Regulation of dietary fatty acid entrapment in subcutaneous adipose tissue and skeletal muscle

Postprandial lipaemia induces an acute decrease of insulin sensitivity in healthy men independently of plasma NEFA levels

AIMS/HYPOTHESIS

Typical Western diets cause postprandial lipaemia for 18 h per day. We tested the hypothesis that postprandial lipaemia decreases insulin sensitivity.

SUBJECTS, MATERIALS AND METHODS

Employing a randomised crossover design, we administered two types of virtually isocaloric meals to ten healthy volunteers on two separate occasions. The meals (Meals 1 and 2) were both designed to produce a rise in triglycerides, but only Meal 1 generated a rise in NEFA, too. Insulin sensitivity, as quantified by an IVGTT with minimal model analysis, was calculated postabsorptively at 08.00 h and postprandially at 13.00 h, i.e. 3 h after meal ingestion.

RESULTS

Triglycerides rose from 0.91+/-0.31 mmol/l postabsorptively to 2.08+/-0.70 mmol/l postprandially with Meal 1 (p=0.005) and from 0.92+/-0.41 to 1.71+/-0.79 mmol/l with Meal 2 (p=0.005). Neither the triglyceride levels at 13.00 h, nor the post-meal AUCs for triglycerides were statistically different between Meal 1 and Meal 2. NEFA rose from 0.44+/-0.17 mmol/l postabsorptively to 0.69+/-0.16 mmol/l postprandially with Meal 1 (p=0.005) and showed no significant change with Meal 2 (0.46+/-0.31 mmol/l postabsorptively vs 0.36+/-0.32 mmol/l postprandially, p=0.09). Both the NEFA level at 13.00 h and the post-meal AUC for NEFA were significantly higher after Meal 1 than Meal 2. Compared with the postabsorptive state, insulin sensitivity decreased postprandially after each of the two meals to a comparable degree (Meal 1: -53%, p=0.02; Meal 2: -45%, p=0.005).

CONCLUSIONS/INTERPRETATION

Our study reveals a drop in insulin sensitivity during postprandial lipaemia and strongly suggests that decreased insulin sensitivity is brought about by elevated plasma levels of triglyceride-rich lipoproteins independently of plasma NEFA levels.

The elevation of ingested lipids within plasma chylomicrons is prolonged in men compared with women

The lipemic response to a high-fat meal is greater in men than in women. However, sex-related differences in the metabolic fate of ingested fat are not well understood. The purpose of this study was to measure the recovery of ingested fat in plasma fractions of chylomicrons (CHYLO), VLDL, and plasma fatty acids, as well as in expired breath (i.e., oxidation) in men and women. Nonobese subjects (n = 10; 5 men, 5 women) consumed 0.7 g fat/kg body weight containing 7 mg/kg of [1,1,1-13C]-trioleate the morning after an overnight fast. Plasma total triglyceride (TG) concentration and 13C recovery in the CHYLO, VLDL, and plasma fatty acid fractions, as well as expired breath samples, were measured over the 11-h period after the meal. Plasma total TG excursion was greater (P < 0.05) in men than in women during the 11-h period after the meal. Similarly, the recovery of the ingested tracer-labeled fat in the CHYLO fraction was greater in men than in women (main effect for sex; P < 0.05). Recovery of ingested tracer-labeled fat in VLDL, the plasma fatty acid fraction, and expired breath did not differ in men and women. Therefore, the elevated postprandial lipemia found in men compared with women was due to a prolonged availability of the lipid in chylomicrons, but was not related to differences in oxidation rates or incorporation of the ingested lipid into VLDL by the liver.

