Women produce fewer but triglyceride-richer very low-density lipoproteins than men

Effect of prior exercise on postprandial lipemia: an updated quantitative review

Reducing postprandial triglycerides (TG) can lower the risk for cardiovascular disease. The purpose of this study was to perform a meta-analytic review of the literature to estimate the effect of prior exercise on postprandial lipemia. A total of 121 effects were found from 76 studies for the total TG response and 70 effects from 44 studies for the incremental area under the curve (iAUC) TG response. The weighted mean effect was moderate for the total TG response, Cohen's d = −0.60 ( P < 0.0001), and for the iAUC response, Cohen's d = −0.59 ( P < 0.0001). Moderator analysis revealed women exhibited a larger reduction ( P < .01) in the total TG response following exercise ( d = −0.96) than men ( d = −0.57); high-intensity interval training induced a larger reduction ( P < .05) in the iAUC response ( d = −1.49) than aerobic ( d = −0.58) or resistance ( d = −0.13) exercise, and participants maintaining an energy deficit following exercise exhibited a greater reduction in the iAUC response ( d = −0.67) compared with participants in energy balance ( d = −0.28). We conclude that prior acute exercise reduces postprandial lipemia, with the magnitude of effect influenced by sex, type of exercise, and energy deficit following exercise.

Sexual dimorphism in the effects of exercise on metabolism of lipids to support resting metabolism

Exercise training is generally a healthful activity and an effective intervention for reducing the risk of numerous chronic diseases including cardiovascular disease and diabetes. This is likely both a result of prevention of weight gain over time and direct effects of exercise on metabolism of lipids and the other macronutrient classes. Importantly, a single bout of exercise can alter lipid metabolism and metabolic rate for hours and even into the day following exercise, so individuals who regularly exercise, even if not performed every single day, overall could experience a substantial change in their resting metabolism that would reduce risk for metabolic diseases. However, resting metabolism does not respond similarly in all individuals to exercise participation, and indeed gender or sex is a major determinant of the response of resting lipid metabolism to prior exercise. In order to fully appreciate the metabolic effects and health benefits of exercise, the differences between men and women must be considered. In this article, the differences in the effects of exercise on resting metabolic rate, fuel selection after exercise, as well as the shuttling of triglyceride and fatty acids between tissues are discussed. Furthermore, concepts related to sex differences in the precision of homeostatic control and sex differences in the integration of metabolism between various organs are considered.

Gender differences in skeletal muscle substrate metabolism - molecular mechanisms and insulin sensitivity

It has become increasingly apparent that substrate metabolism is subject to gender-specific regulation, and the aim of this review is to outline the available evidence of molecular gender differences in glucose and lipid metabolism of skeletal muscle. Female sex has been suggested to have a favorable effect on glucose homeostasis, and the available evidence from hyperinsulinemic-euglycemic clamp studies is summarized to delineate whether there is a gender difference in whole-body insulin sensitivity and in particular insulin-stimulated glucose uptake of skeletal muscle. Whether an eventual higher insulin sensitivity of female skeletal muscle can be related to gender-specific regulation of molecular metabolism will be topic for discussion. Gender differences in muscle fiber type distribution and substrate availability to and in skeletal muscle are highly relevant for substrate metabolism in men and women. In particular, the molecular machinery for glucose and fatty acid oxidative and storage capacities in skeletal muscle and its implications for substrate utilization during metabolic situations of daily living are discussed, emphasizing their relevance for substrate choice in the fed and fasted state, and during periods of physical activity and recovery. Together, handling of carbohydrate and lipids and regulation of their utilization in skeletal muscle have implications for whole-body glucose homeostasis in men and women. 17-β estradiol is the most important female sex hormone, and the identification of estradiol receptors in skeletal muscle has opened for a role in regulation of substrate metabolism. Also, higher levels of circulating adipokines as adiponectin and leptin in women and their implications for muscle metabolism will be considered.

Systemic free fatty acid disposal into very low-density lipoprotein triglycerides

We measured the incorporation of systemic free fatty acids (FFA) into circulating very low-density lipoprotein triglycerides (VLDL-TGs) under postabsorptive, postprandial, and walking conditions in humans. Fifty-five men and 85 premenopausal women with BMI 18-24 (lean) and 27-36 kg/m(2) (overweight/obese) received an intravenous bolus injection of [1,1,2,3,3-(2)H5]glycerol (to measure VLDL-TG kinetics) and either [1-(14)C]palmitate or [9,10-(3)H]palmitate to determine the proportion of systemic FFA that is converted to VLDL-TG. Experiments started at 0630 h after a 12-h overnight fast. In the postabsorptive protocol, participants rested and remained fasted until 1330 h. In the postprandial protocol, volunteers ingested frequent portions of a fat-free smoothie. In the walking protocol, participants walked on a treadmill for 5.5 h at ∼3× resting energy expenditure. Approximately 7% of circulating FFA was converted into VLDL-TG. VLDL-TG secretion rates (SRs) were not statistically different among protocols. Visceral fat mass was the only independent predictor of VLDL-TG secretion, explaining 33-57% of the variance. The small proportion of systemic FFA that is converted to VLDL-TG can confound the expected relationship between plasma FFA concentration and VLDL-TG SRs. Regulation of VLDL-TG secretion is complex in that, despite a broad spectrum of physiological FFA concentrations, VLDL-TG SRs did not vary based on different acute substrate availability.

Independent effects of testosterone on lipid oxidation and VLDL-TG production: a randomized, double-blind, placebo-controlled, crossover study

Low testosterone (T) levels in men have been shown to predict development of the metabolic syndrome, but the effects of T on lipid metabolism are incompletely understood. In a randomized, double-blind, placebo-controlled, crossover study, 12 healthy, young males received gonadotropin-releasing hormone agonist treatment 1 month prior to 3 of 4 trial days to induce castrate levels of T. On trial days, T gel was applied to the body containing either high or low physiological T dose or placebo. On the 4th trial day, participants constituted their own eugonadal controls. Each study comprised a 5-h basal period and a 3-h hyperinsulinemic-euglycemic clamp. Short-term hypogonadism did not affect VLDL triglyceride (TG) secretion, nor did it affect VLDL-TG concentrations. It was, however, characterized by lower total lipid oxidation. In addition, acute rescue with high physiological T increased VLDL-TG secretion during both basal and clamp conditions. These data show that T can act through fast nongenomic pathways in the liver. In addition, the early hypogonadal state is characterized by decreased total lipid oxidation, but whether these changes represent early hypogonadal metabolic dysfunction warrants further investigations. T is not a major determinant of resting VLDL-TG kinetics in men.

Effect of high-intensity interval exercise on basal triglyceride metabolism in non-obese men

A single bout of high-intensity interval aerobic exercise has been shown to produce the same or greater metabolic benefits as continuous endurance exercise with considerably less energy expenditure, but whether this applies to very low density lipoprotein (VLDL) metabolism is not known. We sought to examine the effect of a single bout of high-intensity interval aerobic exercise on basal VLDL-triglyceride (TG) kinetics 14 and 48 h after exercise cessation to determine the acute and time-dependent effects of this type of exercise on VLDL-TG metabolism. Eight healthy sedentary men (age, 23.6 ± 6.1 years; body mass index, 23.1 ± 2.2 kg·m(-2), peak oxygen consumption (V̇O2peak), 36.3 ± 5.5 mL·kg(-1)·min(-1)) participated in three stable isotopically labeled tracer infusion studies: (i) 14 h and (ii) 48 h after a single bout of high-intensity aerobic interval exercise (60% and 90% of V̇O2peak in 4 min intervals for a total of 32 min; gross energy expenditure ∼500 kcal) and (iii) after an equivalent period of rest, in random order. Fasting plasma VLDL-TG concentration was 20% lower at 14 h (P = 0.046) but not at 48 h (P = 1.000) after exercise compared with the resting trial. VLDL-TG plasma clearance rate increased by 21% at 14 h (P < 0.001) but not at 48 h (P = 0.299) after exercise compared with rest, whereas hepatic VLDL-TG secretion rate was not different from rest at any time point after exercise. We conclude that high-intensity interval exercise reduces fasting plasma VLDL-TG concentrations in non-obese men the next day by augmenting VLDL-TG clearance, just like a single bout of continuous endurance exercise. This effect is short-lived and abolished by 48 h after exercise.

