The indirect pathway of hepatic glycogen synthesis and reduction of food intake by metabolic inhibitors

Quantifying hepatic glycogen synthesis by direct and indirect pathways in rats under normal ad libitum feeding conditions

Hepatic glycogen synthesis from intact hexose (direct pathway) relative to that from gluconeogenic precursors (indirect pathway) was quantified in ad libitum-fed rats. Following (2)H(2)O administration and overnight feeding, the livers were removed and glycogen (2)H-enrichment was measured by (2)H NMR. Six controls and six rats rendered hyperglycemic by streptozotocin (STZ; fasting blood glucose = 385 +/- 31 mg/dl) were studied. The indirect pathway contribution, estimated as glycogen hydrogen 5 relative to hydrogen 2 enrichment, was 54% +/- 4% for control rats-similar to values from healthy, meal-fed humans. In STZ-treated rats, the indirect pathway contribution was significantly higher (68% +/- 4%, P < 0.05 vs. controls), similar to that of Type 1 diabetic (T1D) patients. In conclusion, sources of hepatic glycogen synthesis in rats during ad libitum nocturnal feeding were quantified by analysis of glycogen enrichment from (2)H(2)O. STZ caused alterations resembling the pathophysiology of hepatic glycogen synthesis in T1D patients.

Beneficial and deleterious effects of hydroxycitrate in rats fed a high-fructose diet

OBJECTIVE

The present study assessed whether long-term supplementation of a high-fructose diet with hydroxycitrate (HCA), an inhibitor of de novo lipogenesis that is widely used as a non-prescription dietary aid, decreases food intake, visceral fat accumulation, hypertriglyceridemia, and hyperinsulinemia in rats.

METHODS

We examined the effects of HCA (1.8% of diet) on food intake, body weight gain, visceral fat accumulation, hypertriglyceridemia, and hyperinsulinemia in rats during a 4-wk period of ad libitum access to a 50% fructose diet after a 3-wk period of food restriction in which they lost about 20% of their body weight.

RESULTS

HCA decreased food intake and weight gain throughout the test and reduced visceral fat accumulation compared with control rats fed ad libitum (CON). Rats that were pair-fed (PF) to the HCA rats showed similar decreases in weight gain and visceral fat. HCA did not ameliorate the hypertriglyceridemia induced by high-fructose feeding. HCA improved insulin sensitivity on day 10 in comparison with CON rats, but by day 27 insulin levels were similarly higher and liver glycogen levels were similarly lower in HCA and CON rats in comparison with PF rats. Liver lipid content was elevated in HCA rats compared with CON and PF rats.

CONCLUSION

These findings indicate that, although HCA attenuates body weight gain and visceral fat accumulation by reducing food intake under these conditions, it has no lasting beneficial effects on hypertriglyceridemia and hyperinsulinemia and leads to the accumulation of liver lipids.

Comparison of the effects of three different (-)-hydroxycitric acid preparations on food intake in rats

Background

Studies on the effects of (-)-hydroxycitric acid (HCA) in humans are controversial. As differences in the HCA preparations may contribute to this apparent discrepancy, the aim of the current study is to compare different HCA-containing preparations in adult Wistar rats.

Design

The effects of 3 different HCA-containing preparations (Regulator, Citrin K, Super CitriMax HCA-600-SXS, all used at an effective HCA dose of 150 and 300 mg/kg, administered intragastrically) on food intake and body weight were studied in adult male Wistar rats. The efficacy was tested under 2 different experimental conditions: 1) after a single dose administration and 2) during repeated administration for 4 subsequent days.

Results

Regulator and Citrin K significantly reduced food intake in both experimental setups, while Super CitriMax HCA-600-SXS was less effective. When administered for 4 subsequent days Regulator and Citrin K diminished body weight gain.

Conclusion

Regulator and Citrin K were shown to be potent inhibitors of food intake in rats, whereas Super CitriMax HCA-600-SXS showed only small and more inconsistent effects. The striking differences in efficacy between these 3 preparations indicate that low doses of a relatively low-effective HCA preparation may have contributed to the lack of efficacy as found in several human studies.