The adipocyte life cycle hypothesis

The adipocyte life cycle hypothesis states that the metabolic properties of an adipocyte vary predictably during its life cycle: that as an adipocyte matures, it accumulates triacylglycerol (triglyceride) and becomes larger; that the rates of triacylglycerol synthesis and lipolysis are matched within adipocytes and that larger adipocytes, in general, have greater rates of triacylglycerol synthesis and, concurrently, greater rates of lipolysis and, therefore, larger adipocytes have greater rates of transmembrane fatty acid flux; and that the secretion of cytokines can also be related to adipocyte size with larger adipocytes having a more unfavourable profile of cytokine secretion than smaller adipocytes. Adipocyte location is an important modifier of this relationship and the favoured sites of adipocyte proliferation are a function of gender and the position within the life cycle of the organism at which proliferation occurs. The adipocyte life cycle hypothesis posits that the metabolic consequences of obesity depend on whether expansion of adipose tissue is achieved primarily by an increase in adipocyte number or adipocyte size. This hypothesis may explain a variety of previously unanswered clinical puzzles such as the vulnerability of many peoples from South East Asia to the adverse metabolic consequences of obesity.

The in vivo effects of the Pro12Ala PPARgamma2 polymorphism on adipose tissue NEFA metabolism: the first use of the Oxford Biobank

AIMS/HYPOTHESIS

To investigate the phenotypic effects of common polymorphisms on adipose tissue metabolism and cardiovascular risk factors, we set out to establish a biobank with the unique feature of allowing a prospective recruit-by-genotype approach. The first use of this biobank investigates the effects of the peroxisome proliferator-activated receptor (PPAR) Pro12Ala polymorphism on integrative tissue-specific physiology. We hypothesised that Ala12 allele carriers demonstrate greater adipose tissue metabolic flexibility and insulin sensitivity.

MATERIALS AND METHODS

From a comprehensive population register, subjects were recruited into a biobank, which was genotyped for the Pro12Ala polymorphism. Twelve healthy male Ala12 carriers and 12 matched Pro12 homozygotes underwent detailed physiological phenotyping using stable isotope techniques, and measurements of blood flow and arteriovenous differences in adipose tissue and muscle in response to a mixed meal containing [1,1,1-(13)C]tripalmitin.

RESULTS

Of 6,148 invited subjects, 1,072 were suitable for inclusion in the biobank. Among Pro12 homozygotes, insulin sensitivity correlated with HDL-cholesterol concentrations, and inversely correlated with blood pressure, apolipoprotein B, triglyceride and total cholesterol concentrations. Ala12 carriers showed no such correlations. In the meal study, Ala12 carriers had lower plasma NEFA concentrations, higher adipose tissue and muscle blood flow, and greater insulin-mediated postprandial hormone-sensitive lipase suppression along with greater insulin sensitivity than Pro12 homozygotes.

CONCLUSIONS/INTERPRETATION

This study shows that a recruit-by-genotype approach is feasible and describes the biobank's first application, providing tissue-specific physiological findings consistent with the epidemiological observation that the PPAR Ala12 allele protects against the development of type 2 diabetes.

Adipocyte signaling and lipid homeostasis: sequelae of insulin-resistant adipose tissue

For many years adipose tissue was viewed as the site where excess energy was stored, in the form of triglycerides (TGs), and where that energy, when needed elsewhere in the body, was released in the form of fatty acids (FAs). Recently, it has become clear that when the regulation of the storage and release of energy by adipose tissue is impaired, plasma FA levels become elevated and excessive metabolism of FA, including storage of TGs, occurs in nonadipose tissues. Most recently, work by several laboratories has made it clear that in addition to FA, adipose tissue communicates with the rest of the body by synthesizing and releasing a host of secreted molecules, collectively designated as adipokines. Several recent reviews have described how these molecules, along with FA, significantly effect total body glucose metabolism and insulin sensitivity. Relatively little attention has been paid to the effects of adipokines on lipid metabolism. In this review, we will describe, in detail, the effects of molecules secreted by adipose tissue, including FA, leptin, adiponectin, resistin, TNF-alpha, IL-6, and apolipoproteins, on lipid homeostasis in several nonadipose tissues, including liver, skeletal muscle, and pancreatic beta cells.