Acute effects of exercise and calorie restriction on triglyceride metabolism in women

PURPOSE

The mechanisms by which exercise reduces fasting plasma triglyceride (TG) concentrations in women and the effect of negative energy balance independent of muscular contraction are not known.The aim of this study was to evaluate the effects of equivalent energy deficits induced by exercise or calorie restriction on basal VLDL-TG metabolism in women.

METHODS

Eleven healthy women (age = 23.5 ± 2.7 yr, body mass index = 21.6 ± 1.4 kg·m-2; mean ± SD) underwent a stable isotopically labeled tracer infusion study to determine basal VLDL-TG kinetics after performing, in random order, three experimental trials on the previous day: (i) a single exercise bout (brisk walking at 60% of peak oxygen consumption for 123 ± 18 min, with a net energy expenditure of 2.06 ± 0.39 MJ, ∼500 kcal), (ii) dietary energy restriction of 2.10 ± 0.41 MJ, and (iii) a control day of isocaloric feeding and rest (zero energy balance).

RESULTS

Fasting plasma VLDL-TG concentration was approximately 30% lower after the exercise trial compared with the control trial (P < 0.001), whereas no significant change was detected after the calorie restriction trial (P = 0.297 vs control). Relative to the control condition, exercise increased the plasma clearance rate of VLDL-TG by 22% (P = 0.001) and reduced hepatic VLDL-TG secretion rate by approximately 17% (P = 0.042), whereas hypocaloric diet had no effect on VLDL-TG kinetics (P > 0.2).

CONCLUSION

(i) Exercise-induced hypotriglyceridemia in women manifests through a different mechanism (increased clearance and decreased secretion of VLDL-TG) than that previously described in men (increased clearance of VLDL-TG only), and (ii) exercise affects TG homeostasis by eliciting changes in VLDL-TG kinetics that cannot be reproduced by an equivalent diet-induced energy deficit, indicating that these changes are independent of the exercise-induced negative energy balance but instead are specific to muscular contraction.

Estrogen treatment after ovariectomy protects against fatty liver and may improve pathway-selective insulin resistance

Pathway-selective insulin resistance where insulin fails to suppress hepatic glucose production but promotes liver fat storage may underlie glucose and lipid abnormalities after menopause. We tested the mechanisms by which estrogen treatment may alter the impact of a high-fat diet (HFD) when given at the time of ovariectomy (OVX) in mice. Female C57BL/6J mice underwent sham operation, OVX, or OVX with estradiol (E2) treatment and were fed an HFD. Hyperinsulinemic-euglycemic clamps were used to assess insulin sensitivity, tracer incorporation into hepatic lipids, and liver triglyceride export. OVX mice had increased adiposity that was prevented with E2 at the time of OVX. E2 treatment increased insulin sensitivity with OVX and HFD. In sham and OVX mice, HFD feeding induced fatty liver, and insulin reduced hepatic apoB100 and liver triglyceride export. E2 treatment reduced liver lipid deposition and prevented the decrease in liver triglyceride export during hyperinsulinemia. In mice lacking the liver estrogen receptor α, E2 after OVX limited adiposity but failed to improve insulin sensitivity, to limit liver lipid deposition, and to prevent insulin suppression of liver triglyceride export. In conclusion, estrogen treatment may reverse aspects of pathway-selective insulin resistance by promoting insulin action on glucose metabolism but limiting hepatic lipid deposition.

Randomized trial of exercise effect on intrahepatic triglyceride content and lipid kinetics in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) and alterations in hepatic lipoprotein kinetics are common metabolic complications associated with obesity. Lifestyle modification involving diet-induced weight loss and regular exercise decreases intrahepatic triglyceride (IHTG) content and very low density lipoprotein (VLDL) triglyceride (TG) secretion rate. The aim of this study was to evaluate the weight loss-independent effect of following the physical activity guidelines recommended by the Department of Health and Human Services on IHTG content and VLDL kinetics in obese persons with NAFLD. Eighteen obese people (body mass index [BMI]: 38.1 ± 4.6 kg/m(2)) with NAFLD were randomized to 16 weeks of exercise training (45%-55% VO(2peak) , 30-60 minutes × 5 days/week; n = 12) or observation (control; n = 6). Magnetic resonance spectroscopy and stable isotope tracer infusions in conjunction with compartmental modeling were used to evaluate IHTG content and hepatic VLDL-TG and apolipoprotein B-100 (apoB-100) secretion rates. Exercise training resulted in a 10.3% ± 4.6% decrease in IHTG content (P < 0.05), but did not change total body weight (103.1 ± 4.2 kg before and 102.9 ± 4.2 kg after training) or percent body fat (38.9% ± 2.1% before and 39.2% ± 2.1% after training). Exercise training did not change the hepatic VLDL-TG secretion rate (17.7 ± 3.9 μmol/min before and 16.8 ± 5.4 μmol/min after training) or VLDL-apoB-100 secretion rate (1.5 ± 0.5 nmol/min before and 1.6 ± 0.6 nmol/min after training).

CONCLUSION

Following the Department of Health and Human Services recommended physical activity guidelines has small but beneficial effects on IHTG content, but does not improve hepatic lipoprotein kinetics in obese persons with NAFLD.

Testosterone increases the muscle protein synthesis rate but does not affect very-low-density lipoprotein metabolism in obese premenopausal women

Men and women with hyperandrogenemia have a more proatherogenic plasma lipid profile [e.g., greater triglyceride (TG) and total and low-density lipoprotein-cholesterol and lower high-density lipoprotein-cholesterol concentrations] than healthy premenopausal women. Furthermore, castration of male rats markedly reduces testosterone availability below normal and decreases plasma TG concentration, and testosterone replacement reverses this effect. Testosterone is, therefore, thought to be an important regulator of plasma lipid homeostasis. However, little is known about the effect of testosterone on plasma TG concentration and kinetics. Furthermore, testosterone is a potent skeletal muscle protein anabolic agent in men, but its effect on muscle protein turnover in women is unknown. We measured plasma lipid concentrations, hepatic very low density lipoprotein (VLDL)-TG and VLDL-apolipoprotein B-100 secretion rates, and the muscle protein fractional synthesis rate in 10 obese women before and after trandermal testosterone (1.25 g of 1% AndroGel daily) treatment for 3 wk. Serum total and free testosterone concentrations increased (P < 0.05) by approximately sevenfold in response to testosterone treatment, reaching concentrations that are comparable to those in women with hyperandrogenemia, but lower than the normal range for eugonadal men. Except for a small (∼10%) decrease in plasma high-density lipoprotein particle and cholesterol concentrations (P < 0.04), testosterone therapy had no effect on plasma lipid concentrations, lipoprotein particle sizes, and hepatic VLDL-TG and VLDL-apolipoprotein B-100 secretion rates (all P > 0.05); the muscle protein fractional synthesis rate, however, increased by ∼45% (P < 0.001). We conclude that testosterone is a potent skeletal muscle protein anabolic agent, but not an important regulator of plasma lipid homeostasis in obese women.