High dose of Garcinia cambogia is effective in suppressing fat accumulation in developing male Zucker obese rats, but highly toxic to the testis

We investigated the ability of Garcinia cambogia extract containing (-)-hydroxycitric acid (HCA) to suppress body fat accumulation in developing male Zucker obese (fa/fa) rats. We also examined histopathologically the safety of its high doses. Diets containing different levels of HCA (0, 10, 51, 102 and 154 mmol/kg diet) were fed to 6-week-old rats for 92 or 93 days. Each diet group was pair-fed to the 154 mmol HCA/kg diet group. Epididymal fat accumulation and histopathological changes in tissues were observed. The highest dose of HCA-containing Garcinia cambogia (154 mmol HCA/kg diet) showed significant suppression of epididymal fat accumulation in developing male Zucker obese rats, compared with the other groups. However, the diets containing 102 mmol HCA/kg diet and higher (778 and 1244 mg HCA/kg BW/d, respectively) caused potent testicular atrophy and toxicity, whereas diets containing 51 mmol HCA/kg diet (389 mg HCA/kg BW/d) or less did not. Accordingly, 51 mmol HCA/kg diet (389 mg HCA/kg BW/d) was deemed to be the no observed adverse effect level (NOAEL).

Hydroxycitrate has long-term effects on feeding behavior, body weight regain and metabolism after body weight loss in male rats

We examined the long-term effect of hydroxycitrate (HCA) on food intake, meal patterns, body weight regain and energy conversion ratio as well as on different blood and liver variables in rats after substantial body weight loss. Rats were fed a 1% (g/100 g) fat diet (81% carbohydrate, 10% protein, 1% fat) or a 12% (g/100 g) fat diet (76% carbohydrate, 9% protein, 12% fat) in two separate experiments. Supplementing both diets with 3% HCA after 10 d of restrictive feeding reduced body weight regain over the whole subsequent period of ad libitum consumption (22 d) and decreased the energy conversion ratio [body weight regain (g)/energy intake (MJ)] at the end of the experiment. Only in rats fed the 12% fat diet, HCA had a long-term suppression of food intake. The anorectic effect occurred predominately during the light phase, and was due mainly to a reduction in numbers of meals. In rats fed the 12% fat diet, HCA had no effect on plasma beta-hydroxybutyrate (BHB), but reduced plasma triacylglycerol and increased liver fat concentration. In rats fed the 1% fat diet, HCA did not affect any metabolic variable examined. Thus, the suppressive effect of HCA on body weight regain, which was maintained for at least 3 wk, appears to be independent of the dietary fat content. Yet, the fat content of the diet seemed to be important for the long-term suppressive effect of HCA on feeding. The fact that HCA did not change plasma BHB concentration does not support a role for increased hepatic fatty acid oxidation in the anorectic effect of HCA.

The effect of (-)-hydroxycitrate on energy intake and satiety in overweight humans

OBJECTIVE

Assessment of the effects of 2 weeks of daily administration of HCA on energy intake and satiety in overweight men and women.

DESIGN

A 6-week randomized placebo-controlled single-blinded cross-over trial.

SUBJECTS

consumed three times daily for 2 weeks 100 ml tomato juice (placebo) and, separated by a 2 week wash-out period, 100 ml tomato juice with 300 mg HCA.Subjects: Twelve males and 12 females (body mass index 27.5+/-2.0 kg/m(2); age 37+/-10 y).

MEASUREMENTS

After 2 weeks, 24 h energy intake (EI), appetite profile, hedonics, mood and possible change in dietary restraint were assessed in the laboratory restaurant. Prevention of degradation and bio-availability was documented.

RESULTS

Twenty-four-hour EI was decreased by 15-30% (P<0.05) with HCA treatment compared to placebo, without changes in the appetite profile, dietary restraint, mood, taste perception and hedonics, while body weight tended to decrease (P=0.1).

