Glycerol production and utilization measured using stable isotopes

From plasma triglycerides to triglyceride metabolism: effects on mortality in the Copenhagen General Population Study

AIMS

It is unclear whether higher triglyceride metabolism per se contributes to mortality separate from elevated triglyceride-rich lipoproteins and body mass index. This study tested the hypotheses that higher triglyceride metabolism, measured as higher plasma glycerol and β-hydroxybutyrate, is associated with increased all-cause, cardiovascular, cancer, and other mortality.

METHODS AND RESULTS

This study included 30 000 individuals nested within 109 751 individuals from the Copenhagen General Population Study. During a median follow-up of 10.7 years, 9897 individuals died (2204 from cardiovascular, 3366 from cancer, and 2745 from other causes), while none were lost to follow-up. In individuals with glycerol >80 µmol/L (highest fourth) vs. individuals with glycerol <52 µmol/L (lowest fourth), the multivariable adjusted hazard ratio for all-cause mortality was 1.31 (95% confidence interval 1.22-1.40). In individuals with β-hydroxybutyrate >154 µmol/L (highest fourth) vs. individuals with β-hydroxybutyrate <91 µmol/L (lowest fourth), the multivariable adjusted hazard ratio for all-cause mortality was 1.18 (1.11-1.26). Corresponding values for higher plasma glycerol and β-hydroxybutyrate were 1.37 (1.18-1.59) and 1.18 (1.03-1.35) for cardiovascular mortality, 1.24 (1.11-1.39) and 1.16 (1.05-1.29) for cancer mortality, and 1.45 (1.28-1.66) and 1.23 (1.09-1.39) for other mortality, respectively. Results were robust to exclusion of first years of follow-up, to stratification for covariates including plasma triglycerides and body mass index, and to further adjustments.

CONCLUSION

This study observed an increased risk of all-cause, cardiovascular, cancer, and other mortality with higher triglyceride metabolism. This was not explained by higher plasma triglycerides and body mass index. The hypothesis studied in the present paper should be further validated by isotope flux studies.

Mathematical modeling reveals differential dynamics of insulin action models on glycerol and glucose in adolescent girls with obesity

Under healthy conditions, the pancreas responds to a glucose challenge by releasing insulin. Insulin suppresses lipolysis in adipose tissue, thereby decreasing plasma glycerol concentration, and it regulates plasma glucose concentration through action in muscle and liver. Insulin resistance (IR) occurs when more insulin is required to achieve the same effects, and IR may be tissue-specific. IR emerges during puberty as a result of high concentrations of growth hormone and is worsened by youth-onset obesity. Adipose, liver, and muscle tissue exhibit distinct dose-dependent responses to insulin in multi-phase hyperinsulinemic-euglycemic (HE) clamps, but the HE clamp protocol does not address potential differences in the dynamics of tissue-specific insulin responses. Changes to the dynamics of insulin responses would alter glycemic control in response to a glucose challenge. To investigate the dynamics of insulin acting on adipose tissue, we developed a novel differential-equations based model that describes the coupled dynamics of glycerol concentrations and insulin action during an oral glucose tolerance test in female adolescents with obesity and IR. We compared these dynamics to the dynamics of insulin acting on muscle and liver as assessed with the oral minimal model applied to glucose and insulin data collected under the same protocol. We found that the action of insulin on glycerol peaks approximately 67 min earlier (p < 0.001) and follows the dynamics of plasma insulin more closely compared to insulin action on glucose as assessed by the parameters representing the time constants for insulin action on glucose and glycerol (p < 0.001). These findings suggest that the dynamics of insulin action show tissue-specific differences in our IR adolescent population, with adipose tissue responding to insulin more quickly compared to muscle and liver. Improved understanding of the tissue-specific dynamics of insulin action may provide novel insights into the progression of metabolic disease in patient populations with diverse metabolic phenotypes.

Fasting serum free glycerol concentration is a potential surrogate marker of visceral obesity and insulin sensitivity in middle-aged Japanese men

BACKGROUND

Triglyceride (TG) is a tri-ester composed of a glycerol and 3 fatty acids. Degradation of TG in adipose tissue is increased in the fasting state but inhibited in the postprandial state. Although insulin suppresses adipose TG degradation, patients with insulin resistance have high concentrations of insulin and free glycerol (FG) in the fasting state.