Very Low Density Lipoprotein Metabolism in Patients with Chronic Kidney Disease

BACKGROUND: Hypertriglyceridemia is a common metabolic complication of chronic kidney disease (CKD) and an important risk factor for coronary heart disease in this patient population. The mechanisms responsible for the development of hypertriglyceridemia in subjects with CKD are not clear. METHODS: We studied very low density lipoprotein triglyceride (VLDL-TG) and VLDL-apolipoprotein B-100 (VLDL-apoB-100) kinetics in vivo in 6 subjects with non-dialysis-dependent CKD (CKD-ND), 6 subjects with CKD treated with peritoneal dialysis (CKD-PD) and 24 sex-, age- and body mass index-matched control subjects with normal renal function (12 control subjects each matched with the CKD-ND and CKD-PD group, respectively). RESULTS: The secretion rates of VLDL-TG and VLDL-apoB-100 into plasma were not different between CKD-ND or CKD-PD and their respective control groups. The mean residence times of VLDL-TG and VLDL-apoB-100 in plasma, which represents the time VLDL-TG and VLDL-apoB-100 spend in the circulation after secretion by the liver, tended to be greater in subjects with CKD-ND than in control subjects (222 ± 38 vs. 143 ± 21 min, p = 0.07, and 352 ± 102 vs. 200 ± 20 min, p = 0.06, respectively) and were about two-fold greater in subjects with CKD-PD compared with their control group (248 ± 51 vs. 143 ± 21 min and 526 ± 116 vs. 182 ± 16 min, respectively; both p ≤ 0.01). CONCLUSION: Impaired plasma clearance of VLDL-TG and VLDL-apoB-100 is the major abnormality associated with hypertriglyceridemia in patients with either CKD-ND or CKD-PD.

Regulation of the expression of key genes involved in HDL metabolism by unsaturated fatty acids

The cardioprotective effects of HDL have been largely attributed to their role in the reverse cholesterol transport pathway, whose efficiency is affected by many proteins involved in the formation and remodelling of HDL. The aim of the present study was to determine the effects, and possible mechanisms of action, of unsaturated fatty acids on the expression of genes involved in HDL metabolism in HepG2 cells. The mRNA concentration of target genes was assessed by real-time PCR. Protein concentrations were determined by Western blot or immunoassays. PPAR and liver X receptor (LXR) activities were assessed in transfection experiments. Compared with the SFA palmitic acid (PA), the PUFA arachidonic acid (AA), EPA and DHA significantly decreased apoA-I, ATP-binding cassette A1 (ABCA1), lecithin-cholesterol acyltransferase (LCAT) and phospholipid transfer protein mRNA levels. EPA and DHA significantly lowered the protein concentration of apoA-I and LCAT in the media, as well as the cellular ABCA1 protein content. In addition, DHA repressed the apoA-I promoter activity. AA lowered only the protein concentration of LCAT in the media. The activity of PPAR was increased by DHA, while the activity of LXR was lowered by both DHA and AA, relative to PA. The regulation of these transcription factors by PUFA may explain some of the PUFA effects on gene expression. The observed n-3 PUFA-mediated changes in gene expression are predicted to reduce the rate of HDL particle formation and maturation.

Notch-ligand expression by NALT dendritic cells regulates mucosal Th1- and Th2-type responses

Our previous studies showed that an adenovirus (Ad) serotype 5 vector expressing Flt3 ligand (Ad-FL) as nasal adjuvant activates CD11c(+) dendritic cells (DCs) for the enhancement of antigen (Ag)-specific IgA antibody (Ab) responses. In this study, we examined the molecular mechanism for activation of CD11c(+) DCs and their roles in induction of Ag-specific Th1- and Th2-cell responses. Ad-FL activated CD11c(+) DCs expressed increased levels of the Notch ligand (L)-expression and specific mRNA. When CD11c(+) DCs from various mucosal and systemic lymphoid tissues of mice given nasal OVA plus Ad-FL were cultured with CD4(+) T cells isolated from non-immunized OVA TCR-transgenic (OT II) mice, significantly increased levels of T cell proliferative responses were noted. Furthermore, Ad-FL activated DCs induced IFN-γ, IL-2 and IL-4 producing CD4(+) T cells. Of importance, these APC functions by Ad-FL activated DCs were down-regulated by blocking Notch-Notch-L pathway. These results show that Ad-FL induces CD11c(+) DCs to the express Notch-ligands and these activated DCs regulate the induction of Ag-specific Th1- and Th2-type cytokine responses.

Portal vein and systemic adiponectin concentrations are closely linked with hepatic glucose and lipoprotein kinetics in extremely obese subjects

Low systemic plasma adiponectin concentrations are associated with abnormalities in hepatic glucose and lipoprotein metabolism in obese people. However, the relationship between the delivery of adiponectin to the liver via the portal vein and hepatic glucose and lipoprotein metabolism is not known. We examined the relationship between hepatic substrate metabolism (glucose rate of appearance into plasma and hepatic very low-density lipoprotein [VLDL]-triglyceride [TG] and VLDL-apolipoprotein B-100 [apoB-100] secretion rates, determined by using stable isotope-labeled tracer techniques) and portal vein adiponectin concentration, in 8 insulin-resistant, extremely obese subjects (body mass index, 65 ± 7 kg/m(2)). Portal vein adiponectin concentration was inversely associated with basal glucose rate of appearance (r = -0.820, P = .013) and VLDL-TG (r = -0.823, P = .012) and VLDL-apoB-100 (r = -0.787, P = .020) secretion rates. Very similar correlations were obtained for radial artery adiponectin as a result of a mirroring relationship between portal and arterial adiponectin concentrations (r = 0.899, P = .002) and the absence of significant arteriovenous concentration differences (P = .570). Insulin resistance, assessed with the homeostasis model assessment score, was also strongly associated with hepatic glucose and lipid metabolic parameters, as well as with adiponectin concentrations in the portal vein and radial artery. These results suggest that adiponectin delivery to the liver, whether via the portal or the systemic circulation, may be an important regulator of basal hepatic glucose, VLDL-TG, and VLDL-apoB-100 production rates in obese people, possibly through direct effects on the liver or changes in hepatic insulin sensitivity. However, portal vein adiponectin does not appear to be superior to arterial adiponectin as a marker of hepatic metabolic dysregulation. Additional studies are needed to elucidate the mechanism(s) responsible for the strong association we observed between adiponectin and hepatic substrate metabolism.

Postprandial lipid responses to an alpha-linolenic acid-rich oil, olive oil and butter in women: a randomized crossover trial

Background

Postprandial lipaemia varies with gender and the composition of dietary fat due to the partitioning of fatty acids between beta-oxidation and incorporation into triacylglycerols (TAGs). Increasing evidence highlights the importance of postprandial measurements to evaluate atherogenic risk. Postprandial effects of alpha-linolenic acid (ALA) in women are poorly characterized. We therefore studied the postprandial lipid response of women to an ALA-rich oil in comparison with olive oil and butter, and characterized the fatty acid composition of total lipids, TAGs, and non-esterified fatty acids (NEFAs) in plasma.

Methods

A randomized crossover design (n = 19) was used to compare the postprandial effects of 3 meals containing 35 g fat. Blood samples were collected at regular intervals for 7 h. Statistical analysis was carried out with ANOVA (significant difference = P < 0.05).