Rosiglitazone increases indexes of stearoyl-CoA desaturase activity in humans: link to insulin sensitization and the role of dominant-negative mutation in peroxisome proliferator-activated receptor-gamma

Fatty acid desaturases such as steaoryl-CoA desaturase (SCD) convert saturated to unsaturated fatty acids and are involved in lipogenesis. Observational and animal data suggest that SCD-1 activity is related to insulin sensitivity. However, the effects of insulin-sensitizing drugs on SCD gene expression and desaturase activities are unknown in humans. In a randomized, placebo-controlled, double-blind, crossover study, 24 subjects with type 2 diabetes and one subject with partial lipodystrophy and diabetes due to dominant-negative mutation in the peroxisome proliferator-activated receptor-gamma (PPARgamma) gene (P467L) received placebo and rosiglitazone for 3 months. SCD gene expression in adipose tissue was determined in 23 subjects, and in a representative subgroup (n = 10) we assessed fatty acid composition in fasting plasma triglycerides to estimate SCD and delta6- and delta5-desaturase activity, using product-to-precursor indexes. SCD mRNA expression increased by 48% after rosiglitazone (P < 0.01). SCD and delta5-desaturase but not delta6-desaturase activity indexes were increased after rosiglitazone versus placebo (P < 0.01 and P < 0.05, respectively). The change in activity index but not the expression of SCD was associated with improved insulin sensitivity (r = 0.73, P < 0.05). In the P467L PPARgamma carrier, SCD and delta5-desaturase activity indexes were exceptionally low but were restored (52- and 15-fold increases, respectively) after rosiglitazone treatment. This study shows for the first time that rosiglitazone increases SCD activity indexes and gene expression in humans. An increased SCD activity index may reflect increased lipogenesis and might contribute to insulin sensitization by rosiglitazone. The restored SCD activity index after rosiglitazone in PPARgamma mutation supports a pivotal role of PPARgamma function in SCD regulation.

The Third International Symposium on Obesity and Hypertension ISOH'03: 'Genetics and Molecular Mechanisms' (October 23-25, 2003, Berlin Germany)

The Third International Symposium on Obesity and Hypertension (ISOH'03) was held on 23-25, October 2003 at the Max Delbruck Center for Molecular Medicine in Berlin-Buch, Germany. The meeting, which consisted largely of invited lectures, presented a state-of-the-art overview of the genetic and molecular mechanisms that link obesity and hypertension. The over 40 oral presentations were supplemented by around 90 poster presentations from 34 countries. The meeting was attended by around 250 participants from 54 countries. This paper briefly reviews the contents of the invited lectures presented at this meeting covering topics ranging from genetics, molecular mechanisms, pathophysiology, cardiovascular risk, to the management of patients with obesity-related hypertension. Stimulated by the continuing success of these Symposia, the organizers are currently planning to hold a Fourth International Symposium on Obesity and Hypertension (ISOH'05) in Berlin: a tentative date for this meeting has been set for 27-29, October 2005.

Plasma cytokine response during the postprandial period: a potential causal process in vascular disease?

Chronic inflammation of the vascular endothelium produces endothelial dysfunction and ultimately atherogenesis. Postprandial hyperlipidaemia is an independent risk factor for cardiovascular disease. Recent studies show that the magnitude of postprandial lipaemia following a single fatty meal is negatively related to vascular function. This is associated with a transient increase in the concentrations of pro-inflammatory cytokines and soluble adhesion molecules and in pro-oxidant activity. One possible interpretation is that repeated exposure of the blood vessel wall to the activities of pro-inflammatory cytokines and pro-oxidants may damage the vascular endothelium and promote atherogenesis. Based on these results, we propose a model of a causal mechanism to explain how consumption of a fatty meal may impair vascular dysfunction.

Absorption and transport of dietary long-chain fatty acids in cirrhosis: a stable-isotope-tracing study

BACKGROUND

In rats, 30-70% of dietary fatty acids (FAs) are absorbed through the portal vein. Whether this occurs in humans is unknown, but it may occur in persons with cirrhosis, who show a blunted chylomicronemic response to dietary fat without significant steatorrhea.

OBJECTIVE

The objective was to investigate whether portal FA absorption occurs in humans with cirrhosis.

DESIGN

Six control subjects and 10 patients with (n = 5) and without (n = 5) cirrhotic ascites were fed [1-(13)C]palmitic and oleic acids in a test meal. Samples were drawn before and 30, 60, 90, 120, 240, 360, 480, and 720 min afterward for plasma [1-(13)C]-labeled FAs and breath (13)CO(2) assay. Fecal [1-(13)C]-labeled FAs were also measured.