CONCLUSION

HCA treatment reduced 24 h EI in humans while satiety was sustained.

Chemistry and biochemistry of (-)-hydroxycitric acid from Garcinia

(-)-Hydroxycitric acid [(-)-HCA] is the principal acid of fruit rinds of Garcinia cambogia, Garcinia indica, and Garcinia atroviridis. (-)-HCA was shown to be a potent inhibitor of ATP citrate lyase (EC 4.1.3.8), which catalyzes the extramitochondrial cleavage of citrate to oxaloacetate and acetyl-CoA: citrate + ATP + CoA --> acetyl-CoA + ADP + P(i) + oxaloacetate. The inhibition of this reaction limits the availability of acetyl-CoA units required for fatty acid synthesis and lipogenesis during a lipogenic diet, that is, a diet high in carbohydrates. Extensive animal studies indicated that (-)-HCA suppresses the fatty acid synthesis, lipogenesis, food intake, and induced weight loss. In vitro studies revealed the inhibitions of fatty acid synthesis and lipogenesis from various precursors. However, a few clinical studies have shown controversial findings. This review explores the literature on a number of topics: the source of (-)-HCA; the discovery of (-)-HCA; the isolation, stereochemistry, properties, methods of estimation, and derivatives of (-)-HCA; and its biochemistry, which includes inhibition of the citrate cleavage enzyme, effects on fatty acid synthesis and lipogenesis, effects on ketogenesis, other biological effects, possible modes of action on the reduction of food intake, promotion of glycogenesis, gluconeogenesis, and lipid oxidation, (-)-HCA as weight-controlling agent, and some possible concerns about (-)-HCA, which provides a coherent presentation of scattered literature on (-)-HCA and its plausible mechanism of action and is provocative of further research.

Effects of 2-week ingestion of (-)-hydroxycitrate and (-)-hydroxycitrate combined with medium-chain triglycerides on satiety and food intake

The aim of this study was to assess the effects of 2 weeks of supplementation with (-)-hydroxycitrate (HCA) and HCA combined with medium-chain triglycerides (MCT) on satiety and energy intake. The experimental design consisted of three intervention periods of 2 weeks separated by washout periods of 2 or 6 weeks in a double-blind, placebo-controlled, randomized, and crossover design. Seven male and 14 female normal to moderately obese subjects (mean+/-S.D.; age, 43+/-10 years; body mass index, 27.6+/-2.0 kg/m(2)) participated in this study. Subjects consumed three self-selected meals and four isoenergetic snacks daily with either no supplementation (PLA), with 500 mg HCA (HCA), or 500 mg HCA and 3 g MCT (HCA+MCT). Each intervention period ended with a test day, consisting of a standardized breakfast and ad libitum a lunch and a dinner. There was a significant body weight (BW) loss during the 2 weeks of intervention (PLA, -0.5+/-0.3 kg, P<.05; HCA, -0.4+/-0.2 kg, P<.05; HCA+MCT, -0.7+/-0.2 kg, P<.01), but this reduction was not different between treatments. Twenty-four-hour energy intake (PLA, 8.1+/-0.3 MJ; HCA, 8.3+/-0.3 MJ; HCA+MCT, 8.4+/-0.3 MJ) and the area under the curve of the appetite-related parameters during the test day were similar for all treatments. Two weeks of supplementation with HCA and HCA combined with MCT did not result in increased satiety or decreased energy intake compared to placebo in subjects losing BW.

The effects of 2-week ingestion of (--)-hydroxycitrate and (--)-hydroxycitrate combined with medium-chain triglycerides on satiety, fat oxidation, energy expenditure and body weight

OBJECTIVE

Assessment of the effect of 2-week supplementation with (--)-hydroxycitrate (HCA) and HCA combined with medium-chain triglycerides (MCT) on satiety, fat oxidation, energy expenditure (EE) and body weight (BW) loss.

