To investigate the anti-obesity effects of the pomegranate leaf extract (PLE) in a mouse model of high-fat diet induced obesity and hyperlipidemia.
For the anti-obesity experiment, male and female ICR mice were fed with a high-fat diet to induce obesity. When the weight of the high-fat diet group was 20% higher than the normal diet group, the animals were treated with 400 or 800 mg/kg/day of PLE for 5 weeks. Body weight and daily food intake were measured regularly during the experimental period. The various adipose pads were weighed and serum total cholesterol (TC), triglyceride (TG), glucose and high-density lipoprotein cholesterol (HDL-C) were measured after 5 weeks, treatment with PLE. In the fat absorption experiment, both the normal and obese mice were given 0.5 ml lipid emulsion and PLE at a dose of 800 mg/kg at the same time. Serial serum TG levels were measured at times 1, 2, 3, 4 and 6 h after the treatment. TGs in fecal excretions were measured after the mice were orally given a lipid emulsion. Effects of PLE and its isolated compounds (ellagic acid and tannic acid) on pancreatic lipase activity were examined in vitro.
The PLE-treated groups showed a significant decrease in body weight, energy intake and various adipose pad weight percents and serum, TC, TG, glucose levels and TC/HDL-C ratio after 5 weeks treatment. Furthermore, PLE significantly attenuated the raising of the serum TG level and inhibited the intestinal fat absorption in mice given a fat emulsion orally. PLE showed a significant difference in decreasing the appetite of obese mice fed a high-fat diet, but showed no effect in mice fed a normal diet.
PLE can inhibit the development of obesity and hyperlipidemia in high-fat diet induced obese mice. The effects appear to be partly mediated by inhibiting the pancreatic lipase activity and suppressing energy intake. PLE may be a novel appetite suppressant that only affects obesity owing to a high-fat diet.
Obesity is the most common metabolic disease in developed nations and has become a global epidemic in recent years.1 It is associated with a variety of chronic diseases, including hyperlipidemia, diabetes mellitus, hypertension coronary artery disease and certain cancers. Furthermore, obesity, especially abdominal obesity, has an association with dyslipidemia characterized by increasing triglyceride (TG) and decreasing high-density lipoprotein cholesterol (HDL-C) concentrations.2 It is known that an oversupply of fat is associated with the development of obesity in mice.3 Long-term feeding on a high-fat diet can induce obesity with hyperphagia, hypergluconemia, hyperlipidemia and insulin resistance.4, 5 In addition, it has been suggested that high-fat diet induced obesity may contribute to an increase in white and brown adipocytes growth.6
In traditional Chinese medicine, pomegranate preparations, including its roots, barks of trees and roots, and the juice of the fruit, especially the dried peels, have been used to treat acidosis, hemorrhage, diarrhea, helminthiasis and microbial infections.7, 8, 9 Recent investigations have found that the pomegranate extract contains abundant anthocyanins (such as delphinidin, cyanidin and pelargonidin) and hydrolysable tannins (such as punicalin, pedunculagin, punicalagin, gallagic acid, ellagic acid and its esters of glucose) and possesses strong antioxidant, anti-inflammatory properties and anti-tumor effects of tumors promoted by many chemical carcinogens both in vivo and in vitro.10, 11, 12, 13, 14 Some studies have reported that both the pomegranate flowers and fruit extracts exhibited high activity on lowering circulation lipid and modifying heart disease risk factors in diabetic animals and humans with hyperlipidemia,15, 16 but the mechanism is still unknown. In our laboratory, it has been found that the pomegranate leaf extract (PLE), containing abundant tannins, had a strong lipid-lowering action, that is, decreased the plasma levels of total cholesterol (TC) and TG in hyperlipemic animals after a long-term of oral administration. It is well established that hyperlipidemia has a potent association with obesity.17 Based on the above data, we speculated that PLE could modulate the abnormal lipid metabolism and cause weight loss. To test this hypothesis, we investigated the anti-obesity effects of PLE in obese mice induced by a high-fat diet.
Both female and male CD-1(ICR) mice (3 weeks old) were obtained from the Beijing Vital River Laboratory Animal Technological Company (Beijing, China). They were housed for 1 week under a 12/12 h light/dark cycle in a temperature- and humidity-controlled room and freely fed standard laboratory chow with water ad libitum. The standard laboratory chow contained (in weight percent) approximately: 4% fat, 24% protein, 65% carbohydrate, and the high-fat diet contained: 15% lard (saturated fat), 20% olive oil (polyunsaturated fat), 35% protein, 15% carbohydrate, 0.5% vitamin mixture, 3% mineral mixture and 5% cholesterol (primarily egg yolk). Both chows were also obtained from the Laboratory Animal Institute of Chinese Academy of Medical Sciences (SCXK Jing 2001-0003).