RESULTS

[1-(13)C]-Labeled FAs increased in chylomicrons in all groups, but less in ascitic cirrhotic patients, because their median area under the curve from 120 to 720 min was significantly lower than in the control subjects for labeled palmitate [520 (interquartile range: 192-1137) compared with 2862 (2674-4175) micromol . min/L] and oleate [829 (781-1263) compared with 3119 (2939-4986) micromol . min/L]. [1-(13)C]-Labeled FA enrichment of VLDL was also lower in cirrhotic patients. [1-(13)C]-Labeled FA in free FAs peaked earlier in ascitic than in nonascitic patients and control subjects, mainly for [1-(13)C]oleate, and the median area under the curve from 0 to 120 min was significantly higher in ascitic patients than in control subjects [301 (255-400) compared with 48 (34-185) micromol . min/L]. Fecal excretion of [1-(13)C]-labeled FA was negligible and not significantly different between groups.

CONCLUSIONS

The low [1-(13)C]-labeled FA concentrations in chylomicrons and VLDL, without increased fecal losses, confirm previous data in cirrhotic patients with the use of an unlabeled fat load. The earlier [1-(13)C]-labeled FA appearance in free FAs supports the portal absorption of dietary fat in patients with advanced cirrhosis with spontaneous portal-systemic shunting.

The effects of rosiglitazone on fatty acid and triglyceride metabolism in type 2 diabetes

AIMS/HYPOTHESIS

We investigated the effects of rosiglitazone on NEFA and triglyceride metabolism in type 2 diabetes.

METHODS

In a double-blind, placebo-controlled, cross-over study of rosiglitazone in diet-treated type 2 diabetic subjects, we measured arteriovenous differences and tissue blood flow in forearm muscle and subcutaneous abdominal adipose tissue, used stable isotope techniques, and analysed gene expression. Responses to a mixed meal containing [1,1,1-(13)C]tripalmitin were assessed.

RESULTS

Rosiglitazone induced insulin sensitisation without altering fasting NEFA concentrations (-6.6%, p=0.16). Postprandial NEFA concentrations were lowered by rosiglitazone compared with placebo (-21%, p=0.04). Adipose tissue NEFA release was not decreased in the fasting state by rosiglitazone treatment (+24%, p=0.17) and was associated with an increased fasting hormone-sensitive lipase rate of action (+118%, p=0.01). Postprandial triglyceride concentrations were decreased by rosiglitazone treatment (-26%, p<0.01) despite unchanged fasting concentrations. Rosiglitazone did not change concentrations of triglyceride-rich lipoprotein remnants. Adipose tissue blood flow increased with rosiglitazone (+32%, p=0.03). Postprandial triglyceride [(13)C]palmitic acid concentrations were unchanged, whilst NEFA [(13)C]palmitic acid concentrations were decreased (p=0.04). In muscle, hexokinase II mRNA expression was increased by rosiglitazone (+166%, p=0.001) whilst the expression of genes involved in insulin signalling was unchanged. Adipose tissue expression of FABP4, LPL and FAT/CD36 was increased.

CONCLUSIONS/INTERPRETATION

Rosiglitazone decreases postprandial NEFA and triglyceride concentrations. This may represent decreased spillover of NEFAs from adipose tissue depots. Decreased delivery of NEFAs to the liver may lead to lowered postprandial triglyceride concentrations. Upregulation of hexokinase II expression in muscle may contribute to insulin sensitisation by rosiglitazone.

Diabetes, lipids, and adipocyte secretagogues

That obesity is associated with insulin resistance and type II diabetes mellitus is well accepted. Overloading of white adipose tissue beyond its storage capacity leads to lipid disorders in non-adipose tissues, namely skeletal and cardiac muscles, pancreas, and liver, effects that are often mediated through increased non-esterified fatty acid fluxes. This in turn leads to a tissue-specific disordered insulin response and increased lipid deposition and lipotoxicity, coupled to abnormal plasma metabolic and (or) lipoprotein profiles. Thus, the importance of functional adipocytes is crucial, as highlighted by the disorders seen in both "too much" (obesity) and "too little" (lipodystrophy) white adipose tissue. However, beyond its capacity for fat storage, white adipose tissue is now well recognised as an endocrine tissue producing multiple hormones whose plasma levels are altered in obese, insulin-resistant, and diabetic subjects. The consequence of these hormonal alterations with respect to both glucose and lipid metabolism in insulin target tissues is just beginning to be understood. The present review will focus on a number of these hormones: acylation-stimulating protein, leptin, adiponectin, tumour necrosis factor alpha, interleukin-6, and resistin, defining their changes induced in obesity and diabetes mellitus and highlighting their functional properties that may protect or worsen lipid metabolism.