Results

No significant difference was seen in incremental area under the curve (iAUC) plasma-TAG between the meals. ALA and oleic acid levels were significantly increased in plasma after ALA-rich oil and olive oil meals, respectively. Palmitic acid was significantly increased in plasma-TAG after the butter meal. The ratios of 18:2 n-6 to18:3 n-3 in plasma-TAGs, three and seven hours after the ALA-rich oil meal, were 1.5 and 2.4, respectively. The corresponding values after the olive oil meal were: 13.8 and 16.9; and after the butter meal: 9.0 and 11.6.

Conclusions

The postprandial p-TAG and NEFA response in healthy pre-menopausal women was not significantly different after the intake of an ALA-rich oil, olive oil and butter. The ALA-rich oil significantly affected different plasma lipid fractions and improved the ratio of n-6 to n-3 fatty acids several hours postprandially.

Sex differences in lipid and lipoprotein metabolism: it's not just about sex hormones

It is commonly thought that sex hormones are important regulators of plasma lipid kinetics and are responsible for sexual dimorphism in the plasma lipid profile. Here we discuss the findings from studies evaluating lipid and lipoprotein kinetics in men and women in the context of what we know about the effects of exogenous sex hormone administration, and we conclude that it is more complicated than that. It has become clear that normal physiological alterations in the hormonal milieu (i.e. due to menopause or throughout the menstrual cycle) do not significantly affect plasma lipid homeostasis. Furthermore, parenterally administered estrogens have either no effect or only very small beneficial effects, whereas orally administered estrogens raise plasma triglyceride concentrations--a phenomenon that is not consistent with the observed sex differences and likely results from the hepatic "first-pass effect." The effects of progestogens and androgens mimic only in part the differences in plasma lipids between men and women. Thus, the underlying physiological modulators of plasma lipid metabolism responsible for the differences between men and women remain to be elucidated.

Young women partition fatty acids towards ketone body production rather than VLDL-TAG synthesis, compared with young men

Before the menopause, women are relatively protected against CVD compared with men. The reasons for this sex difference are not completely understood, but hepatic fatty acid metabolism may play a role. The present study aimed to investigate the utilisation of plasma NEFA by the liver and to determine whether they are partitioned differently into ketone bodies and VLDL-TAG in healthy, lean young men and women. Volunteers were studied during a prolonged overnight fast (12-19 h) using an intravenous infusion of [U-¹³C]palmitate. After 12 h fasting, the women had a more advantageous metabolic profile with lower plasma glucose (P < 0·05) and TAG (P < 0·05) but higher plasma NEFA (P < 0·05) concentrations. Plasma 3-hydroxybutyrate (3-OHB) concentrations rose more in women than in men, and the transfer of ¹³C from [U-¹³C]palmitate to plasma [¹³C]3-OHB reached a plateau 6-7 h after the start of the infusion in women but was still increasing at 6 h in men. This implies a slower 3-OHB production rate and/or dilution by other precursor pools in men. In women, the high isotopic enrichment of plasma 3-OHB suggested that systemic plasma fatty acids were the major source of 3-OHB production. However, in men, this was not observed during the course of the study (P < 0·01). There were no sex differences for the incorporation of ¹³C into VLDL1- or VLDL2-TAG. The ability of young women to partition fatty acids towards ketone body production rather than VLDL-TAG may contribute to their more advantageous metabolic profile compared with young men.

Effects of dietary fat modification on insulin sensitivity and on other risk factors of the metabolic syndrome--LIPGENE: a European randomized dietary intervention study

BACKGROUND

Excessive energy intake and obesity lead to the metabolic syndrome (MetS). Dietary saturated fatty acids (SFAs) may be particularly detrimental on insulin sensitivity (SI) and on other components of the MetS.

OBJECTIVE

This study determined the relative efficacy of reducing dietary SFA, by isoenergetic alteration of the quality and quantity of dietary fat, on risk factors associated with MetS.

DESIGN

A free-living, single-blinded dietary intervention study.

SUBJECTS AND METHODS

MetS subjects (n = 417) from eight European countries completed the randomized dietary intervention study with four isoenergetic diets distinct in fat quantity and quality: high-SFA; high-monounsaturated fatty acids and two low-fat, high-complex carbohydrate (LFHCC) diets, supplemented with long chain n-3 polyunsaturated fatty acids (LC n-3 PUFAs) (1.2 g per day) or placebo for 12 weeks. SI estimated from an intravenous glucose tolerance test (IVGTT) was the primary outcome measure. Lipid and inflammatory markers associated with MetS were also determined.

RESULTS

In weight-stable subjects, reducing dietary SFA intake had no effect on SI, total and low-density lipoprotein cholesterol concentration, inflammation or blood pressure in the entire cohort. The LFHCC n-3 PUFA diet reduced plasma triacylglycerol (TAG) and non-esterified fatty acid concentrations (P < 0.01), particularly in men.

CONCLUSION

There was no effect of reducing SFA on SI in weight-stable obese MetS subjects. LC n-3 PUFA supplementation, in association with a low-fat diet, improved TAG-related MetS risk profiles.

Increased whole-body adiposity without a concomitant increase in liver fat is not associated with augmented metabolic dysfunction

Aim of this study was to determine whether an increase in adiposity, without a concomitant increase in intrahepatic triglyceride (IHTG) content, is associated with a deterioration in metabolic function. To this end, multiorgan insulin sensitivity, assessed by using a two-stage hyperinsulinemic-euglycemic clamp procedure in conjunction with stable isotopically labeled tracer infusion, and very low-density lipoprotein (VLDL) kinetics, assessed by using stable isotopically labeled tracer infusion and mathematical modeling, were determined in 10 subjects with class I obesity (BMI: 31.6 +/- 0.3 kg/m(2); 37 +/- 2% body fat; visceral adipose tissue (VAT): 1,225 +/- 144 cm(3)) and 10 subjects with class III obesity (BMI: 41.5 +/- 0.5 kg/m(2); 43 +/- 2% body fat; VAT: 2,121 +/- 378 cm(3)), matched on age, sex, and IHTG content (14 +/- 4 and 14 +/- 3%, respectively). No differences between class I and class III obese groups were detected in insulin-mediated suppression of palmitate (67 +/- 3 and 65 +/- 3%, respectively; P = 0.635) and glucose (67 +/- 3 and 73 +/- 5%, respectively; P = 0.348) rates of appearance in plasma, and the insulin-mediated increase in glucose disposal (218 +/- 18 and 193 +/- 30%, respectively; P = 0.489). In addition, no differences between class I and class III obese groups were detected in secretion rates of VLDL-triglyceride (6.5 +/- 1.0 and 6.0 +/- 1.4 micromol/l x min, respectively; P = 0.787) and VLDL-apolipoprotein B-100 (0.40 +/- 0.05 and 0.41 +/- 0.04 nmol/l x min, respectively; P = 0.866), and plasma clearance rates of VLDL-triglyceride (31 (16-59) and 29 (18-46) ml/min, respectively; P = 0.888) and VLDL-apolipoprotein B-100 (15 (11-19) and 17 (11-25) ml/min, respectively; P = 0.608). We conclude that increased adiposity without a concomitant increase in IHTG content does not cause additional abnormalities in adipose tissue, skeletal muscle, and hepatic insulin sensitivity, or VLDL metabolism.