OBJECTIVE

We examined whether the fasting FG concentration reflects visceral obesity and insulin sensitivity in middle-aged Japanese men.

METHODS

We measured the fasting serum FG concentration in 72 males aged 30 to 50 years using a simple enzymatic method. The subjects were divided into tertiles according to their homeostasis model assessment of insulin resistance (HOMA-IR). Besides routine glucose- and lipid-related parameters, we determined insulin sensitivity as the rate of glucose disappearance in a 2-step hyperinsulinemic-euglycemic clamp and the abdominal visceral fat area (VFA) by magnetic resonance imaging.

RESULTS

The highest HOMA-IR tertile group had a higher fasting FG concentration than the middle- and lowest-tertile groups (0.077 ± 0.024 vs 0.063 ± 0.017 and 0.061 ± 0.016 mmol/L, P < .05 and P < .01). The FG concentration was positively correlated with VFA (rs = 0.36; P < .01) and the HOMA-IR score (rs = 0.26, P < .05) but negatively correlated with insulin sensitivity (rs = -0.26, P < .05). Multivariate regression analysis revealed that the FG concentration is independently associated with VFA and insulin sensitivity.

CONCLUSION

The fasting FG concentration reflects VFA and insulin sensitivity in middle-aged Japanese men. The fasting FG concentration may be a potential surrogate marker of visceral obesity and insulin resistance in outpatients.

Methods for quantifying adipose tissue insulin resistance in overweight/obese humans

BACKGROUND/OBJECTIVES

Insulin resistance of adipose tissue is an important feature of obesity-related metabolic disease. However, assessment of lipolysis in humans requires labor-intensive and expensive methods, and there is limited validation of simplified measurement methods. We aimed to validate simplified methods for the quantification of adipose tissue insulin resistance against the assessment of insulin sensitivity of lipolysis suppression during hyperinsulinemic-euglycemic clamp studies.

SUBJECTS/METHODS

We assessed the insulin-mediated suppression of lipolysis by tracer-dilution of [1,1,2,3,3-²H₅]glycerol during hyperinsulinemic-euglycemic clamp studies in 125 overweight or obese adults (85 men, 40 women; age 50±11 years; body mass index 38±7 kg m^-2). Seven indices of adipose tissue insulin resistance were validated against the reference measurement method.

RESULTS

Low-dose insulin infusion resulted in suppression of the glycerol rate of appearance ranging from 4% (most resistant) to 85% (most sensitive), indicating a good range of adipose tissue insulin sensitivity in the study population. The reference method correlated with (1) insulin-mediated suppression of plasma glycerol concentrations (r=0.960, P<0.001), (2) suppression of plasma non-esterified fatty acid (NEFA) concentrations (r=0.899, P<0.001), (3) the Adipose tissue Insulin Resistance (Adipo-IR) index (fasting plasma insulin-NEFA product; r=-0.526, P<0.001), (4) the fasting plasma insulin-glycerol product (r=-0.467, P<0.001), (5) the Adipose Tissue Insulin Resistance Index (fasting plasma insulin-basal lipolysis product; r=0.460, P<0.001), (6) the Quantitative Insulin Sensitivity Check Index (QUICKI)-NEFA index (r=0.621, P<0.001), and (7) the QUICKI-glycerol index (r=0.671, P<0.001). Bland-Altman plots showed no systematic errors for the suppression indices but proportional errors for all fasting indices. Receiver-operator characteristic curves confirmed that all indices were able to detect adipose tissue insulin resistance (area under the curve ⩾0.801, P<0.001).

CONCLUSIONS

Adipose tissue insulin sensitivity (that is, the antilipolytic action of insulin) can be reliably quantified in overweight and obese humans by simplified index methods. The sensitivity and specificity of the Adipo-IR index and the fasting plasma insulin-glycerol product, combined with their simplicity and acceptable agreement, suggest that these may be most useful in clinical practice.

Convergence in insulin resistance between very severely obese and lean women at the end of pregnancy

AIMS

Disrupted intermediary metabolism may contribute to the adverse pregnancy outcomes in women with very severe obesity. Our aim was to study metabolism in such pregnancies.