PLE (containing ellagic acid 10.6%, prepared by our laboratory with the match number 040818) was used and the voucher specimen (031006) was preserved in our laboratory for future reference. TG, glucose, HDL-C and TC test kits were purchased from Beijing Zhongsheng High-tech Bioengineering Company (Beijing, China). Lipid emulsion was purchased from Beijing Green Cross Company (Beijing, China).
Effects of PLE on obesity in mice induced by a high-fat diet
Mice were divided into two groups, the obese group was fed high-fat chow and the normal control group was fed standard laboratory chow for 6 weeks. By the end of 6 weeks period, the body weight of the obese group was approximately 20% higher than the normal control group. Then, 24 male obese mice or 20 female obese mice were randomly divided into four groups, with each group having six (male) or five (female) mice. The high-dose PLE group was treated with PLE at a dose of 800 mg/kg. The low-dose PLE group was given PLE at a dose of 400 mg/kg. The sibutramine group was treated with sibutramine at a dose of 4.6 mg/kg, and the control group was given normal saline. PLE and sibutramine were orally administered to the mice by gavage every day. All the obese groups were continually fed the high-fat diet during this experiment. The control groups of normal mice, including both females and males, were daily given water and continually fed standard laboratory chow. Animals were allowed free access to water and food throughout the experiment. Body weight was measured at least two times a week, and daily food intake was recorded once a week. After an 8-h fast, the blood of each mouse was taken under anesthesia with ether at the end of 5 weeks treatment. The serum was separated and frozen at −80°C until analysis. Then each mouse was killed. The body length (from nose to anus) was measured, and various visceral adipose pads and inguinal subcutaneous adipose pad were quickly removed and weighed. The Lee's obesity index was calculated as follows: body weight (g) ⅓/nasal-anal length (cm)*1000.
Effect of PLE on the appetite in mice fed a normal diet or a high-fat diet
Mice were randomly divided into four groups (seven male mice and seven female mice per group): the high-fat diet plus PLE group, the high-fat diet control group, the normal diet plus PLE group and the normal diet control group. The PLE groups were orally administered PLE by gavage at a dose of 800 mg/kg/day for 5 weeks. Body weight and food intake were monitored every week. Body length was measured and various adipose pads were removed and weighed at the end of this experiment.
Effects of PLE on serum TG levels in normal mice after a single oral administration of lipid emulsion
Male ICR mice (20.03±1.97 g) were divided into three groups, with each group matched for body weight (n=6). The lipid emulsion was prepared with 5 ml olive oil and 10 ml 1% Tween-20 solution sonicated for 10 min.14 After all the mice had fasted 12 h, they were orally given the lipid emulsion at a dose of 0.5 ml/mouse (containing 0.046 mmol TG). At the same time that the PLE group was orally given PLE at a dose of 800 mg/kg, tannic acid group was given the tannic acid at a dose of 200 mg/kg, and the control group was given the same volume of water. Serial blood samples were taken from the orbital sinus at times of 0, 1, 2, 4 and 6 h after the administration of the lipid emulsion. The serum was separated by centrifuging at 4°C and stored at −20°C for a later assay of TG.
Effects of PLE on serum TG levels in obese mice after an oral administration of lipid emulsion
High-fat diet induced obese ICR mice (male, 39.9±2.77 g) were divided into two groups, the PLE group and the control group, with each group matched for body weight (n=6). The normal group was fed standard laboratory chow. Lipid emulsion was prepared as we described above. After giving the lipid emulsion (0.5 ml/mouse) by gavage, the PLE group was given PLE at a dose of 800 mg/kg, and both the obese control group and the normal group were given normal saline. Serial blood samples were taken from the orbital sinus, while the mice were under anesthesia with ether, at times of 0, 1, 2, 4 and 6 h after administration of the lipid emulsion. The serum was separated by centrifuge at 4°C and stored at −80°C for later assays.