Acute resistance exercise attenuates fasting and postprandial triglyceridemia in women by reducing triglyceride concentrations in triglyceride-rich lipoproteins

A single bout of endurance exercise lowers fasting and postprandial triglyceride (TG) concentrations in both men and women, by reducing TG in triglyceride-rich lipoproteins (TRLs). The effect of resistance exercise on TRL-TG metabolism is not known; previous studies only measured total plasma TG concentrations and provide conflicting results. Furthermore, none has specifically examined women. We therefore sought to evaluate the effect of a single bout of resistance exercise on TRL-TG metabolism in women. We measured the concentrations of TG in total plasma and TRLs in the fasting state and during an oral fat tolerance test in five healthy untrained women (age: 32 ± 5 years; body mass index: 21.5 ± 1.7 kg/m(2); peak oxygen consumption: 31 ± 4 mL/kg min) in the morning, on two separate occasions: once after a single ~95-min bout of moderate-intensity whole-body resistance exercise (energy expenditure: 2.9 ± 0.1 MJ) and once after an equivalent period of rest, on the preceding afternoon. Fasting plasma TG and TRL-TG concentrations were 22 ± 12 and 40 ± 21% lower, respectively, and postprandial plasma TG and TRL-TG areas-under-the-curve were 24 ± 13 and 27 ± 10% lower, respectively, after exercise than rest (all P values <0.05). Effect sizes ranged from -0.52 to -0.90. Non-TRL-TG concentrations in the fasting and postprandial states were not different between trials (P > 0.60). We conclude that a single bout of resistance exercise attenuates fasting and postprandial triglyceridemia in women by reducing TRL-TG concentrations.

Estrogen deficiency after menopause does not result in male very-low-density lipoprotein metabolism phenotype

CONTEXT

Sex differences in lipid metabolism result in a less proatherogenic plasma lipid profile in premenopausal women than men. The mechanisms responsible for this are unclear but are thought to be related to differences in the sex hormone milieu in men and women.

OBJECTIVE

Our objective was to evaluate the effect of endogenous sex hormones on very-low-density lipoprotein (VLDL) triglyceride (TG) and apolipoprotein B-100 (apoB-100) metabolism. EXPERIMENTAL DESIGN AND MAIN OUTCOME MEASURES: We measured basal VLDL-TG and VLDL-apoB-100 concentrations and kinetics by using stable isotope-labeled tracers.

SETTING AND PARTICIPANTS

Eight premenopausal women [age, 43 + or - 8 yr; body mass index (BMI), 35 + or - 4 kg/m(2); mean + or - sd], eight postmenopausal women (age, 55 + or - 4 yr; BMI, 34 + or - 4 kg/m(2)), and eight men (age, 41 + or - 13 yr; BMI, 34 + or - 4 kg/m(2)) were studied at Washington University School of Medicine, St. Louis, MO.

RESULTS

VLDL-TG secretion rate was approximately double (P < 0.05) in postmenopausal women and men compared with premenopausal women but not different in postmenopausal women and men. The secretion rate of VLDL-apoB-100 was not different in pre- and postmenopausal women but was greater (P < 0.05) in men than in women.

CONCLUSIONS

Endogenous ovarian sex steroids are responsible for sexual dimorphism in VLDL-TG secretion, whereas VLDL-apoB-100 secretion is not regulated by female reproductive hormones.

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.

What we know and do not know about sex and cardiac disease

Cardiovascular disease (CVD) remains the single leading cause of death in both men and women. A large proportion of the population with CVD will die with a diagnosis of congestive heart failure (CHF). It is becoming increasingly recognized that sex differences exist in the etiology, development, and outcome of CHF. For example, compared to male counterparts, women that present with CHF are typically older and have systolic cardiac function that is not impaired. Despite a growing body of literature addressing the underlying mechanisms of sex dimorphisms in cardiac disease, there remain significant inconsistencies reported in these studies. Given that the development of CHF results from the complex integration of genetic and nongenetic cues, it is not surprising that the elucidation and subsequent identification of molecular mechanisms remains unclear. In this review, key aspects of sex differences in CVD and CHF will be highlighted with an emphasis on some of the unanswered questions regarding these differences. The contention is presented that it becomes critical to reference cellular mechanisms within the context of each sex to better understand these sex dimorphisms.

Metabolic actions of insulin in men and women

Insulin is an important regulator of glucose, lipid, and protein metabolism. It suppresses hepatic glucose and triglyceride production, inhibits adipose tissue lipolysis and whole-body and muscle proteolysis, and stimulates glucose uptake in muscle. In this review we discuss what is currently known about the control of substrate metabolism by insulin in men and women. The data available so far indicate that women are more sensitive to insulin with regards to glucose metabolism (both in the liver and in muscle), whereas there are no differences between men and women in insulin action on lipolysis. Potential differences exist in the regulation of plasma triglyceride concentration and protein metabolism by insulin and in changes in insulin action in response to stimuli (e.g., weight loss and exercise) that are known to alter insulin sensitivity. However, these areas have not been studied comprehensively enough to draw firm conclusions.

Nonalcoholic fatty liver disease as the transducer of hepatic oversecretion of very-low-density lipoprotein-apolipoprotein B-100 in obesity

OBJECTIVE

To examine the association between liver fat content and very low-density lipoprotein (VLDL)-apolipoprotein (apo) B-100 kinetics and the corresponding responses to weight loss in obese subjects.

METHODS AND RESULTS

VLDL-apoB-100 kinetics were assessed using stable isotope tracers, and the fat content of the liver and abdomen was determined by magnetic resonance techniques in 25 obese subjects. In univariate analysis, liver fat content was significantly (P<0.05 in all) associated with body mass index (r=0.65), visceral fat area (r=0.45), triglycerides (r=0.40), homeostasis model assessment score (r=0.40), VLDL-apoB-100 concentrations (r=0.44), and secretion rate (r=0.45). However, liver fat content was not associated with plasma concentrations of retinol-binding protein 4, fetuin A, adiponectin, interleukin-6, and tumor necrosis factor-alpha. Of these 25 subjects, 9 diagnosed as having nonalcoholic fatty liver disease (which is highly prevalent in obese individuals and strongly associated with dyslipidemia) underwent a weight loss program. The low-fat diet achieved significant reduction in body weight, body mass index, liver fat, visceral and subcutaneous fat areas, homeostasis model assessment score, triglycerides, VLDL-apoB-100 concentrations, and VLDL-apoB-100 secretion rate. The percentage reduction of liver fat with weight loss was significantly associated with the corresponding decreases in VLDL-apoB-100 secretion (r=0.67) and visceral fat (r=0.84).

CONCLUSION

In patients with obesity, hepatic steatosis increases VLDL-apoB-100 secretion. Weight loss can help reduce this abnormality.

ACAT2 and human hepatic cholesterol metabolism: identification of important gender-related differences in normolipidemic, non-obese Chinese patients

OBJECTIVE

ACAT2 is a major cholesterol esterification enzyme specifically expressed in hepatocytes and may control the amount of hepatic free (unesterified) cholesterol available for secretion into bile or into HDL. This study aims to further elucidate physiologic roles of ACAT2 in human hepatic cholesterol metabolism.

METHODS AND RESULTS

Liver biopsies from 40 normolipidemic, non-obese gallstone patients including some gallstone-free patients (female/male, 18/22) were collected and analyzed for microsomal ACAT2 activity, protein and mRNA expression. Plasma HDL-cholesterol (HDL-C) was significantly higher in females than in males, while triglycerides were significantly lower. ACAT2 activity in females was significantly lower than observed in males, regardless of the presence of gallstone disease. Moreover, the activity of ACAT2 correlated negatively with plasma levels of HDL-C (r=-0.57, P<0.05) and with Apo AI (r=-0.49, P<0.05).

