Particularly skeletal muscle tissue has been investigated as a source of circulating exercise factors, and several myokines have been identified. However, exercise also has an impact on other tissues. The liver is interposed between energy storing and energy utilising tissues and is highly active during exercise, maintaining energy homeostasis. Recently, a novel group of exercise factors-termed hepatokines-has emerged. These proteins (fibroblast growth factor 21, follistatin, angiopoietin-like protein 4, heat shock protein 72, insulin-like growth factor binding protein 1) are released from the liver and increased in the bloodstream during or in the recovery after an exercise bout. In this narrative review, we evaluate this new group of exercise factors focusing on the regulation and potential function in exercise metabolism and adaptations. These hepatokines may convey some of the beneficial whole-body effects of exercise that could ameliorate metabolic diseases, such as obesity or type 2 diabetes.

Integrative Physiology of Fasting

Extended bouts of fasting are ingrained in the ecology of many organisms, characterizing aspects of reproduction, development, hibernation, estivation, migration, and infrequent feeding habits. The challenge of long fasting episodes is the need to maintain physiological homeostasis while relying solely on endogenous resources. To meet that challenge, animals utilize an integrated repertoire of behavioral, physiological, and biochemical responses that reduce metabolic rates, maintain tissue structure and function, and thus enhance survival. We have synthesized in this review the integrative physiological, morphological, and biochemical responses, and their stages, that characterize natural fasting bouts. Underlying the capacity to survive extended fasts are behaviors and mechanisms that reduce metabolic expenditure and shift the dependency to lipid utilization. Hormonal regulation and immune capacity are altered by fasting; hormones that trigger digestion, elevate metabolism, and support immune performance become depressed, whereas hormones that enhance the utilization of endogenous substrates are elevated. The negative energy budget that accompanies fasting leads to the loss of body mass as fat stores are depleted and tissues undergo atrophy (i.e., loss of mass). Absolute rates of body mass loss scale allometrically among vertebrates. Tissues and organs vary in the degree of atrophy and downregulation of function, depending on the degree to which they are used during the fast. Fasting affects the population dynamics and activities of the gut microbiota, an interplay that impacts the host's fasting biology. Fasting-induced gene expression programs underlie the broad spectrum of integrated physiological mechanisms responsible for an animal's ability to survive long episodes of natural fasting.

Gender differences in physical and psychological stress responses among college judoists undergoing weight reduction

Gender-related differences in anthropometry, blood biochemistry, psychological parameters, and energy intake during prematch weight reduction were studied in 22 men and 7 women college judoists who lost weight by combining judo training, restricting food and fluid, and sweating. Body weight (BW) decreased significantly by 2.2±1.4 kg in men and 2.0±1.4 kg in women 2 weeks after weight reduction started - not significandy different. Body fat, relative body fat and total energy intake also decreased significandy in both groups 2 weeks after weight reduction started. Lean body mass decreased significandy 2 weeks after weight reduction started only in men. Men had significantly decreased blood lipids, immunoglobulins, complements, hematocrit, white blood cell count, and serum electrolytes, and significant increases in blood uric nitrogen, creatinine, and hemoglobin, while women showed no such changes. The score for vigor in the Profile of Mood States (POMS) decreased in both groups 2 weeks after weight reduction started, but with no statistically gender difference. In women, scores for anxiety in the State-Trait Anxiety Inventory (STAI) and confusion in POMS increased significandy. Although the men and women had the same BW reduction, significant physical stress response was seen only in men, and psychological stress due to weight reduction and mental pressure of an upcoming competition were seen more in women.

Acute hypoxia induces hypertriglyceridemia by decreasing plasma triglyceride clearance in mice

Obstructive sleep apnea (OSA) induces intermittent hypoxia (IH) during sleep and is associated with elevated triglycerides (TG). We previously demonstrated that mice exposed to chronic IH develop elevated TG. We now hypothesize that a single exposure to acute hypoxia also increases TG due to the stimulation of free fatty acid (FFA) mobilization from white adipose tissue (WAT), resulting in increased hepatic TG synthesis and secretion. Male C57BL6/J mice were exposed to FiO(2) = 0.21, 0.17, 0.14, 0.10, or 0.07 for 6 h followed by assessment of plasma and liver TG, glucose, FFA, ketones, glycerol, and catecholamines. Hypoxia dose-dependently increased plasma TG, with levels peaking at FiO(2) = 0.07. Hepatic TG levels also increased with hypoxia, peaking at FiO(2) = 0.10. Plasma catecholamines also increased inversely with FiO(2). Plasma ketones, glycerol, and FFA levels were more variable, with different degrees of hypoxia inducing WAT lipolysis and ketosis. FiO(2) = 0.10 exposure stimulated WAT lipolysis but decreased the rate of hepatic TG secretion. This degree of hypoxia rapidly and reversibly delayed TG clearance while decreasing [(3)H]triolein-labeled Intralipid uptake in brown adipose tissue and WAT. Hypoxia decreased adipose tissue lipoprotein lipase (LPL) activity in brown adipose tissue and WAT. In addition, hypoxia decreased the transcription of LPL, peroxisome proliferator-activated receptor-γ, and fatty acid transporter CD36. We conclude that acute hypoxia increases plasma TG due to decreased tissue uptake, not increased hepatic TG secretion.