Effects of PLE on total fat absorption in normal mice after a single oral administration of lipid emulsion
Male ICR mice (19.56±1. 48 g) were divided into four groups (n=6). After a 12-h fasting, they were given the lipid emulsion by gavage at a dose of 0.5 ml/mouse. The lipid emulsion was prepared as described above. At the same time that the PLE group was orally given PLE at a dose of 800 mg/kg, the tannic acid group was given tannic acid at a dose of 200 mg/kg, the ellagic acid group was given ellagic acid at a dose of 80 mg/kg and the control group was given the same volume of water. Thereafter, the standard rodent chow was given at 1.0 g/mouse. The feces were collected during the following 12 h and were air-dried overnight at room temperature. Each fecal sample was weighed and pulverized. The amount of fat in feces was analyzed according to the method of Kamer et al.,18 and the dietary fat absorption was expressed as a percentage of ingested fat.
Effects of PLE on pancreatic lipase activity in vitro
Pancreatic lipase activity was determined by measuring the rate of free fatty acid releasing from TG. At first, the lipid emulsion was diluted with 0.1 mol/l Tris-HCl buffer (pH 8.4) to obtain a final concentration of 30 μM TG. The diluted emulsion (0.1 ml) was incubated with 0.05 ml pancreatic lipase (final concentration 5 U per tube) and 0.1 ml of various concentrations of sample solutions for 15 min at 37°C. Pancreatic lipase activity was determined as millimole of fatty acid released per milliliter reaction mixture. Fatty acid release rate is detailed elsewhere.19
All values are reported as mean±s.d. Statistical comparison between groups of mice was calculated using SPSS software. Analysis of variance with Dunnett's test was used to analyze the data, and the criterion for statistical significance was P<0.05.
Effects of PLE in high-fat diet induced obese mice
In this study, obesity was induced in normal mice by feeding a high-fat diet for 6 weeks. Compared with mice fed a standard diet, mice fed a high-fat diet increased their body weight by 20.0% after 6 weeks of feeding and became 21.8% heavier after 11 weeks of feeding. Parallel to the body weight change, the weights of regional adipose pads (including genital adipose pad, perirenal adipose pad and inguinal subcutaneous adipose pad) were significantly higher in obese mice than in normal mice at the end of 11 weeks. In addition, serum TC, TG and glucose levels and TC/HDL-C ratio were significantly increased in mice feeding on a high-fat diet for 11 weeks, compared with mice fed a standard diet. As shown in Figure 1, the PLE group at the dose of 800 mg/kg significantly suppressed the increase in body weight compared with obese control mice. The food intake per day per mouse was weighed in all groups for calculation of energy intake. As shown in Table 1, the mean energy consumption was significantly different between the normal mice and the control group of obese mice. Both the female and male mice treated with PLE at a dose of 800 mg/kg had significantly reduced energy intakes as compared with the obese controls.
The Lee's index and various adipose pad weight percents are shown in Table 2. Compared with the control group of obese mice, Lee's index was significantly decreased in both PLE and sibutramine-treated groups. The final adipose pad weight percents were significantly increased by feeding a high-fat diet compared to the normal diet, and were significantly reduced in mice fed a high-fat diet plus PLE at the dose of 800 or 400 mg/kg compared to the high-fat diet.
As shown in Table 3, the high-fat diet group exhibited a significant increase in serum glucose, TG and TC levels. As compared to the high-fat diet mice, the high dose of PLE mice showed a marked decrease in serum glucose, TG and TC concentrations and a significant decrease in TC/HDL-C ratio. Sibutramine, the anti-obesity drug used in the clinic, had significant effects not only on reducing body weight and various adipose pad weight percents but also on lowering circulation TG and glucose concentrations and TC/HDL-C ratio.
Effect of PLE on the appetite in mice fed a normal diet or a high-fat diet
As shown in Table 4, after treatment with PLE for 5 weeks, the normal diet plus PLE group did not have a significant decrease in body weight or energy intake compared to the normal diet control group. Furthermore, there was no significant difference of the adipose pad weight percents between the two groups of mice fed standard laboratory chow (data not shown). There was a significant difference in body weight and energy intake between the two groups fed a high-fat diet. Both genders of the PLE groups showed a significant decrease in body weight and energy intake compared to the high-fat diet control group.
Effects of PLE on serum TG levels in normal mice after an oral administration of lipid emulsion
Figure 2 shows the serial changes in serum TG concentration when the lipid emulsion with or without PLE was administered orally to mice. At 1, 2, 4 and 6 h of administration of PLE, the serum TG concentrations were significantly lower than those in the control group.