Validation of deuterium-labeled fatty acids for the measurement of dietary fat oxidation during physical activity

Measurement of 13C-labeled fatty acid oxidation is hindered by the need for acetate correction, measurement of the rate of CO2 production in a controlled environment, and frequent collection of breath samples. The use of deuterium-labeled fatty acids may overcome these limitations. Herein, d31-palmitate was validated against [1-13C]palmitate during exercise. Thirteen subjects with body mass index of 22.9 +/- 3 kg/m2 and body fat of 19.6 +/- 11% were subjected to 2 or 4 h of exercise at 25% maximum volume oxygen consumption (VO2max). The d31-palmitate and [1-13C] palmitate were given orally in a liquid meal at breakfast. The d3-acetate and [1-13C]acetate were given during another visit for acetate sequestration correction. Recovery of d31-palmitate in urine at 9 h after dose was compared with [1-13C] palmitate recovery in breath. Cumulative recovery of d31-palmitate was 10.6 +/- 3% and that of [1-13C]palmitate was 5.6 +/- 2%. The d3-acetate and [1-13C]acetate recoveries were 85 +/- 4% and 54 +/- 4%, respectively. When [1-13C]acetate recovery was used to correct 13C data, the average recovery differences were 0.4 +/- 3%. Uncorrected d31-palmitate and acetate-corrected [1-13C]palmitate were well correlated (y=0.96x + 0; P <0.0001) when used to measure fatty acid oxidation during exercise. Thus, d31-palmitate can be used in outpatient settings as it eliminates the need for acetate correction and frequent sampling.

Effect of exercise performed immediately before a meal of moderate fat content on postprandial lipaemia

The majority of the studies that have found a lowering effect of exercise on postprandial lipaemia have employed exercise 12-18 h before a test meal of exaggerated fat content (over 60 % total energy). The aim of the present study was to investigate whether this effect is manifest when exercise is performed immediately before a test meal of moderate fat content. Eleven healthy young men cycled for 45 min at 62 % maximal heart rate or rested, and, immediately afterwards, consumed a meal of moderate fat content (35 % total energy, 0.65 g/kg body mass) in a random counterbalanced design. Blood samples were drawn before exercise, before the meal, and for 8 h postprandially. No significant differences were observed in plasma triacylglycerol concentrations and areas under the triacylglycerol concentration v. time curves between exercise and rest, although exercise reduced the postprandial lipaemic response by 17 %. Non-esterified fatty acids, glucose, and insulin did not differ significantly between the trials. In conclusion, moderate exercise performed immediately before a meal of a fat content typical to the Western diet had only a modest effect on postprandial lipaemia.

Effect of meal sequence on postprandial lipid, glucose and insulin responses in young men

OBJECTIVE

To investigate whether the postprandial changes in plasma triacylglycerol (TAG), nonesterified fatty acids (NEFA), glucose and insulin concentrations in young men were the same if an identical meal was fed at breakfast and lunch, and if the response to lunch was modified by consumption of breakfast.

METHODS

In two trials (1 and 2) healthy subjects (age 22+/-1 y, body mass index 22+/-2 kg/m(2)) were fed the same mixed macronutrient meal at breakfast at 08:00 h and lunch at 14:00 h. In the third trial, no breakfast was fed and the overnight fast extended until lunch at 14:00 h. Addition of [1,1,1-(13)C]tripalmitin to one meal in each trial was used to distinguish between endogenous and meal-derived lipids.