DESIGN

Three intervention periods of 2 weeks separated by washout periods of 4 weeks. Double-blind, placebo-controlled, randomised and cross-over design.

SUBJECTS

Eleven overweight male subjects (mean+/-s.d.; age, 47+/-16 y; body mass index, 27.4 +/- 8.2 kg/m(2)).

INTERVENTION

Subjects consumed three self-selected meals and four iso-energetic (420 kJ) snacks daily with either no supplementation (PLA), 500 mg HCA (HCA) or 500 mg HCA and 3 g MCT (HCA+MCT). Each intervention ended with a 36 h stay in the respiration chamber.

RESULTS

There was a significant BW loss during the 2 weeks of intervention (PLA, -1.0 +/- 0.4 kg, P<0.05; HCA, -1.5 +/- 0.5 kg, P<0.01; HCA+MCT, -1.3 +/- 0.2 kg, P<0.001), but this reduction was not different between treatments. 24 h EE (PLA, 11.8 +/- 0.2 MJ; HCA, 11.7 +/- 0.1 MJ; HCA+MCT, 11.5 +/- 0.1 MJ), 24 h RQ (0.85 +/- 0.00 in all treatments) and the area under the curve of the appetite-related parameters were not different between treatments.

CONCLUSION

Two-week supplementation with HCA and HCA combined with MCT did not result in increased satiety, fat oxidation, 24 h EE or BW loss compared to PLA, in subjects losing BW.

Gas chromatography/mass spectrometry method to quantify blood hydroxycitrate concentration

Hydroxycitrate (HCA), a popular dietary supplement for weight loss, is a competitive inhibitor of ATP-citrate lyase, an extramitochondrial enzyme involved in the initial steps of de novo lipogenesis (DNL). Although animal studies have shown that HCA effectively inhibits DNL and induces weight loss, these findings have not been consistent in humans. This raises the possibility that the bioavailability of HCA may differ among species. We developed a new GC/MS method to measure HCA levels in blood, using [U-(13)C]citrate (CA*) as internal standard to account for losses associated with the isolation, derivatization, and measurement of HCA. HCA and CA* were derivatized with BSTFA + 10% TMCS and analyzed using PCI/GC/MS (CA*, m/z 471; and HCA, m/z 553). The plasma HCA concentration was measured over a 3.5-h period in four subjects having ingested 2 g of HCA. Their plasma HCA concentration ranged from 0.8 to 8.4 microg/ml 30 min and 2 h after ingestion, respectively. These results demonstrate that when taken acutely, HCA is absorbed, yet present in small quantities in human plasma. This simple method requiring minimal sample preparation is able to measure trace amounts of HCA with accuracy and precision.

Effects of acute (-)-hydroxycitrate supplementation on substrate metabolism at rest and during exercise in humans

BACKGROUND

(-)-Hydroxycitrate (HCA), a competitive inhibitor of ATP-citrate lyase, should reduce the extramitochondrial acetyl-CoA pool. It has been hypothesized that HCA ingestion can reduce malonyl-CoA concentrations and consequently increase fatty acid oxidation in vivo.

OBJECTIVE

This study investigated the acute effects of HCA supplementation on substrate utilization at rest and during exercise in endurance-trained humans.

DESIGN

Ten cyclists [x+/- SD) age: 24 +/- 2 y, weight: 73 +/- 2 kg, maximal oxygen uptake: 4.95 +/- 0.11 L/min, maximal work output (W:max): 408 +/- 8 W] were studied at rest and during 2 h of exercise at 50% W:max on 2 occasions. Both 45 and 15 min before exercise and 30 and 60 min after the start of exercise, 3.1 mL/kg body wt of an HCA solution (19 g/L) or placebo was ingested. Total fat and carbohydrate oxidation rates were assessed. Blood samples were collected at 15-min intervals at rest and every 30 min during exercise.

