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* Department of Food Engineering and Biotechnology, Technion, Israel Institute of Technology, Haifa, Israel; and the 3 Department of Pathology, Carmel Medical Center and The Bruce Rappaport Faculty of Medicine, Technion, Israel Institute of Technology, Haifa, Israel
2To whom correspondence should be addressed. E-mail: werman{at}tx.technion.ac.il.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • avocado • dietary fiber • weight gain • cholesterol • hepatic fat
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), and its nutritional composition depends strongly on fruit variety and the season of the year (2
). Avocado contains one to two times more protein than any other fruit, is high in manganese, phosphorous, iron and potassium, but low in sodium, and also contains vitamin E, vitamin C, ß-carotene, thiamin, riboflavin, nicotinic acid and folate (1
). Avocado is rich in oil (15–30 g/100 g fresh fruit) that is mainly monounsaturated (3
) and is a good source of the essential linoleic acid (4
). The amount of simple sugars in the avocado fruit is low, but in contrast, it contains appreciable levels of dietary fiber (DF)4
and is the highest in fiber among fruits (4
). Dietary fibers are very heterogeneous in nature and found mostly in fruit, vegetables, nuts, grains, seeds and legumes (5
). Avocado contains several structural polysaccharides, including insoluble (cellulose and lignin) and soluble (hemicelluloses and pectin) DF (6
). Due to their physicochemical properties and indigestible characteristics, DF directly affect the digestive track with further indirect systematic physiological consequences. DF impose positive modifications on viscosity, motility, nutrient absorption, content, transit time, empting, and probiotic properties of the entire digestive track (7
). These modifications may resolve constipation, reduce fat absorption, lower glycemic index and plasma insulin levels, alter colon fermentation and microbial proliferation, and reduce plasma cholesterol (7
,8
). Therefore, adding recommended levels of DF to the diet is considered vital for normal intestine performance, good health, and for controlling major risk factors for diabetes, obesity, gallstones, hypercholesterolemia and heart disease (9
–13
). The purpose of this research was to study the influence of dry, defatted avocado pulp as a DF source on some dietary and metabolic measures such as food consumption, weight gain, relative cecum weight, hepatic fat content, and plasma and hepatic cholesterol levels in rats.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Local avocado fruits of the Ettinger variety were sliced and a sample was taken for analyzing total (14
), insoluble (15
), and soluble (16
) fiber levels. The flesh (no seed) was freeze-dried, finely ground, and then extracted with petroleum ether to remove oil. The dried defatted avocado pulp was analyzed for total dietary fiber (14
), moisture (17
), protein (18
), starch (19
), residual oil (19
), and water holding capacity (20
), and was used as the avocado fiber source.
Animal studies.
Weanling, Sprague-Dawley male rats were obtained from the animal colony of the Department of Food Engineering & Biotechnology (Technion, Haifa, Israel). The rats were treated according to the Ethics Committee of the Technion.
Rats were housed individually in stainless steel cages, fitted with wire-mesh bottoms, in a temperature-controlled room (21°C) with 12-h light and dark periods. They were allowed free access to water. In study 1, 40 rats (45–50 g, each) were randomly assigned to two groups (cellulose vs. avocado) and then equally subdivided to consume diets with or without cholesterol and cholic acid. Rats were allowed free access to a semipurified diet containing (g/kg diet): casein, 200; coconut oil, 150; AIN-93 (21
); mineral mixture, 35; vitamin mixture, 10; DL-methionine, 3; cellulose, 100 (or 230 g avocado pulp providing 100 g fibers); cornstarch, to complete 1000 g. One subgroup of each fiber group was supplemented (replacing starch) with cholesterol (10 g/kg diet) and cholic acid (5 g/kg diet), to induce hypercholesterolemia. In study 2, sixty rats (55–60 g, each) were randomly assigned to two groups (cellulose vs. avocado). The rats was then equally divided into three subgroups (30, 60 or 100 g fiber/kg diet) and fed diets with or without cholesterol and cholic acid as described above. Avocado-fed rats were allowed free access to their respective diets, while cellulose-fed rats were pair-fed to the group mean of their counterpart avocado-fed rats. Food consumption was recorded daily, and feces were collected during the last 5 d, dried and weighed. After 28 d, rats were deprived of food for 14 h and killed by CO2 asphyxiation. The abdomen was opened, blood was collected from the abdominal aorta, and plasma (2500 x g for 25 min, 4°C) was stored at -80°C. Liver, cecum, kidney, and pancreas were removed, blotted, and weighed. Plasma and hepatic cholesterol levels were determined according to Seary and Bergquist, (22
).