CONCLUSION

This is the first description of a gender-related difference in hepatic ACAT2 activity in normolipidemic non-obese Chinese patients suggesting a possible role for ACAT2 in the regulation of cholesterol metabolism in humans. The negative correlation between ACAT2 activity and HDL-C or Apo AI may reflect this regulation. Since ACAT2 activity generally has been found to be pro-atherogenic in animal models, the observed sex-related difference may contribute to female protection from complications of coronary heart disease (CHD).

Free fatty acid kinetics in the late phase of postexercise recovery: importance of resting fatty acid metabolism and exercise-induced energy deficit

Free fatty acid (FFA) availability increases several-fold during exercise and remains significantly elevated for at least 3 to 6 hours after exercise cessation. Little, however, is known regarding the duration of the postexercise rise in FFA flux. In the present study, we used stable isotope-labeled palmitate infusion to examine fatty acid metabolism in 27 healthy untrained men and women (age, 29 +/- 7 years; body mass index, 25 +/- 4 kg/m2) between 13 to 16 hours and 21 to 24 hours after a single bout of moderate-intensity endurance exercise (1-2 hours at 60% of peak oxygen consumption), performed in the evening, and after a time-matched resting trial. Postabsorptive FFA rate of appearance (Ra) and FFA concentration in plasma were significantly greater after exercise than rest throughout the recovery period (P < .015), but the exercise-induced increases declined from approximately 40% at 13 to 16 hours to approximately 10% at 21 to 24 hours postexercise (P = .001). The magnitude of the exercise-induced increase in plasma FFA concentration was proportional to the increase in FFA Ra. Correlation analysis demonstrated that exercise-induced changes in plasma FFA Ra at 13 to 16 hours are (1) negatively associated with resting plasma FFA Ra and (2) positively associated with the net energy expenditure of exercise and the exercise-induced changes in whole-body fat oxidation rate (all P values < .05). In multivariate stepwise linear regression analysis, baseline plasma FFA Ra (P < or = .008) and net energy expenditure of exercise (P < or = .005) independently predicted the exercise-induced change in plasma FFA Ra at 13 to 16 hours. We conclude that the exercise-induced increase in FFA mobilization is (1) long-lived, persisting for 12 to 24 hours after exercise, with a progressive decline with time; (2) greater in subjects with low than high resting plasma FFA availability; and (3) greater after exercise with high than low energy demand.

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.

Stable isotope-labeled tracers for the investigation of fatty acid and triglyceride metabolism in humans in vivo

Understanding lipid metabolism and its regulation requires information on the rates at which lipids are produced within the body, absorbed (dietary lipids) into the body, transported within the body, and utilized by various tissues. This article focuses on the use of stable isotope-labeled tracers for the quantitative evaluation of major pathways of fatty acid and triglyceride metabolism in humans in vivo. Adipose tissue lipolysis and free fatty acid appearance in plasma, fatty acid tissue uptake and oxidation, and hepatic very low-density lipoprotein triglyceride secretion are among the metabolic pathways that can be studied by using stable isotope labeled tracers, and will be discussed in detail. The methodology has been in use for many years and is constantly being refined. A variety of tracers and analytical approaches are available and can be used; knowing the advantages, assumptions, and limitations of each is essential for the planning of studies and the interpretation of data, which can provide unique insights into human lipid metabolism.

A single bout of whole-body resistance exercise augments basal VLDL-triacylglycerol removal from plasma in healthy untrained men

A single bout of prolonged aerobic exercise lowers plasma TAG (triacylglycerol) concentrations the next day by increasing the efficiency of VLDL (very-low-density lipoprotein)-TAG removal from the circulation. The effect of resistance exercise on VLDL-TAG metabolism is not known. Therefore we evaluated VLDL-TAG kinetics by using stable isotope-labelled tracers in eight healthy untrained men (age, 25.3+/-0.8 years; body mass index, 24.5+/-0.6 kg/m(2)) in the post-absorptive state in the morning on two separate occasions: once after performing a single 90-min bout of strenuous isokinetic resistance exercise (three sets x ten repetitions, 12 exercises at 80% of maximum peak torque production, with a 2-min rest interval between exercises) on the preceding afternoon and once after an equivalent period of rest. Fasting plasma VLDL-TAG concentrations in the morning after exercise were significantly lower than in the morning after rest (0.23+/-0.04 compared with 0.33+/-0.06 mmol/l respectively; P=0.001). Hepatic VLDL-TAG secretion rate was not different (P=0.31), but plasma clearance rate of VLDL-TAG was significantly higher (by 26+/-8%) after exercise than rest (31+/-3 compared with 25+/-3 ml/min respectively; P=0.004), and the mean residence time of VLDL-TAG in the circulation was significantly shorter (113+/-10 compared with 144+/-18 min respectively; P=0.02). Fasting plasma NEFA (non-esterified fatty acid; 'free' fatty acid) and serum beta-hydroxybutyrate concentrations were both significantly higher after exercise than rest (P<0.05), whereas plasma glucose and serum insulin concentrations were not different (P>0.30). We conclude that, in healthy untrained men, a single bout of whole-body resistance exercise lowers fasting plasma VLDL-TAG concentrations by augmenting VLDL-TAG removal from plasma. The effect appears to be qualitatively and quantitatively similar to that reported previously for aerobic exercise.

Plasma lipid transfer enzymes in non-diabetic lean and obese men and women

There are considerable differences in the plasma lipid profile between lean and obese individuals and between men and women. Little, however, is known regarding the effects of obesity and sex on the plasma concentration of enzymes involved in intravascular lipid remodeling. Therefore, we measured the immunoreactive protein mass of lipoprotein lipase (LPL), hepatic lipase (HL), cholesterol-ester transfer protein (CETP) and lecithin-cholesterol acyl transferase (LCAT) in fasting plasma samples from 40 lean and 40 obese non-diabetic men and premenopausal women. Women, compared with men, had approximately 5% lower plasma LCAT (p < 0.041), approximately 35% greater LPL (p = 0.001) and approximately 10% greater CETP (p = 0.085) concentrations. Obese, compared with lean individuals of both sexes, had approximately 30% greater plasma LCAT (p < 0.001), approximately 20% greater CETP (p < 0.001) and approximately 20% greater LPL (p = 0.071) concentrations. Plasma HL concentration was not different in lean men and women. Obesity was associated with increased (by approximately 50%) plasma HL concentration in men (p = 0.018) but not in women; consequently, plasma HL concentration was lower in obese women than obese men (p = 0.009). In addition, there were direct correlations between plasma lipid transfer enzyme concentrations and lipoprotein particle concentrations and sizes. There are considerable differences in basal plasma lipid transfer enzyme concentrations between lean and obese subjects and between men and women, which may be partly responsible for respective differences in the plasma lipid profile.