RESULTS

Plasma HCA concentrations increased after HCA ingestion up to 0.39 +/- 0.02 mmol/L (82.0 +/- 4.8 mg/L). However, no significant differences in total fat and carbohydrate oxidation rates were observed between trials. Accordingly, plasma glucose, glycerol, and fatty acid concentrations did not differ between trials. Plasma lactate concentrations were significantly lower in the HCA than in the placebo trial after 30 min of exercise but at the end of the exercise period they did not differ between trials.

CONCLUSION

HCA, even when provided in large quantities, does not increase total fat oxidation in vivo in endurance-trained humans.

Chronic (-)-hydroxycitrate administration spares carbohydrate utilization and promotes lipid oxidation during exercise in mice

(-)-Hydroxycitrate (HCA) is an active ingredient that is extracted from the rind of the Indian fruit, Garcinia cambogia, which is available as an herbal supplement and is used to lose weight. In this study, the acute and chronic effects of HCA on energy metabolism were examined in male Std ddY mice. Mice were placed into metabolic chambers and administered 10 mg HCA or water (control) orally. Serum free fatty acid levels were significantly higher 100 min after administration in the HCA group, but the respiratory exchange ratio was not different from that in the control group. The concentration of glycogen in the gastrocnemius muscle was higher in the HCA group 16 h after administration, and in a separate study, the maximum swimming time until fatigue was slightly longer (P: = 0. 21) than that in the control group on d 1. The difference was significant on d 3 after 3 d of HCA or water administration. Other mice were administered 10 mg HCA or water orally twice a day for 25 d. On d 26, they were placed into metabolic chambers after administration and allowed to rest for 1 h, followed by 1 h of running at 15 m/min. Respiratory gas was monitored. The respiratory exchange ratio was significantly lower in the HCA group during both resting and exercising conditions. These results suggest that chronic administration of HCA promotes lipid oxidation and spares carbohydrate utilization in mice at rest and during running.

Utility of metformin as an adjunct to hydroxycitrate/carnitine for reducing body fat in diabetics

Excessive exposure of tissues to fatty acids is likely to be the chief cause of the various dysfunctions that lead to sustained hyperglycemia in type II diabetes. These dysfunctions are likely to be substantially reversible if body fat and dietary fat can be greatly reduced. Disinhibition of hepatic fatty acid oxidation with hydroxycitrate (HCA) and carnitine has considerable potential as a new weight-loss strategy, but in diabetics runs the risk of further enhancing excessive hepatic gluconeogenesis. Since the clinical utility of metformin in diabetes is probably traceable to inhibition of gluconeogenesis, its use as an adjunct to HCA/carnitine treatment of obesity in diabetics deserves evaluation, particularly as metformin therapy itself tends to reduce body weight. A consideration of relevant evidence suggests that metformin therapy will not impede the activation of fatty acid oxidation by HCA/carnitine, and is likely to potentiate the appetite-suppressant and thermogenic benefits of this strategy. Indeed, since metformin has been reported to lower body weight and improve cardiovascular risk factors in obese non-diabetics, a broader application of a metformin/HCA/carnitine therapy for obesity can be contemplated.

Acetaminophen glucuronidation accurately reflects gluconeogenesis in fasted dogs

To assess whether acetaminophen glucuronide accurately reflects uridyl diphosphate-glucose (UDP-glucose) derived from gluconeogenesis during fasting, three mongrel dogs received infusions of [U-14C]lactate, [1-13C]galactose, and [6-3H]glucose (after fasting overnight or for 2.5 days). After initiation of the isotopes (3 h), acetaminophen was given, and the urinary acetaminophen glucuronide was isolated. The mean plasma [14C]glucose specific activity (SA) was similar to the mean urinary acetaminophen glucuronide SA both after fasting overnight [299 +/- 19 vs. 296 +/- 14 disintegrations.min-1 (dpm).mumol-1, respectively] and after 2.5 days of fasting (511 +/- 8 vs. 562 +/- 32 dpm/mumol, respectively). Mean plasma glucose flux calculated using [6-3H]glucose decreased (P < 0.05) with two additional days of fasting (18.7 +/- 1.2 vs. 13.6 +/- 0.6 mumol.kg-1.min-1), as did intrahepatic (P < 0.05) UDP-glucose flux measured using [1-13C]galactose (8.6 +/- 0.7 vs. 5.5 +/- 0.3 mumol.kg-1.min-1). We conclude that, in fasted dogs, plasma glucose and UDP-glucose, as sampled by acetaminophen, equally reflect gluconeogenesis and appear to come from the same pool of glucose 6-phosphate. In addition, cycling of glucose moieties through UDP-glucose and glycogen decreases with an increased period of fasting.