Histomorphometry.
Liver slices were fixed in 0.05 mmol/L saline phosphate buffer containing 4% formaldehyde. Paraffin embedded specimens were longitudinally sectioned (6 µm) and stained with Hematoxylin and Eosin followed by Sudan Black for fat (23
). Positively stained hepatocytes showed intracytoplasmic red granules, and nonstained tissue appeared light blue by light microscopy. An Image analyzer (trichip RGB video camera; Sony, Tokyo, Japan) installed on a light microscope (Zeiss, Jena, Germany) and attached to a computer equipped with a frame grabber was used to analyze the extension of positively stained liver tissue. Images were captured, digitized and displayed on a high-resolution colored monitor. Artifacts were avoided, and the ten most intensely stained fields were analyzed at a power lens of 20 x 10. Images were loaded on screen buffers having a resolution of 760 x 570 pixels, and measured in standardized frames (62993 µm2). Image Pro Plus 4 software (Media Cybernetics, Baltimore, MD) was used to assess extension of positively stained cells after segmentation and thresholding. Fat staining extension was represented by the percentage of positively stained areas per microscopic field.
Statistics.
Rat data were analyzed using SAS 6.12. Data were analyzed by two-way ANOVA with fiber and cholesterol as main effects in study 1, while main effects in study 2 were fiber and fiber level. Interactions were analyzed by the general linear model procedure and Duncan’s Multiple Range Test was used to separate means. Differences with P values < 0.05 were considered significant. Histomorphometric data were analyzed using SPSS 6.0 (Chicago, IL). Positively stained cases in two groups were compared by the Mann-Whitney U test. Comparison between cases used the nonparametric Kruskal-Wallis ANOVA followed by a corrected Mann-Whitney U test for multiple comparisons. Differences with two tailed P values < 0.05 were considered significant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Levels of total, insoluble and soluble fibers in fresh avocado fruit were 5.21, 3.95, and 1.34 g/100 g, respectively. The avocado pulp, used as a fiber source, contained (g/100 g): 43.4, total fiber; 15.8, moisture; 12.8, protein; 2.5, ash; and traces of starch. The pulp had a water-holding capacity of 17.7 compared with 20.9 g/g of cellulose.
Rat studies.
In study 1 daily food consumption, feces excretion, and body weight gains were lower in rats consuming avocado pulp (Table 1
). Rats consuming the avocado diet with cholesterol required more food to gain a gram of body weight than all other groups. Among rats fed normal or cholesterol-containing diets, relative cecum weights were significantly higher in those fed avocado pulp, while no differences were observed in relative pancreas weights (data not shown). Regardless of the fiber source, relative liver weights were higher in rats fed cholesterol (data not shown). In both fiber groups, plasma and hepatic cholesterol levels were higher in rats fed cholesterol-containing diets. Hepatic cholesterol levels did not differ, while plasma cholesterol levels of avocado-fed rats were greater than those of cellulose-fed rats when diets included cholesterol.
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) body weight gains decreased as more fibers were consumed, but this phenomenon was more prominent in the avocado-fed groups. In general, as dietary fiber levels increased, more food was needed to gain a gram of body weight. However, avocado-fed rats required significantly more food compared with those fed cellulose. Cecum relative weights increased as more avocado was consumed, while regardless of the fiber level, cecum relative weights were higher in avocado-fed rats than in their cellulose pair-fed counterparts. At all fiber levels, relative weights of the liver, kidney, or pancreas did not differ among groups (data not shown). At all fiber levels, hepatic cholesterol levels did not differ, while plasma cholesterol levels of avocado-fed rats were greater than in cellulose-fed rats.