Gut-liver interaction in triglyceride-rich lipoprotein metabolism

The liver and intestine have complementary and coordinated roles in lipoprotein metabolism. Despite their highly specialized functions, assembly and secretion of triglyceride-rich lipoproteins (TRL; apoB-100-containing VLDL in the liver and apoB-48-containing chylomicrons in the intestine) are regulated by many of the same hormonal, inflammatory, nutritional, and metabolic factors. Furthermore, lipoprotein metabolism in these two organs may be affected in a similar fashion by certain disorders. In insulin resistance, for example, overproduction of TRL by both liver and intestine is a prominent component of and underlies other features of a complex dyslipidemia and increased risk of atherosclerosis. The intestine is gaining increasing recognition for its importance in affecting whole body lipid homeostasis, in part through its interaction with the liver. This review aims to integrate recent advances in our understanding of these processes and attempts to provide insight into the factors that coordinate lipid homeostasis in these two organs in health and disease.

Decrease in hepatic very-low-density lipoprotein-triglyceride secretion after weight loss is inversely associated with changes in circulating leptin

AIM

Although weight loss usually decreases very-low-density lipoprotein-triglyceride (VLDL-TG) secretion rate, the change in VLDL-TG kinetics is not directly related to the change in body weight. Circulating leptin also declines with weight loss and can affect hepatic lipid metabolism. The aim of this study was to determine whether circulating leptin is associated with weight loss-induced changes in VLDL-TG secretion.

METHODS

Ten extremely obese subjects were studied. VLDL-TG secretion rate and the contribution of systemic (derived from lipolysis of subcutaneous adipose tissue TG) and non-systemic fatty acids (derived primarily from lipolysis of intrahepatic and intraperitoneal TG, and de novo lipogenesis) to VLDL-TG production were determined by using stable isotopically labelled tracer methods before and 1 year after gastric bypass surgery.

RESULTS

Subjects lost 33 +/- 12% of body weight, and VLDL-TG secretion rate decreased by 46 +/- 23% (p = 0.001), primarily because of a decrease in the secretion of VLDL-TG from non-systemic fatty acids (p = 0.002). Changes in VLDL-TG secretion rates were not significantly related to reductions in body weight, body mass index, plasma palmitate flux, free fatty acid or insulin concentrations. The change in VLDL-TG secretion was inversely correlated with the change in plasma leptin concentration (r = -0.72, p = 0.013), because of a negative association between changes in leptin and VLDL-TG secretion from non-systemic fatty acids (r = -0.95, p < 0.001).

CONCLUSIONS

Weight loss-induced changes in plasma leptin concentration are inversely associated with changes in VLDL-TG secretion rate. Additional studies are needed to determine whether the correlation between circulating leptin and VLDL-TG secretion represents a cause-and-effect relationship.

Improved metabolic control after supplemented fasting in overweight type II diabetic patients

Twenty obese type II diabetes patients were treated in a metabolic ward during 4 weeks with a very low calorie regimen (200 kcal/day). They were given dietary advice and reexamined 3 months after discharge. Mean body weight decreased by 10 kg during fasting, blood glucose was normalized, urinary glucose disappeared and the K-value at i.v. glucose tolerance test increased. Fasting serum insulin concentrations decreased by 54%. Serum triglycerides (TG) decreased by 65%, serum cholesterol (Chol) by 28% and high density lipoprotein (HDL) Chol by 14%. Three months later, only serum TG remained significantly decreased (-47%) while HDL Chol was significantly higher than on admission (+11%). Fasting blood glucose remained significantly lower (-25%) with a low urinary glucose excretion. Supplemented fasting appears to be a safe and useful tool in the treatment of obese type II diabetics. It causes, at least during a limited follow-up period, a significant improvement in glucose control and lipoprotein metabolism in spite of a concomitant reduction of the antidiabetic medication.