Glycogen synthesis in amphibian oocytes: evidence for an indirect pathway

Glycogen is the main end product of glucose metabolism in amphibian oocytes. However, in the first few minutes after [U-14C]glucose microinjection most of the label is found in lactate. The burst of lactate production and the shape of the time curves for the labelling of glucose 6-phosphate, fructose 6-phosphate, glucose 1-phosphate and glycogen suggest a precursor-product relationship of lactate with respect to glycogen and its intermediates. Expansion (by microinjection) of the pool of glycolytic intermediates, such as dihydroxyacetone phosphate, glyceraldehyde 3-phosphate, 3-phosphoglycerate or phosphoenolpyruvate, results in a marked decrease in [U-14C]glucose incorporation into glycogen. After co-injection of doubly labelled glucoses, extensive detritiation (93%) of the glucosyl units of glycogen was observed with [2-3H, U-14C]glucose and partial detritiation with [3-3H,U-14C]glucose (34%) or [5-3H,U-14C]glucose (46%). After injection of [6-3H,U-14C]glucose, a small but significant and reproducible detritiation (13%) in glycogen was observed. Co-injection of [U-14C]glucose and 3-mercaptopicolinate resulted in marked inhibition of glycogen labelling. Half-maximal inhibition was observed at 0.58 mM 3-mercaptopicolinate, which agrees with the IC50 value (0.47 mM) for the inhibition in vitro of phosphoenolpyruvate carboxykinase activity. We concluded that in frog oocytes most of the glucosyl units are incorporated into glycogen by an indirect pathway involving breakdown of glucose to lactate, which is then converted into glycogen via gluconeogenesis. Both processes, glycolytic degradation of glucose to lactate and subsequent reconversion of the latter into hexose phosphates and glycogen, occur in the same cell.

Hepatic glycogen accurately reflected by acetaminophen glucuronide in dogs refed after fasting

To validate a method to "biochemically biopsy" the immediate precursor of intrahepatic glycogen [uridyl diphosphate (UDP)-glucose] using acetaminophen and to assess how fasting affects the direct and indirect pathways of glycogen synthesis, dogs were fasted overnight (group 1, n = 5) or for 2.5 days (group 2, n = 5) and then given a 4-h duodenal infusion of unlabeled glucose, [3-3H]glucose, and [U-14C]lactate to label hepatic glycogen via the direct and indirect pathways, respectively, and [1-13C]galactose to measure intrahepatic UDP-glucose flux. After 3 h for equilibration, acetaminophen was given and urine was collected for acetaminophen glucuronide. Multiple liver biopsies were obtained. The mean 3H/14C ratios of glucose derived from glycogen (10.4 +/- 4.1 and 1.1 +/- 0.3 for groups 1 and 2, respectively) and glucose derived from acetaminophen glucuronide (11.5 +/- 4.0 and 1.0 +/- 0.1 for groups 1 and 2, respectively) were similar. Fasting significantly increased UDP-glucose flux, the rate of glycogen synthesis, and the contribution of the indirect pathway. We conclude that, in dogs, 1) no functional hepatic zonation exists with regard to acetaminophen glucuronidation and liver glycogen synthesis and 2) with appropriate choice of isotopic tracers and study design, UDP-glucose flux can accurately reflect rates of hepatic glycogen synthesis.