METHODS

We recruited a longitudinal cohort of very severely obese (n = 190) and lean (n = 118) glucose-tolerant women for anthropometric and metabolic measurements at early, mid and late gestation and postpartum. In case-control studies of very severely obese and lean women we measured glucose and glycerol turnover during low- and high-dose hyperinsulinaemic-euglycaemic clamps (HEC) at early and late pregnancy and in non-pregnant women (each n = 6-9) and body fat distribution by MRI in late pregnancy (n = 10/group).

RESULTS

Although greater glucose, insulin, NEFA and insulin resistance (HOMA-IR), and greater weight and % fat mass (FM) was observed in very severely obese vs lean participants, the degree of worsening was attenuated in the very severely obese individuals with advancing gestation, with no difference in triacylglycerol (TG) concentrations between very severely obese and lean women at term. Enhanced glycerol production was observed in early pregnancy only in very severely obese individuals, with similar intrahepatic FM in very severely obese vs lean women by late gestation. Offspring from obese mothers were heavier (p = 0.04).

CONCLUSIONS/INTERPRETATION

Pregnancies complicated by obesity demonstrate attenuation in weight gain and insulin resistance compared with pregnancies in lean women. Increased glycerol production is confined to obese women in early pregnancy and obese and lean individuals have similar intrahepatic FM by term. When targeting maternal metabolism to treat adverse pregnancy outcomes, therapeutic intervention may be most effective applied early in pregnancy.

GSK256073, a selective agonist of G-protein coupled receptor 109A (GPR109A) reduces serum glucose in subjects with type 2 diabetes mellitus

AIMS

This clinical trial assessed whether a potent, selective GPR109A agonist, GSK256073, could, through inhibition of lipolysis, acutely improve glucose homeostasis in subjects with type 2 diabetes mellitus.

METHODS

Thirty-nine diabetic subjects were enrolled in the randomized, single-blind, placebo-controlled, three-period crossover trial. Each subject received placebo and two of four regimens of GSK256073 for 2 days. GSK256073 was dosed 5 mg every 12 h before breakfast and supper (BID), 10 mg every 24 h before breakfast (QD), 25 mg BID and 50 mg QD.

RESULTS

The change from baseline weighted mean glucose concentration for an interval from 24 to 48 h after the initial drug dose was significantly reduced for all GSK256073 regimens, reaching a maximum of -0.87 mmol/l (-1.20, -0.52) with the 25 mg BID dose. Sustained suppression of non-esterified fatty acid (NEFA) and glycerol concentrations was observed with all GSK256073 doses throughout the 48-h dosing period. Serum insulin and C-peptide concentrations fell in concert with glucose concentrations and calculated HOMA-IR scores decreased 27-47%, consistent with insulin sensitization. No marked differences were evident between either 10 and 50 mg total daily doses or QD versus BID dosing.

CONCLUSIONS

Administration of a GPR109A agonist for 2 days significantly decreased serum NEFA and glucose concentrations in diabetic subjects. Glucose improvements were associated with decreased insulin concentrations and measures of enhanced insulin sensitivity. Improved glucose control occurred with GSK256073 doses that were generally safe and not associated with events of flushing or gastrointestinal disturbances.

Endocrine, metabolic, and morphologic alterations of adipose tissue during critical illness

OBJECTIVE

Observational studies report lower mortality in obese than in lean critically ill patients, an association referred to as the "obesity paradox." This may suggest a possible protective role for adipose tissue during severe illness.

DATA SOURCES

Relevant publications were identified based on searches in PubMed and on secondary searches of their bibliographies.

DATA SYNTHESIS

The endocrine functions of adipose tissue might play a role in the adaptation to critical illness. In the acute phase of illness, the anti-inflammatory adiponectin is reduced, whereas proinflammatory cytokine expression in adipose tissue is up-regulated. In the prolonged phase of critical illness, both adiponectin and anti-inflammatory cytokine production are increasing. Studies on the proinflammatory adipokine leptin during critical illness are inconsistent, possibly due to confounders such as gender, body mass index, and feeding. Morphologically, adipose tissue of critically ill patients reveals an increased number of newly differentiated, smaller adipocytes. Accentuated macrophage accumulation showing a phenotypic switch to M2-type suggests an adaptive response to the microenvironment of severe illness. Functionally, adipose tissue of critically ill patients develops an increased ability to store glucose and triglycerides.