Comparison of the effect of hyperinsulinemia on acyl-CoA:cholesterol acyltransferase activity in the liver and intestine of the rat

Recent studies have suggested that cholesteryl ester synthesis plays a critical role in the assembly of VLDL apo B and triacylglycerol in the liver. Chronic hyperinsulinemia is associated with increased TG production and since cholesteryl ester synthesis depends on acyl CoA:cholesterol acyltransferase (ACAT), we investigated the possibility that chronic hyperinsulinemia might increase ACAT activity. We also measured ACAT activity in the intestinal mucosa, where it has been suggested to play a role in induction of diabetes-associated hypercholesterolemia. Chronically hyperinsulinemic rats were produced by injecting insulin (2 weeks, 6U/day). To prevent profound hypoglycemia, these rats were given 10% sucrose in place of drinking water. Acute hyperinsulinemia was produced by injecting a single dose of 0.5 U insulin ip. Chronic hyperinsulinemia led to a significant increase in free cholesterol, cholesteryl esters, triacylglycerols and phospholipids in the whole liver (27%, P < 0.05; 60%, P < 0.05; 70%, P < 0.01; 37%, P < 0.01, respectively) and an increase in hepatic microsomal triacylglycerol (P < 0.05). In contrast, the microsomal lipids of the intestinal mucosa decreased significantly. In chronically hyperinsulinemic rats there was no change in hepatic ACAT, while ACAT in the intestine actually decreased (26%-50%, P < 0.01).

CONCLUSIONS

since the effect of chronic hyperinsulinemia on hepatic ACAT did not parallel that seen previously on VLDL secretion, the effect of chronic hyperinsulinemia on VLDL production cannot be explained by its effect on hepatic ACAT. However, chronic hyperinsulinemia was associated with reduced ACAT activity in the intestine and this might result in decreased lipoprotein production in the hyperinsulinemic intestine.

Lipoprotein disorders in diabetes mellitus

In IDDM or NIDDM, the total plasma cholesterol and triglycerides are usually within normal limits when the blood glucose is controlled. Marked hypertriglyceridemia can develop with loss of glycemic control and is often due to superimposed genetic abnormalities in lipoprotein metabolism. Tight control in IDDM usually reduces LDL and VLDL to normal levels and may raise HDL above the normal range. Low HDL cholesterol and mild to moderate elevations of VLDL triglyceride are common in NIDDM if obesity or proteinuria is also present. Both HDL and LDL may be smaller and more dense and may be enriched with triglyceride as compared with cholesterol. These abnormalities may require weight loss for control. The increased incidence of cardiovascular disease in diabetes is unexplained but is amplified by the well-defined cardiovascular risk factors. The new American Diabetes Association guidelines call for treatment of high triglycerides and LDL cholesterol to be aggressively reduced. Triglycerides should be under 200 mg/dL, are considered borderline high between 200 and 400 mg/dL, and high when above 400 mg/dL. Low HDL is defined as less than 35 mg/dL. Control of obesity with diet and exercise and reduced intake of saturated fat and cholesterol are important first steps. If needed, drug therapy is appropriate to reduce LDL to levels below 130 mg/dL in all adult diabetics and below 100 mg/dL in those with cardiovascular disease.

Measurement of de novo hepatic lipogenesis in humans using stable isotopes

Direct measurement of de novo lipogenesis has not previously been possible in humans. We measured de novo hepatic lipogenesis in normal men by means of stable isotopes and by combining the acetylated-xenobiotic probe technique with mass isotopomer analysis of secreted very low density lipoprotein-fatty acids (VLDL-FA). Sulfamethoxazole (SMX) was administered with [13C]acetate during an overnight fast followed by refeeding with intravenous glucose (7-10 mg/kg of weight per min), oral Ensure (7-10 mg of carbohydrate/kg of weight per min), or a high-carbohydrate mixed-meal breakfast (3.5 g of carbohydrate/kg of weight). Respiratory quotients remained less than 1.0. High-performance liquid chromatography/mass spectrometry-determined enrichments in SMX-acetate attained stable plateau values, and hepatic acetyl-coenzyme A (CoA) dilution rate did not increase with refeeding (approximately 0.024 mmol/kg per min). The fraction of VLDL-palmitate derived from de novo lipogenesis was only 0.91 +/- 0.27% (fasted) and 1.64-1.97% (fed). For stearate, this was 0.37 +/- 0.08% and 0.47-0.64%. Precursor enrichments predicted from isotopomer ratios were close to measured SMX-acetate enrichments, indicating that SMX-acetate samples the true lipogenic acetyl-CoA pool. Stearate synthesis was less than palmitate and the two did not move in parallel. Estimated total VLDL-FA synthesis is less than 500 mg/day. Thus, de novo hepatic lipogenesis is a quantitatively minor pathway, consistent with gas exchange estimates; fatty acid futile cycling (oxidation/resynthesis) is not thermogenically significant; and synthesis rates of different nonessential fatty acids by human liver are not identical in nonoverfed normal men. The contribution and regulation of de novo lipogenesis in other settings can be studied using this technique.