RESULTS

The postprandial changes in TAG, NEFA and glucose concentrations were similar in trials 1 and 2. The change in plasma total TAG concentration was about two fold less (P<0.05) after lunch compared to breakfast. Postprandial NEFA suppression was the same after breakfast and lunch. Glucose and insulin responses were significantly greater following lunch suggesting decreasing insulin sensitivity during the day. Consumption of breakfast did not alter the postprandial total TAG or NEFA responses after lunch. Measurement of [(13)C]palmitic acid concentration showed that handling of TAG and NEFA from the meal was the same after breakfast and lunch, and was not altered by consumption of breakfast.

CONCLUSIONS

Overall, these data suggest that in young, healthy men regulation of plasma TAG from endogenous sources, principally VLDL, but not chylomicrons during the postprandial period leads to differences in the magnitude of lipaemic response when the same meal was consumed at breakfast or at lunch 6 h later.

Effect of prior exercise on lipemia after a meal of moderate fat content

OBJECTIVE

Prior exercise has been repeatedly shown to reduce lipemia after meals of exaggerated fat content (over 60% of total energy). The aim of the present study was to investigate whether the same applies to meals closer to the composition of the typical Western diet and explore whether exercise affects the release of dietary fat into the bloodstream.

DESIGN

Randomized counterbalanced.

SETTING

Laboratory.

SUBJECTS

Nine healthy young male volunteers.

INTERVENTION

Subjects consumed a meal of moderate fat content (35% of total energy, 0.66 g/kg body mass) 14 h after having either cycled for 1 h at 70-75% of maximal heart rate or rested. Macadamia nuts were used as the main source of dietary fat to trace its entry into the circulation because of their unusual fatty acid composition. Blood samples were drawn before the meal and for 8 h postprandially.

RESULTS

Plasma triacylglycerol concentrations and total area under the triacylglycerol concentration vs time curve (AUC) were significantly lower after exercise (P = 0.001 and 0.003, respectively; effect size for the latter, 0.84). However, incremental (above baseline) AUC was not affected by exercise significantly. When controlling for differences in baseline plasma concentrations, only the fatty acids that were more abundant in the meal than in plasma triacylglycerols were decreased in the early postprandial period following exercise, implying either a suppressive effect of exercise on the rate of triacylglycerol release from the intestine or a more rapid chylomicron clearance after meal consumption.

CONCLUSIONS

Exercise performed between 15 and 14 h before a meal of moderate fat content reduced postprandial lipemia, mainly by lowering fasting triacylglycerols. The effect of exercise on postprandial triacylglycerol metabolism may be mediated, at least in part, by attenuated release of dietary fat from the intestine.

Systemic and forearm triglyceride metabolism: fate of lipoprotein lipase-generated glycerol and free fatty acids

Little is known about the fate of the lipolytic products produced by the action of lipoprotein lipase (LPL) on circulating triglyceride-rich lipoproteins in humans. We studied eight lean, healthy male subjects after an overnight fast. Subjects received infusions of lipid emulsions containing triolein labeled with (3)H on both the glycerol backbone and the fatty acid portion of the molecule; (14)C glycerol and (14)C oleate were coinfused to quantify the systemic and forearm release of (3)H glycerol and (3)H oleate resulting from LPL action. There was significant forearm uptake of both whole plasma triglyceride (presumed to represent primarily VLDL; extraction fraction 2.6 +/- 0.6%, P < 0.005 vs. zero) and radiolabeled triglyceride derived from the lipid emulsion (a surrogate for chylomicrons; extraction fraction 31 +/- 4%, P < 0.005 vs zero). Systemic clearance and forearm fractional extraction of glycerol was greater than that of oleate (P < 0.001 and P < 0.02, respectively). The systemic and forearm fractional release of LPL-generated glycerol were similar at 51 +/- 4 and 59 +/- 1%, respectively (NS). In contrast, the forearm fractional release of LPL-generated oleate was less than systemic fractional release (14 +/- 2 vs. 36 +/- 4%, P < 0.0001). These results indicate that there is escape, or spillover, of the lipolytic products of LPL action on triglyceride-rich lipoproteins in humans. They further suggest that LPL-mediated fatty acid uptake is an inefficient process, but may be more efficient in muscle than in adipose tissue.