Basal adipose tissue and hepatic lipid kinetics are not affected by a single exercise bout of moderate duration and intensity in sedentary women

Hypertriacylglycerolaemia is an important risk factor for cardiovascular disease. In men, we have shown that the effects of evening exercise on basal VLDL (very-low-density lipoprotein) metabolism are dose-dependent: a single prolonged bout of aerobic exercise [2 h at 60% of VO(2 peak) (peak oxygen consumption)] reduces fasting plasma TAG [triacylglycerol (triglyceride)] concentrations, via enhanced clearance of VLDL-TAG from the circulation, whereas the same exercise performed for 1 h has no effect on VLDL-TAG metabolism and concentration. We hypothesized that women are more sensitive to the TAG-lowering effect of exercise because they reportedly use more intramuscular TAG as an energy source during exercise, and depletion of muscle TAG stores has been linked to reciprocal changes in skeletal muscle LPL (lipoprotein lipase) activity. To test our hypothesis, we measured basal VLDL-TAG and VLDL-apoB-100 (apolipoprotein B-100), and plasma NEFA [non-esterified fatty acid ('free fatty acid')] kinetics, by using stable isotope-labelled tracer techniques, on the morning after a single session of evening exercise of moderate duration and intensity (1 h at 60% of VO(2 peak)) in eight sedentary pre-menopausal women (age, 28+/-3 years; body mass index, 27+/-2 kg/m(2); body fat, 34+/-3%; values are means+/-S.E.M.). Compared with an equivalent period of evening rest, exercise had no effect on post-absorptive NEFA concentrations and the rate of appearance in plasma, VLDL-TAG and VLDL-apoB-100 concentrations, hepatic VLDL-TAG and VLDL-apoB-100 secretion and plasma clearance rates (all P>0.05). We conclude that, in women, as in men, a single session of exercise of moderate intensity and duration is not sufficient to bring about the alterations in VLDL metabolism that have been linked to post-exercise hypotriacylglycerolaemia.

Effect of obesity on the plasma lipoprotein subclass profile in normoglycemic and normolipidemic men and women

OBJECTIVE

To determine the effect of obesity without the confounding effect of metabolic complications on the lipoprotein subclass profile in men and women.

DESIGN

Cross-sectional study.

SUBJECTS

A total of 40 lean (body mass index (BMI): 18.5-25 kg/m(2)) and 40 obese (BMI: 30-45 kg/m(2)) subjects, with blood pressure <140/90 mm Hg, fasting plasma glucose concentration <100 mg per 100 ml and total triglyceride concentration <150 mg per 100 ml; all obese subjects had normal oral glucose tolerance.

MEASUREMENTS

Fasting concentrations of very low-, intermediate-, low- and high-density lipoproteins (VLDL, IDL, LDL, and HDL, respectively) and average VLDL, LDL and HDL particle sizes were evaluated by using proton nuclear magnetic resonance spectroscopy.

RESULTS

Obese compared with lean individuals of both sexes had increased plasma concentrations of VLDL (by approximately 50%), IDL (by approximately 100%), LDL (by approximately 50%), and to some extent HDL (by approximately 10%) particles (P<0.05). The contribution of large VLDL to total VLDL concentration, small LDL to total LDL concentration, and small HDL to total HDL concentration was greater in obese than lean subjects (P<0.05), resulting in larger average VLDL size but smaller average LDL and HDL sizes (P<0.05). Women, compared with men, had reduced concentrations of total VLDL particles (by approximately 10%) due to lower concentrations of large and medium VLDL and a shift toward large at the expense of small HDL particles (P<0.05), with no difference in total HDL particle concentration. IDL and total LDL concentrations and LDL subclass distribution were not different between men and women.

CONCLUSION

Obesity is associated with pro-atherogenic alterations in the lipoprotein subclass profile, which may increase cardiovascular disease risk even in the absence of classical metabolic risk factors. On the other hand, the female cardiovascular disease risk advantage is probably largely related to differences in traditional lipid risk factors (plasma triglyceride and HDL-cholesterol concentrations) because sex differences in the plasma lipoprotein subclass profile are minimal.

High-intensity interval aerobic training reduces hepatic very low-density lipoprotein-triglyceride secretion rate in men

A single bout of strenuous endurance exercise reduces fasting plasma triglyceride (TG) concentrations the next day (12-24 h later) by augmenting the efficiency of very low-density lipoprotein (VLDL)-TG removal from the circulation. Although much of the hypotriglyceridemia associated with training is attributed to the last bout of exercise, the relevant changes in VLDL-TG metabolism have never been investigated. We therefore examined basal VLDL-TG kinetics in a group of sedentary young men (n=7) who underwent 2 mo of supervised high-intensity interval training (3 sessions/wk; running at 60 and 90% of peak oxygen consumption in 4-min intervals for a total of 32 min; gross energy expenditure: 446+/-29 kcal) and a nonexercising control group (n=8). Each subject completed two stable isotope-labeled tracer infusion studies in the postabsorptive state, once before and again after the intervention (approximately 48 h after the last exercise bout in the training group). Peak oxygen consumption increased by approximately 18% after training (P <or= 0.05), whereas body weight and body composition were not altered. Fasting plasma VLDL-TG concentration was reduced after training by approximately 28% (P <or= 0.05), and this was due to reduced hepatic VLDL-TG secretion rate (by approximately 35%, P <or= 0.05) with no changes (<5%, P>0.7) in VLDL-TG plasma clearance rate and the mean residence time of VLDL-TG in the circulation. No significant changes in VLDL-TG concentration and kinetics were observed in the nonexercising control group (all P >or= 0.3). We conclude that a short period of high-intensity interval aerobic training lowers the rate of VLDL-TG secretion by the liver in previously sedentary men. This is different from the mechanism underlying the hypotriglyceridemia of acute exercise; however, it remains to be established whether our finding reflects an effect of the longer time lapse from the last exercise bout, an effect specific to the type of exercise performed, or an effect of aerobic training itself.

Gender-specific differences in the kinetics of nonfasting TRL, IDL, and LDL apolipoprotein B-100 in men and premenopausal women

OBJECTIVE

To investigate mechanisms underlying gender differences in serum lipoprotein concentrations, the kinetic behavior of apoB-100 was assessed.

METHODS AND RESULTS

Twenty subjects (<50 years; 12 men and 8 premenopausal women) were provided a Western diet for 4 to 6 weeks, after which the kinetics of apoB-100 in triglyceride-rich, intermediate-density, and low-density lipoprotein (TRL, IDL, and LDL) were determined in the fed state. Nonfasting plasma TC, LDL-C, and triglyceride concentrations were 23%, 34%, and 57% lower, respectively, in the women compared with men. Plasma TRL and LDL apoB 100 pool sizes were lower by 40% and 30%, respectively. These differences were accounted for by higher TRL and LDL apoB 100 fractional catabolic rates (FCR), rather than differences in production rates (PR). Plasma TRL-C and LDL-C were positively correlated with TRL and LDL apoB 100 concentrations and pool size, and negatively correlated with TRL and LDL apoB 100 FCR (women: r=-0.59, P<0.01 and r=-0.54, P<0.04, and men: r=-0.43, P<0.05 and r=-0.44, P<0.05). No significant associations were observed between plasma TRL-C and LDL-C and PR.

CONCLUSIONS

These data suggest the mechanism for lower TRL-C and LDL-C concentrations in women was determined predominantly by higher TRL and LDL FCR rather than lower PR. This could explain, in part, the lower CVD risk in premenopausal women relative to men.

Acute exercise-induced changes in basal VLDL-triglyceride kinetics leading to hypotriglyceridemia manifest more readily after resistance than endurance exercise

Resistance training is considered less effective than endurance training in lowering plasma triglyceride (TG) concentrations. Acutely, however, a single bout of strenuous exercise, whether endurance or resistance, increases the efficiency of very low-density lipoprotein (VLDL)-TG removal from the circulation and leads to hypotriglyceridemia. The comparative effects of these two types of exercise on VLDL-TG metabolism are not known. We therefore examined basal VLDL-TG kinetics by using stable isotope-labeled tracers in seven healthy, nonobese, untrained young men in the postabsorptive state, the morning after a single 90-min bout of either low-intensity endurance exercise (approximately 30% of peak oxygen consumption) or high-intensity resistance exercise (3 sets of 10 repetitions for 12 exercises at 80% of peak torque production), matched for total energy expenditure (approximately 400 kcal), or an equivalent period of rest on the preceding afternoon. Compared with rest, resistance exercise lowered fasting plasma VLDL-TG concentration by -28 +/- 10% (P = 0.034), increased VLDL-TG plasma clearance rate by 30 +/- 8% (P = 0.003), and shortened the mean residence time (MRT) of VLDL-TG in the circulation by -36 +/- 11 min (P = 0.016), whereas endurance exercise had no effect (all P > 0.05). Basal VLDL-TG plasma clearance rate was greater (P = 0.003) and VLDL-TG MRT was shorter (P = 0.012) the morning after resistance than endurance exercise. We conclude that, for the same total energy expenditure, resistance exercise is more potent than endurance exercise in eliciting changes in VLDL-TG metabolism that have been linked with hypotriglyceridemia, and it should thus be considered as an alternative to or in addition to endurance exercise for the control of plasma TG concentrations.