Postprandial triglyceride response in type 1 (insulin-dependent) diabetes mellitus is not altered by short-term deterioration in glycaemic control or level of postprandial insulin replacement

The effect of deteriorating glycaemic control on the lipoprotein responses to the ingestion of a high fat meal was investigated in seven normolipidaemic Type 1 (insulin-dependent) diabetic patients and the results were compared with corresponding responses in seven normolipidaemic control subjects. In addition, the importance of insulin in regulating the postprandial lipoprotein responses was examined by comparing the results obtained from the diabetic patients maintained on a basal infusion of insulin throughout the study with those obtained when a step-up, step-down insulin infusion was administered following the meal. Vitamin A was added to the test meal in all subjects to trace the metabolism of the chylomicron (Sf greater than 1000) and non-chylomicron (Sf less than 1000) fractions in the postprandial period. No differences in fasting and postprandial triglyceride levels nor in the concentration of the chylomicron and non-chylomicron fractions were observed between diabetic and control subjects. In the diabetic patients short-term (two-week) deterioration in glycaemic control did not have any adverse influence on the basal and postprandial lipid responses. However, while the amount of insulin administered after the meal in the diabetic patients did not have any effect on the postprandial triglyceride or chylomicron responses, the concentration of non-esterified fatty acids was significantly higher (p less than 0.0005) when only a basal infusion of insulin was administered.

IN CONCLUSION

1) Short-term deterioration in glycaemic control does not adversely affect lipoprotein concentrations in Type 1 diabetes. 2) Non-esterified fatty acids appear to be a more sensitive index of insulinization post-prandially than triglycerides.

Stabilisation of plasma substrate concentrations: A model for conducting metabolic studies

Metabolic studies involving changes in plasma substrate concentrations are frequently carried out after an overnight fast. This condition, however, is a transition between the post-prandial period and the beginning of starvation, and thus is associated with rapid changes in the plasma concentration of many substrates. Such alterations might interfere with the interpretation of modifications in plasma concentrations resulting from experimental manipulations. Infusion of glucose at a rate of 250 mg kg(-1) h(-1) for 1h and subsequently at 162 mg kg(-1) h(-1) together with amino-acids at 50 mg kg(-1) h(-1) is used to stabilise the plasma concentration of most substrates within 3 h, a condition which is maintained for the subsequent 5 h or more. This study offers a model which is more suitable for many metabolic investigations than overnight fasting and which takes little time or expense to prepare.

Weight set-point theory and the high-density lipoprotein concentrations of long-distance runners

Long-distance runners have higher high-density lipoprotein (HDL)-cholesterol concentrations and lower adiposity than sedentary men. Most cross-sectional studies claim that the runners' elevated HDL-cholesterol is not due to the runners' leanness. However, when cross-sectional studies use analysis of covariance (ANCOVA) to adjust for adiposity, or when they compare runners with lean sedentary men, they make an incorrect tacit assumption. They assume that the relationship between change in adiposity and change in HDL-cholesterol in men who have lost fat by running is the same as the cross-sectional difference in HDL-cholesterol between naturally fat and lean sedentary men. Regression slopes for HDL-cholesterol versus adiposity during and at the end of 1 year of running in 35 initially sedentary men suggest this assumption is incorrect; the increase in HDL-cholesterol that accompanies weight loss (-4.28 +/- 1.01 mg/100 mL per kg/m2) is considerably greater than the increase in HDL-cholesterol that is associated with lower adiposity cross-sectionally (-0.78 +/- 0.46 mg/100 mL per kg/m2). These results suggest the following theory: long-distance runners have the HDL metabolism of men who are below their sedentary set-point weight rather than the HDL metabolism of men who are naturally lean without exercising or dieting. This theory was applied to data from 23 published comparisons between long-distance runners and sedentary men.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of insulin in triglyceride turnover in rats

Exogenously induced hyperinsulinemia can increase in vivo triglyceride production in rats receiving dietary fructose, either as monosaccharide or as sucrose, but not in those receiving only glucose. Thus, in the presence of fructose, but not glucose, insulin stimulates triglyceride production. Dietary fructose can also impair the removal of circulating triglyceride. Exogenous insulin overcomes this fructose-associated impairment of triglyceride removal. On the other hand, streptozotocin-diabetic rats showed a suppressed triglyceride secretion rate (TgSR) but their plasma triglyceride level was unchanged. Therefore, insulin deficiency may result in not only decreased production of triglyceride but also impaired triglyceride removal from the circulation. Fructose-fed diabetic rats showed higher plasma triglyceride levels than chow-fed diabetic rats without a concomitant increase in TgSR, suggesting impaired triglyceride removal from the circulation induced by fructose in diabetic rats. Glucose-fed diabetic rats did not differ in TgSR or plasma triglyceride level from chow-fed diabetic rats. These observations indicate that circulating insulin and dietary fructose, but not glucose, have a key role in very-low-density lipoprotein triglyceride turnover in rats.