CONCLUSIONS

Endocrine, metabolic, and morphologic properties of adipose tissue change during critical illness. These alterations may suggest a possible adaptive, protective role in optimizing chances of survival. More research is needed to understand the exact role of adipose tissue in lean vs. obese critically ill patients, in order to understand how illness-associated alterations contribute to the obesity paradox.

Decreased hepatic futile cycling compensates for increased glucose disposal in the Pten heterodeficient mouse

Despite altered regulation of insulin signaling, Pten(+/-) heterodeficient standard diet-fed mice, approximately 4 months old, exhibit normal fasting glucose and insulin levels. We report here a stable isotope flux phenotyping study of this "silent" phenotype, in which tissue-specific insulin effects in whole-body Pten(+/-)-deficient mice were dissected in vivo. Flux phenotyping showed gain of function in Pten(+/-) mice, seen as increased peripheral glucose disposal, and compensation by a metabolic feedback mechanism that 1) decreases hepatic glucose recycling via suppression of glucokinase expression in the basal state to preserve hepatic glucose production and 2) increases hepatic responsiveness in the fasted-to-fed transition. In Pten(+/-) mice, hepatic gene expression of glucokinase was 10-fold less than wild-type (Pten(+/+)) mice in the fasted state and reached Pten(+/+) values in the fed state. Glucose-6-phosphatase expression was the same for Pten(+/-) and Pten(+/+) mice in the fasted state, and its expression for Pten(+/-) was 25% of Pten(+/+) in the fed state. This study demonstrates how intra- and interorgan flux compensations can preserve glucose homeostasis (despite a specific gene defect that accelerates glucose disposal) and how flux phenotyping can dissect these tissue-specific flux compensations in mice presenting with a "silent" phenotype.

Parenteral and enteral metabolism of anaplerotic triheptanoin in normal rats

A new chronic treatment for inherited disorders of long-chain fatty acid oxidation involves administering up to one-third of dietary calories as triheptanoin, a medium-odd-chain triglyceride (Roe CR, Sweetman L, Roe DS, David F, and Brunengraber H. J Clin Invest 110: 259-269, 2002). Heptanoate and C(5)-ketone bodies derived from its partial oxidation in liver are precursors of anaplerotic propionyl-CoA in peripheral tissues. It was hypothesized that increasing anaplerosis in peripheral tissues would boost energy production. In the present study, we tested the potential of a triheptanoin emulsion as an intravenous nutrient. Normal rats were infused with triheptanoin intravenously or intraduodenally at up to 40% of caloric requirement. The blood concentration ratio (heptanoate/C(5)-ketone bodies) was high with intravenous and low with intraduodenal triheptanoin infusion. During intravenous infusion of triheptanoin, lipolysis was stimulated but appeared compensated by fatty acid reesterification. During intraduodenal infusion of triheptanoin, lipolysis was not stimulated. Our data support the hypothesis that intravenous triheptanoin could be used to treat decompensated patients with long-chain fatty acid oxidation disorders.

Stable isotopes in obesity research

Obesity is recognized as a major public health problem. Obesity is a multifactorial disease and is often associated with a wide range of comorbidities including hypertension, non-insulin dependent (Type II) diabetes mellitus, and cardiovascular disease, all of which contribute to morbidity and mortality. This review deals with stable isotope mass spectrometric methods and the application of stable isotopes to metabolic studies of obesity. Body composition and total energy expenditure (TEE) can be measured by mass spectrometry using stable isotope labeled water, and the metabolism of protein, lipid, and carbohydrate can be measured using appropriate labeled tracer molecules.