ASP enhances in situ lipoprotein lipase activity by increasing fatty acid trapping in adipocytes

Acylation-stimulating protein (ASP) increases triglyceride (TG) storage (fatty acid trapping) in adipose tissue and plays an important role in postprandial TG clearance. We examined the capacity of ASP and insulin to stimulate the activity of lipoprotein lipase (LPL) and the trapping of LPL-derived nonesterified fatty acid (NEFA) in 3T3-L1 adipocytes. Although insulin increased total LPL activity (secreted and cell-associated; P < 0.001) in 3T3-L1 adipocytes, ASP moderately stimulated secreted LPL activity (P = 0.04; 5% of total LPL activity). Neither hormone increased LPL translocation from adipocytes to endothelial cells in a coculture system. However, ASP and insulin increased the V(max) of in situ LPL activity ([(3)H]TG synthetic lipoprotein hydrolysis and [(3)H]NEFA incorporation into adipocytes) by 60% and 41%, respectively (P </= 0.01) without affecting K(m). Tetrahydrolipstatin (LPL inhibitor) diminished baseline, ASP-, and insulin-stimulated in situ LPL activity, resulting in [(3)H]TG accumulation (P < 0.0001). Unbound oleate inhibited in situ LPL activity (P < 0.0001) but did not eliminate the ASP stimulatory effect. Therefore, 1) the clearance of TG-rich lipoproteins is enhanced by ASP through increasing TG storage and relieving NEFA inhibition of LPL; and 2) the effectiveness of adipose tissue trapping of LPL-derived NEFAs determines overall LPL activity, which in turn determines the efficiency of postprandial TG clearance.

Selective partitioning of dietary fatty acids into the VLDL TG pool in the early postprandial period

Circulating triacylglycerol (TG) arises mainly from dietary fat. However, little is known about the entry of dietary fat into the major TG pool, very low-density lipoprotein (VLDL) TG. We used a novel method to study the specific incorporation of dietary fatty acids into postprandial VLDL TG in humans. Eight healthy volunteers (age 25.4 +/- 2.2 years, body mass index 22.1 +/- 2.3 kg/m2) were fed a mixed meal containing 30 g fish oil and 600 mg [1-13C]palmitic acid. Chylomicrons and VLDL were separated using immunoaffinity against apolipoprotein B-100. The fatty acid composition of lipoproteins was analyzed by gas chromatography/mass spectrometry. [1-13C]palmitic acid started to appear in VLDL TG 3 h after meal intake, and a similar delay was observed for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Approximately 20% of dietary fatty acids entered the VLDL TG pool 6 h after meal intake. DHA was clearly overincorporated into this pool compared with [1-13C]palmitic acid and EPA. This seemed to depend on a marked elevation of this fatty acid in the nonesterified fatty acid pool. In summary, the contribution of dietary fatty acids to early postprandial VLDL TG is substantial. The role of DHA in VLDL TG production will require further investigation.

Contribution of fatty acids released from lipolysis of plasma triglycerides to total plasma fatty acid flux and tissue-specific fatty acid uptake

There is controversy over the extent to which fatty acids (FAs) derived from plasma free FAs (FFAs) or from hydrolysis of plasma triglycerides (TGFAs) form communal or separate pools and what the contribution of each FA source is to cellular FA metabolism. Chylomicrons and lipid emulsions were labeled with [(3)H]triolein, injected into mice, and appearance in plasma of [(3)H]oleic acid was estimated, either through a steady-state approach or by compartmental modeling. [(14)C]oleic acid was included to trace plasma FFA. Eighty to 90% of triglyceride (TG) label was recovered in plasma, irrespective of tracer method or TG source. The contribution of TG lipolysis to total plasma FA turnover was 10-20%. After infusion of [(3)H]TG and [(14)C]FA, the retention of these labels varied substantially among liver, adipose tissue, and skeletal and heart muscle. Retention of TG label changed during fasting in the same direction as lipoprotein lipase (LPL) activity is regulated. We propose a model that reconciles the paradoxical 80-90% loss of TG label into plasma with LPL-directed differential uptake of TGFA in tissues. In this model, TGFAs mix locally at the capillaries with plasma FFAs, where they would lead to an increase in the local FA concentration, and hence, FA uptake. Our data indicate that a distinction between TG-derived FA and plasma FFA cannot be made.