Triglyceride-rich lipoprotein metabolism during acute hyperinsulinemia in hypertriglyceridemic humans

Chronic endogenous hyperinsulinemia is associated with increased rates of triglyceride production in humans. The effect of acute exogenous hyperinsulinemia on triglyceride production was studied in seven hypertriglyceridemic men before and during six hours of hyperinsulinemic-euglycemic clamping, and in two men before and during six hours of hyperinsulinemic-hyperglycemic clamping. Apparent triglyceride production rates were assessed qualitatively by examining the rate of decline of 3H-glycerol-labeled plasma triglyceride specific activity in the preclamp period, and again when a new steady state had occurred, during the final three hours of the clamp. During the euglycemic (91.2 +/- 3.0 mg glucose/dL plasma) clamps, plasma insulin levels were increased by 700% (0.76 +/- 0.12 to 5.3 +/- 0.29 ng/mL, P less than .001) and plasma glucagon levels decreased by 19%, compared with baseline. The apparent triglyceride production rate did not increase in five of six men during the euglycemic-hyperinsulinemic clamp, or in either man during the hyperglycemic-hyperinsulinemic clamp. During the clamp period the triglyceride declined in the plasma by 23.4 +/- 3.1%; and the apolipoprotein B by 10.5 +/- 1.5%. Hyperinsulinemia with euglycemia was also associated with a decline in the ratio of triglyceride to apolipoprotein B in the triglyceride-rich lipoproteins. This was more pronounced in very low density lipoprotein (VLDL) than in intermediate density lipoprotein (IDL). In this study, hyperinsulinemia led to a decrease in the plasma glucagon concentration. This decrease was positively correlated with the decrease in the slope of the triglyceride specific activity v time curve. Hence, the changes in triglyceride production were not due to an increase in plasma glucagon concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Diabetes and atherosclerosis. Metabolic links

Atherosclerosis is the most frequent complication of diabetes. Two of the many factors that may contribute to this are dyslipoproteinaemias and hyperinsulinaemia. The most frequent dyslipoproteinaemia in diabetics is hypertriglyceridaemia. This probably reflects an increase in intermediate density lipoprotein (IDL) concentration. Plasma levels of triglyceride and insulin are often correlated. This partly reflects a vicious circle in which hypertriglyceridaemia itself can cause insulin resistance and compensatory hyperinsulinaemia. The chronic hyperinsulinaemic state in vivo is, in turn, associated with an increase in the rate of production of very low density lipoprotein (VLDL) triglyceride. Such a vicious circle is of potential importance in atherogenesis in view of the association of both hypertriglyceridaemia and hyperinsulinaemia with early macrovascular disease.

The use of isotopic tracers in studying lipid metabolism in human subjects

We have attempted to evaluate some of the tracer methodologies involved in studying lipid metabolism in humans. The magnitude of this subject prohibits a comprehensive review of all areas. Since the major function of adipose tissue appears to be to supply the body with energy, we have particularly emphasized the approaches used to study the mobilization and oxidation of fat. The importance of these issues, as well as the increasing availability of non-radioactive tracers, suggest an optimistic future for this area of research.

Portal v peripheral hyperinsulinemia and very low density lipoprotein triglyceride kinetics

The effect of subcutaneously delivered insulin on the kinetics of rat plasma triglycerides was compared to that of intraperitoneally delivered insulin. The former route delivered insulin primarily extrahepatically and the latter, intraportally. In comparison to the intraperitoneally delivered insulin, the subcutaneously delivered insulin was associated with a higher peripheral serum insulin, lower serum glucose, lower serum FFA, lower serum triglycerides, and similar rate of triglyceride secretion. The activity of adipose tissue lipoprotein lipase was directly related to the serum insulin concentration. The pattern of serum triglycerides and lipoprotein lipase in the rats receiving subcutaneous insulin suggested that their rate of triglyceride removal exceeded that seen in the rats receiving intraperitoneal insulin. These observations indicate that the route of insulin delivery can influence the balance between the hepatic and extrahepatic effects of insulin on triglyceride kinetics.