Effects of PLE on serum TG levels in obese mice after an oral administration of lipid emulsion
Figure 3 shows the serial changes in serum TG concentration when lipid emulsion with or without PLE was administered to obese mice. Compared to the standard diet mice, the high-fat diet mice exhibited a significant increase in serum TG after administration of a lipid emulsion. At 1 and 3 h after administration of the lipid emulsion, serum TG concentrations in PLE-treated mice were significantly lower than those in obese control mice.
Effects of PLE and its isolated compounds on total intestinal absorption of fat in normal mice after a single dose of lipid emulsion
As shown in Table 5, the total fat intestinal absorption was very efficient in the control group (95.7% of the applied dose) and decreased significantly in the group treated with PLE at the dose of 800 mg/kg (88.3% of the applied dose). Tannic acid and ellagic acid moderately decreased the total fat absorption in mice, but showed no significant difference compared to the control group.
Effects of PLE and its isolated compounds on pancreatic lipase activity in vitro
As shown in Table 6, PLE significantly inhibited the lipase activity, with 50% inhibition of enzyme activity compared to the control at a concentration of 1 g/l. However, ellagic acid, the important compound isolated from PLE, showed a mild inhibition of enzyme activity at the highest concentration of 0.01 g/l, because of its poor solution. In addition, tannic acid significantly reduced pancreatic lipase activity, with nearly 100% inhibition of enzyme activity at a concentration of 0.1 g/l.
Our study, which is the first report about the weight loss effect of pomegranate extract on obese animals, shows that PLE had a significant anti-obesity effect on the development of diet-induced obesity. Oral administration of PLE at a dose of 800 mg/kg not only reduced the body weight, Lee's index and various adipose pad weight percents, but also decreased the serum TC, TG, glucose levels and the TC/HDL-C ratio, which is defined as the main risk factor for dyslipidemia.2 PLE reduced both the dyslipidemia of obesity and cardiovascular risk factors because it exhibited an especially significant decrease in abdominal fat pad weight percent and the serum TG level compared to the control mice.
Further investigations found that the administration of PLE significantly reduced the elevation in serum TG level and the total cumulative absorption of TG after an oral administration of lipid emulsion over 6 h in both normal and obese mice. The data also demonstrated that obese mice had higher TG absorption from the intestine than normal mice. This may have made the obesity worse and harder to treat. Inhibiting the absorption of dietary TG may play an important role on weight loss because an excessive intake of TG from the diet is relevant to the development of obesity. It is well known that dietary fat is not directly absorbed from the intestine unless it has been subjected to the action of pancreatic lipase.20 It has been clinically reported that the pancreatic lipase inhibitor Orlistrat prevented obesity and hyperlipidemia by decreasing the absorption of dietary fat into blood and increasing the fat excretion in feces.21 PLE and its compounds, ellagic acid and tannic acid, also showed a significant effect on inhibiting the pancreatic lipase activity in vitro and increasing the fecal fat excretion, which suggests that it is one of the mechanisms responsible for decreasing the serum TG concentrations after oral administration of a lipid emulsion to both normal and obese mice. We hypothesized that tannins might be the main bioactive fraction of PLE for two reasons: first, tannins (tannic acid and ellagic acid) exhibited significant inhibitory effects on pancreatic lipase activity and fat absorption from intestine. Second, total tannins are the major constituent of PLE (60% in weight) and may be hydrolyzed to tannic acid and ellagic acid when orally administered. Moreover, ellagic acid, which was the most abundant compound in total tannins, may be the main active compound of PLE.
The present study also showed that PLE may be a new advanced energy intake suppressant. In the high-fat diet induced obese mice, the effect of PLE on energy intake was similar to that of sibutramine, an appetite suppressant that has been used in the clinic. PLE could markedly decrease the calorie intake of mice fed a high-fat diet; however, at the same time, the appetite of mice fed a normal diet was not the least affected by PLE. When we monitored the daily activity of the mice, it was noticed that the PLE groups were less active than the sibutramine groups. It also suggested that the mechanism of the effect of PLE on appetite suppression might be different from sibutramine. We speculate that it may have a close affinity with the obesity induced by the high-fat diet. The precise mechanism of the effect of PLE on appetite in mice fed the high-fat diet is still unknown, and further investigation should be undertaken.
In conclusion, PLE had marked effects on inhibiting the development of obesity and hyperlipidemia in obese mice fed the high-fat diet. It suggested that suppressing energy intake and inhibiting the intestinal absorption of dietary fat via inhibition of pancreatic lipase activity might be two possible mechanisms for the anti-obesity effect of PLE.
The study was supported by Natural Science Foundation of China (30572340).