Fat metabolism in exercise - with special reference to training and growth hormone administration

Despite abundance of fat, exclusive dependency on fat oxidation can only sustain a metabolic rate corresponding to 50-60% of VO(2max) in humans. This puzzling finding has been subject to intense research for many years. Lately, it has gained renewed interest as a consequence of increased obesity and physical inactivity imposed by Western lifestyle. Why are humans so poor at metabolizing fat? Can fat metabolism be manipulated by exercise, training, diet and hormones? And why is fat stored in specialized adipose tissue and not just as lipid droplets inside muscle cells? In the present review, human fat metabolism is discussed in relation to how human fat metabolism is designed. Limitations in this design are explored and examples of different designs for fat metabolism from animal physiology are included to illustrate these limitations. Various means of manipulating fat metabolism are discussed with special emphasis on exercise, training, growth hormone (GH) physiology and GH administration. It is concluded that fat stores, non-esterified fatty acids (NEFAs) availability and enzymes for fat oxidation can be increased substantially. However, it is almost impossible to increase fat oxidation during endurance exercise at higher intensities. It seems that, for some reason, the human being is far from optimally designed for fat oxidation during exercise. Acute GH administration has several unexpected effects on fat and carbohydrate metabolism during aerobic exercise, and future research in this area is likely to provide valuable information with respect to GH physiology and the regulation of fat and carbohydrate metabolism during aerobic exercise.

Peroxisome proliferator-activated receptor alpha (PPARalpha) influences substrate utilization for hepatic glucose production

The hypoglycemia seen in the fasting PPARalpha null mouse is thought to be due to impaired liver fatty acid beta-oxidation. The etiology of hypoglycemia in the PPARalpha null mouse was determined via stable isotope studies. Glucose, lactate, and glycerol flux was assessed in the fasted and fed states in 4-month-old PPARalpha null mice and in C57BL/6 WT maintained on standard chow using a new protocol for flux assessment in the fasted and fed states. Hepatic glucose production (HGP) and glucose carbon recycling were estimated using [U-(13)C(6)]glucose, and HGP, lactate, and glycerol turnover was estimated utilizing either [U-(13)C(3)]lactate or [2-(13)C]glycerol infused subcutaneously via Alza miniosmotic pumps. At the end of a 17-h fast, HGP was higher in the PPARalpha null mice than in WT by 37% (p < 0.01). However, recycling of glucose carbon from lactate back to glucose was lower in the PPARalpha null than in WT (39% versus 51%, p < 0.02). The lack of conversion of lactate to glucose was confirmed using an [U-(13)C(3)]lactate infusion. In the fasted state, HGP from lactate and lactate production were decreased by 65 and 55%, respectively (p < 0.05) in PPARalpha null mice. In contrast, when [2-(13)C]glycerol was infused, glycerol production and HGP from glycerol increased by 80 and 250%, respectively (p < 0.01), in the fasted state of PPARalpha null mice. The increased HGP from glycerol was not suppressed in the fed state. While little change was evident for phosphoenolpyruvate carboxykinase (PEPCK) expression, pyruvate kinase expression was decreased 16-fold in fasted PPARalpha null mice as compared with the wild-type control. The fasted and fed insulin levels were comparable, but blood glucose levels were lower in the PPARalpha null mice than in controls. In conclusion, PPARalpha receptor function creates a setpoint for a metabolic network that regulates the rate and route of HGP in the fasted and fed states, in part, by controlling the flux of glycerol and lactate between the triose-phosphate and pyruvate/lactate pools.

Functional foods and food supplements for athletes: from myths to benefit claims substantiation through the study of selected biomarkers

The development of the sports food market and industrial involvement have led to numerous nutritional studies to define the type of nutrients that are most suited to support energy metabolism, fluid balance and muscle function. The key question in many of these studies was: 'Does the product lead to a significant product/consumer benefit that can be used as a claim on the package?' New methods and techniques have been developed, partly with sponsorship of the food industry, with the goal of measuring the effects of specific nutrients and supplements on athletic performance and metabolism. In line with this development, a wide variety of supplements and sports foods/drinks labelled with various performance or health benefit statements have been launched on the sports nutrition market. Although a variety of products have been tested clinically, there are also many products on the market with benefit claims that cannot be supported by sound nutritional and sports physiological science. The current short review highlights some of the methods and biomarkers that are used to substantiate product/consumer benefit claims for foods and drinks that are marketed as functional foods for athletes.