Hypertriglyceridemia and carbohydrate intolerance: interrelations and therapeutic implications

Atherosclerosis, the most frequent complication of diabetes, could be the result of hyperlipidemia, among other factors. Mounting evidence suggests that reducing the concentration of triglyceride-rich lipoprotein, which influences the production of the possibly atherogenic intermediate density lipoprotein (IDL), might diminish the circulating level of potentially atherogenic lipoproteins. Hypertriglyceridemia, even in the absence of obesity, is associated with insulin resistance. To compensate, pancreatic B cells respond to glucose challenge by producing hyperinsulinemia. If the B cells cannot respond adequately, carbohydrate intolerance ensues. Insulin-treated diabetics may also become hyperinsulinemic because routine insulin injection may not reflect physiologic need and because the insulin is administered peripherally rather than portally. Hyperinsulinemia increases the production of circulating triglyceride. It appears to do this in rats by causing the production of more triglyceride-rich lipoprotein particles rather than by increasing the triglyceride content of each particle. Further, at least in rats, the insulin-induced increase in triglyceride production requires the presence of supplementary dietary fructose. Hyperinsulinemia also increases the activity of adipose tissue lipoprotein lipase and the degradation of very low density lipoprotein (VLDL). The concentration of VLDL depends on balance of production and degradation. Accelerated VLDL degradation leads to an increase in IDL production. Because there is mounting evidence that IDL may be atherogenic, this cycle could accelerate atherogenesis. As such, it is reasonable to postulate that reducing the concentration of triglyceride-rich lipoproteins would break this cycle and would diminish the circulating level of potentially atherogenic lipoproteins.

Serum lipoprotein and lipoprotein lipase in overweight, type II diabetics during and after supplemented fasting

Poorly controlled, obese, Type II diabetics were studied before, during, and 3 months after a weight reduction program that used supplemented fasting (200 kcal or 0.9 MJ/day). During fasting, the very low density lipoprotein (VLDL) triglycerides (TG) decreased, as did the adipose tissue lipoprotein lipase (AT-LPLA) and skeletal muscle lipoprotein lipase (SM-LPLA) activities. Three months later VLDL TG remained low (-59%), while high density lipoprotein cholesterol was higher (+11%) and blood glucose control improved compared with values on admission. The fractional removal rate (K2) at the i.v. fat tolerance test (IVFTT) and the SM-LPLA were unchanged, while AT-LPLA (expressed per gram of wet weight, but not as whole-body AT-LPLA) increased by 25%. There were no significant correlations between AT-LPLA and the lipoprotein TG concentrations or K2-IVFTT, although there were significant positive correlations between SM-LPLA and K2-IVFtt, both on admission and after body weight stabilization. This may indicate that SM-LPLA is more directly related to the capacity to remove lipoprotein TG, at least in obese diabetic patients. K2-IVFTT was inversely correlated to the VLDL TG and cholesterol concentrations both before and 3 months after fasting. Because both SM-LPLA and K2-IVFTT were unchanged after body weight reduction, the change in VLDL TG may be mainly due to a reduced rate of lipoprotein synthesis.

Changes in lipoprotein metabolism during a supplemented fast and an ensuing vegetarian diet period

The effect on lipoprotein metabolism of a 2-week modified fast and an immediately ensuing 3-week period on a vegetarian diet was studied under metabolic ward conditions in 21 non-obese female and 6 male patients. The very low calorie diet induced reductions of the cholesterol concentration in all serum lipoprotein classes. In the female patients, who were all normolipoproteinaemic, the triglycerides in serum showed a slight increase during the fast, reflecting small changes in very low (VLDL) and low density lipoprotein triglycerides. This may probably be explained partly by simultaneous significant reductions of both the adipose tissue and skeletal muscle tissue lipoprotein lipase activities (LPLA). In contrast, in the male patients who had a higher VLDL level at admission, the VLDL triglycerides decreased without significant changes of high density lipoprotein (HDL) cholesterol and of LPLA in muscle. The female patients, whose weights were stable during the vegetarian diet, ended up with a lower HDL cholesterol than at the start of the trial. This effect was probably partly due to the high content of polyunsaturated fatty acids in the vegetarian diet. It is concluded that the changes of lipoprotein metabolism during supplemented fasting are quantitatively and qualitatively different in several respects in females and males.