Alterations in adipose tissue and hepatic lipid kinetics in obese men and women with nonalcoholic fatty liver disease

BACKGROUND & AIMS

Steatosis in patients with nonalcoholic fatty liver disease (NAFLD) is due to an imbalance between intrahepatic triglyceride (IHTG) production and export. The purpose of this study was to evaluate TG metabolism in adipose tissue and liver in NAFLD.

METHODS

Fatty acid, VLDL-TG, and VLDL-apolipoprotein B-100 (apoB100) kinetics were assessed by using stable isotope tracers in 14 nondiabetic obese subjects with NAFLD (IHTG, 22.7% +/- 2.0%) and 14 nondiabetic obese subjects with normal IHTG content (IHTG, 3.4% +/- 0.4%), matched on age, sex, body mass index, and percent body fat.

RESULTS

Compared with the normal IHTG group, the NAFLD group had greater rates of palmitate release from adipose tissue into plasma (85.4 +/- 6.6 and 114.1 +/- 8.1 micromol/min, respectively; P = .01) and VLDL-TG secretion (11.4 +/- 1.1 and 24.3 +/- 3.1 micromol/min, respectively; P = .001); VLDL-apoB100 secretion rates were not different between groups. The increase in VLDL-TG secretion was primarily due to an increased contribution from "nonsystemic" fatty acids, presumably derived from lipolysis of intrahepatic and intra-abdominal fat and de novo lipogenesis. VLDL-TG secretion rate increased linearly with increasing IHTG content in subjects with normal IHTG but reached a plateau when IHTG content was >/=10% (r = 0.618, P < .001).

CONCLUSIONS

Obese persons with NAFLD have marked alterations in both adipose tissue (increased lipolytic rates) and hepatic (increased VLDL-TG secretion) TG metabolism. Fatty acids derived from nonsystemic sources are responsible for the increase in VLDL-TG secretion. However, the increase in hepatic TG export is not adequate to normalize IHTG content.

The contribution of splanchnic fat to VLDL triglyceride is greater in insulin-resistant than insulin-sensitive men and women: studies in the postprandial state

OBJECTIVE

We aimed to determine differences in the postprandial contributions of different fatty acid sources to VLDL triglycerides (TGs) in healthy men and women with varying degrees of insulin resistance.

RESEARCH DESIGN AND METHODS

Insulin-resistant (n = 11) and insulin-sensitive (n = 11) men and women (n = 6) were given an intravenous infusion of [(2)H(2)]palmitic acid to investigate systemic nonesterified fatty acid (NEFA) incorporation into VLDL TGs. Participants were also fed a mixed meal containing [U-(13)C]palmitic acid to investigate the contribution of dietary fatty acids to VLDL TG production. Blood samples were taken over the following 6 h. Separation of VLDL was performed by density gradient ultracentrifugation and immunoaffinity techniques specific to apolipoprotein B-100.

RESULTS

Insulin-resistant and insulin-sensitive men had similar postprandial chylomicron and chylomicron remnant TG concentrations, but insulin-resistant men had higher postprandial VLDL TG concentrations (median [range]; area under the curve 485 micromol/l [123-992] vs. 287 micromol/l [162-510]; P < 0.05). At 360 min, most of the difference in VLDL TGs was accounted for by an additional contribution from splanchnic fat (means +/- SE; 331 +/- 76 micromol/l vs. 89 +/- 25 micromol/l; P < 0.01). The contribution of fatty acids from endogenous systemic NEFAs was similar across the groups, as were dietary fatty acids. There was no difference in the VLDL TG concentration or the contribution of different fatty acid sources between insulin-sensitive men and women.

CONCLUSIONS

In the postprandial period, the only sources of fatty acids for VLDL TG production to differ in the insulin-resistant compared with the insulin-sensitive men are those derived from splanchnic sources.

Gender-related differences in the metabolic response to fasting

CONTEXT

Free fatty acids (FFA) may induce insulin resistance via synthesis of intramyocellular ceramide. During fasting, women have lower plasma glucose levels than men despite higher plasma FFA, suggesting protection from FFA-induced insulin resistance.

OBJECTIVE

We studied whether the relative protection from FFA-induced insulin resistance during fasting in women is associated with lower muscle ceramide concentrations compared with men. MAIN OUTCOME MEASURES AND DESIGN: After a 38-h fast, measurements of glucose and lipid fluxes and muscle ceramide and fatty acid translocase/CD36 were performed before and after a hyperinsulinemic euglycemic clamp.

RESULTS

Plasma glucose levels were significantly lower in women than men with a trend for a lower endogenous glucose production in women, whereas FFA and lipolysis were significantly higher. Insulin-mediated peripheral glucose uptake was not different between sexes. There was no gender difference in muscle ceramide in the basal state, and ceramide did not correlate with peripheral glucose uptake. Muscle fatty acid translocase/CD36 was not different between sexes in the basal state and during the clamp.

CONCLUSION

After 38 h of fasting, plasma FFA were higher and plasma glucose was lower in women compared with men. The higher plasma FFA did not result in differences in peripheral insulin sensitivity, possibly because of similar muscle ceramide and fatty acid translocase/CD36 levels in men and women. We suggest that during fasting, women are relatively protected from FFA-induced insulin resistance by preventing myocellular accumulation of ceramide.

Reproducibility of stable isotope-labeled tracer measures of VLDL-triglyceride and VLDL-apolipoprotein B-100 kinetics

To gain insight into the mechanisms regulating plasma lipid homeostasis, FFA, VLDL-triglyceride (TG), and VLDL-apolipoprotein B-100 (apoB-100) kinetics are commonly assessed using stable isotope-labeled tracer methods. The reproducibility of these measurements, which is critical for the experimental design, is unknown. Therefore, we investigated the repeatability of plasma FFA, VLDL-TG, and VLDL-apoB-100 kinetics in eight healthy men using stable isotope-labeled tracer techniques. There were no systematic differences in plasma FFA, VLDL-TG, and VLDL-apoB-100 concentrations and kinetics between the two studies. Intraindividual day-to-day variability for various outcome variables ranged from 15% to 25%, and almost all of this was of biological origin. The most robust outcome variables were FFA rate of appearance and hepatic VLDL-TG and VLDL-apoB-100 secretion rates; the least robust were VLDL-TG and VLDL-apoB-100 plasma clearance rates and mean residence times. Overall, physiologically meaningful differences in mean values (i.e., 25-30% in magnitude) can be obtained with a sample size of 6-10 subjects for paired studies and 12-20 subjects per group for cross-sectional studies, assuming a type I error rate of 0.05 and a type II error rate of 0.20 (i.e., 80% power). These findings will be useful for future studies investigating FFA, VLDL-TG, and VLDL-apoB-100 kinetics with the methods described.