Hyperinsulinemia and in vivo very-low-density lipoprotein-triglyceride kinetics

The effect of hyperinsulinemia (2 wk of twice daily NPH insulin) on the kinetics of very-low-density lipoprotein (VLDL)-triglyceride (TG) was studied in rats. To avoid profound hypoglycemia the rats were allowed sucrose ad libitum. Two control groups were needed: chow only and ad libitum sucrose-supplemented (high-CHO). The insulin-treated rats had 15 times higher IRI and 50% lower plasma glucose levels than either control group. Their TG production exceeded and their TG concentrations were less than those of either control group. This indicated that their TG removal was increased even more than their TG production. This increase in TG production occurred despite lower plasma free fatty acid (FFA) levels, suggesting that a greater proportion of TG fatty acids came from a source other than FFA. Compared with chow controls, high-CHO controls had the same peripheral IRI, a slight increase in TG production, and an increase in TG concentration. The differences between the effects of CHO supplementation alone or together with injected insulin may relate to the IRI and/or the route of access of insulin (peripheral vs. portal). The present studies indicate that hyperinsulinemia, either directly or indirectly, accelerates triglyceride turnover.

Changes in high density lipoprotein cholesterol after initiation of insulin therapy in non-insulin dependent diabetes mellitus: relationship to changes in body weight

To determine the interrelationships of serum lipoproteins and body weight changes with insulin therapy, 55 diabetics (mean age 52.0 +/- 2.2 years), had measurements of body weight, sum of skinfold thickness, fasting serum glucose, serum high density (HDL-C) and low density (LDL-C) lipoprotein cholesterol and serum triglycerides before and 6 weeks after beginning insulin. At entry HDL-C had a significant (p less than 0.05) negative correlation with indices of body weight-body mass index, relative weight and a significant positive correlation with serum triglycerides but no significant association with sum of skinfold thickness, fasting serum glucose or LDL-C. The relationships were seen mainly in patients who previously had not been receiving oral hypoglycemic medications. Insulin significantly increased HDL-C and body weight, and decreased glucose, triglycerides and LDL-C. Changes in HDL-C showed a significant inverse association with changes in triglycerides and direct correlation with changes in body weight, but no significant association with changes in fasting serum glucose or LDL-C. Thus, insulin therapy of diabetes mellitus favorably affects HDL-C concentration perhaps by altering triglyceride metabolism and represents a unique situation where increases in body weight are associated with increases in serum HDL-C concentration.

Glycogenolysis in the fasting dog

Glycogen concentrations were determined in liver-biopsy specimens which were taken from four dogs during five consecutive days of fasting. It was found that maximal depletion of liver glycogen occurred between the second and the third day. Starvation-induced glycogenolysis was much slower in the dog than in men and rats. Fuel fluxes are discussed and it is tentatively concluded that in the fasting dog larger amounts of glycerol are available for gluconeogenesis than in other species.

Hyperinsulinemia in patients with low fractional catabolic rate of triglycerides

In non-diabetic persons whose serum triglyceride (TG) concentrations ranged from normal to very high levels, endogenous TG turnover was measured using the radioglycerol method of Farquhar and coworkers. Insulin, FFA, and glucose concentrations were estimated during an oral glucose tolerance test. Stimulated insulin levels were correlated positively to TG concentrations and absolute TG turnover rates, and negatively to fractional TG catabolic rates. FFA concentrations had similar relationships, also in non-insulin-dependent diabetics. A more detailed analysis showed that elevated insulin and FFA levels - as an expression of peripheral insulin resistance - are typical finding in the kind of patients whose fractional TG catabolic rate is low [less than or equal to 0.210 (h-1)], irrespective of actual serum TG concentration. Our data do not suggest a stimulatory role of insulin for TG production.

Metabolic consequences of fasting in old lean and obese Zucker rats

The effects of fasting on lipid and carbohydrate metabolism and plasma insulin and glucagon levels were compared in lean and obese Zucker rats. Sixteen-month-old female and male rats were fasted for periods of 2, 4, 6 and 12 days. Fasting produced significant decreases in hepatic rates of lipid, cholesterol, and glycogen synthesis, as well as circulating levels of triglycerides, cholesterol, phospholipids, and insulin. Significant increases in hepatic lipid levels and serum free fatty acids were noted. When compared to lean rats, obese rats had elevated rates of hepatic lipid and glycogen synthesis, hepatic lipid and glycogen stores, serum triglycerides, cholesterol, phospholipids, and plasma insulin. Lean rats had higher plasma glucagon levels. Sex differences in several parameters were observed. Females demonstrated higher levels of lipid and cholesterol synthesis and serum free fatty acids, whereas serum cholesterol levels and hepatic glycogen stores were higher in males. Following a 12-day fast, carcass fat and protein content were decreased in both lean and obese rats, but the obese animals maintained an obese body composition. It is concluded that fasting results in qualitatively similar metabolic and hormonal changes in both lean and obese rats, but that abnormalities in carbohydrate and lipid metabolism persist in obese rats even after a 12-day fast.

Effects of caloric restriction on lipid metabolism in man: changes of tissue lipoprotein lipase activities and of serum lipoproteins

Heparin-releasable lipoprotein lipase (LPL) activity was measured in biopsy samples of adipose tissue and skeletal muscle of 8 normal healthy females, first during an isocaloric diet and then after 2 and 7 days on a 400-kcal diet. In adipose tissue the LPL activity expressed per tissue weight fell to 38% and to 22% of the initial level after 2 and 7 days' caloric restriction, respectively. In skeletal muscle the LPL activity rose slightly after two days (+24%) but decreased to 49% of the initial value after seven days on diet. The estimated total body LPL activity decreased to 50% and to 20% of the baseline value after 2 and 7 days, respectively, but the relative contribution of skeletal muscle to the total LPL increased from 10 to 30%. The triglyceride and VLDL triglyceride concentrations were not significantly changed during the low calorie diet but the LDL triglyceride increased and the HDL cholesterol decreased significantly (P less than 0.01). It is concluded that substantial restriction of calorie intake results in a decrease of over-all triglyceride removal capacity but in an increase of the fraction removed by skeletal muscle. The decrease of HDL cholesterol is probably a consequence of the low turnover of exogenous and endogenous triglyceride-rich lipoproteins.

Effect of obesity on the relationship between very low density lipoprotein production rate and plasma triglyceride concentration in normal and hypertriglyceridemic subjects

The effect of obesity on the relationship between very low density lipoprotein (VLDL)-triglyceride (TG) production rate and concentration was studied in 80 subjects whose TG concentrations ranged from 41-1315 mg/100 ml and whose relative weight varied from 0.74-1.46. There was a positive correlation between VLDL-TG production rate and plasma TG concentration in all 80 patients (r = 0.78), which was highly statistically significant (p less than 0.001). Both male and female subjects were subdivided into quartiles on the basis of relative weight; the relationship between VLDL-TG production rate and plasma TG concentration was found to be comparable in all subgroups. These observations suggest that moderate degrees of obesity do not affect VLDL-TG kinetics in patients whose plasma TG levels vary over an extremely wide range.

The metabolic response to hypocaloric protein diets in obese man

Exogenous protein in the absence of other calories can cause protein-sparing, but the mechanisms involved are controversial. It has been postulated that low insulin and high fat-derived substrate levels are necessary and sufficient conditions for such protein-sparing. We therefore established such conditions with differing protocols of protein input to define the role of protein input in mediating the response. Three groups of obese, nondiabetic subjects received the following diets: (1) 82.5+/-1.0 g protein/day (400 cal/day) for 21 days, n = 7; (2) the same, but as a refeeding diet for 7 days after 21-28 days of total fasts, n = 7; and (3) commencing with the same input, but with daily stepwise decrements over 14 days to 19.4+/-2.2 g/day, then maintained an additional 7 days, n = 4. Diet 3 gave approximately the amount and pattern of protein lost during total fasting. The circulating hormone and substrate responses of diets 1 and 3 were comparable and resembled those of total fasts, in that plasma glucose and insulin fell and free fatty acids rose. Blood levels of alanine, pyruvate, and other glucogenic amino acids fell and blood levels of branched-chain amino acids rose transiently. Blood 3-hydroxybutyrate levels and urinary excretion were greater in diet 3 than diet 1, but less than in total fasting. Nitrogen balance in diet 1 was transiently negative, but in equilibrium from 12 to 21 days. In diet 3, it was constantly negative at -6 g/day, the values also observed at 21 days of fasting. Mean 3-methylhistidine excretion decreased by 170 mumol/day in diet 1 and 107 mumol/day in diet 3, reflecting decreased muscle protein catabolism. The refed, protein-depleted subjects, diet 2, showed an increase in plasma glucose without alteration in insulin levels. Free fatty acid and ketone body levels decreased to those of the steady state observed in diet 1. Glucogenic and branched-chain amino acids decreased transiently. Nitrogen balance became positive, and the low 3-methylhistidine excretion increased by 152 mumol/day. The differing responses of nitrogen balance could not be accounted for on the basis of levels of insulin or of fat-derived substrates. The primary determinants of the protein-sparing observed appeared to be the protein supply itself, and the magnitude of the decrease in endogenous protein catabolism. The positive balance on refeeding after prior depletion of protein stores was likely due to the exogenous supply, combined with decreased catabolism and considerably increased reutilization.