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In study 1, in both fiber groups, more fat was histologically observed, following Sudan black staining, in the liver of rats consuming diets with cholesterol compared with those without cholesterol. However, among the rats consuming diets without cholesterol, less fat was observed in the livers of those fed avocado (Fig. 1A
). In study 2, in rats fed diets containing cholesterol, hepatic fat content significantly decreased as the intake of avocado pulp increased (Fig. 1
B). This trend was less pronounced in rats fed cellulose.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Animal studies are required to assess the physiological characteristics of DF, with the notion that a pure fiber exerts metabolic consequences that might differ from those observed when the fiber is an integrated part of a complex system (5
). We made no attempt to purify the fibers from avocado, but to use them as a part of the defatted avocado pulp.
In study 1 we showed that consuming avocado pulp in diets with or without cholesterol reduced food intake and body weight gains compared with cellulose (Table 1)
. Based on these results, we performed a second rat study in which three levels of avocado pulp were added to diets containing cholesterol, and control rats were pair-fed to their avocado-fed counterparts, thus eliminating the influence of food consumption. In study 2 we demonstrated that as the dietary levels of avocado pulp increased, food intakes decreased. Furthermore, in rats fed both fibers, body weight gains decreased as the amount of the fiber increased, but avocado pulp showed a more pronounced effect than cellulose (Table 2)
. Trying to explain these phenomena, we considered several issues.
Taste.
Rats are very sensitive to the bitter taste, and the possible occurrence of such substances in avocado pulp could have resulted in decreased food intake, as actually observed (Table 2)
. A bitter taste in the avocado pulp may have developed during drying or oil extraction as a result of enzymatic and/or nonenzymatic browning processes. However, when a panel of volunteers tasted the avocado pulp, no criticism of any bitter taste was raised.
Energy density.
We assumed that throughout the feeding period, rats stopped eating whenever they fulfilled their energy and nutrient needs. The hypothalamus, using hormonal and biochemical signals, directs the eating behavior of the animal to meet both energy (amount) and nutrient (quality) requirements (24
). Rats consume more food when diets contained less DF compared with a nonfiber control group (25
). Our diets differed mainly in the fiber source, with minor differences in other components. However, because avocado pulp is not a pure fiber, like cellulose, each avocado diet contained 1.3 times more avocado than cellulose to equal fiber levels and, thus, provided additional nutrients that could have increased the energy density of the meal. However, calculations revealed that avocado diets were less energy dense than the cellulose diets, and differences were minimized as the amount of avocado pulp decreased. Avocado pulp contains a soluble fiber fraction that could have been fermented in the cecum to provide some metabolic energy, but this is far less than the energy that would have been expected from the replaced starch.
Satiety.
Low food intakes observed in avocado-fed rats may also be explained by the swelling of fibers in the stomach and the small intestine, achieving early satiety signals in these rats. This is attributed to the water-holding capacity and induced viscosity of soluble fibers. Although the tested fibers did not differ in water-holding capacity, we cannot rule out the satiety-induced feeling of the soluble avocado fibers. Jackson et al. (26
) related low food in
Weight control agent.
Low food intakes led us to assume that avocado may contain a certain weight control agent. Extracting most of the oil from the avocado suggests that this agent is unlikely part of the fat moiety. Furthermore, it does not interfere with protein digestion because no signs of trypsin inhibitor activity were evident in the pulp (data not shown). The occurrence of such a potential agent in the avocado is reinforced by results obtained in the pair-feeding study in which variations in food intakes were eliminated, but avocado-fed rats gained less body weight and required more food to gain a gram of body weight than their counterpart cellulose-fed rats (Table 2)
. Bergh (4
) suggested a similar approach explaining that in humans, losing weight due to avocado intake is related to a weight control factor, not yet identified, that accelerates the basal metabolic rate.
Including avocado pulp in diets containing cholesterol resulted in rats with higher plasma cholesterol levels compared with those fed cellulose (Table 2)
. In addition, although no differences were noted in hepatic cholesterol levels between avocado- and cellulose-fed rats, hepatic fat content, as evaluated histologically, was reduced by avocado. Lowering plasma cholesterol is attributed to soluble DF, although exceptions are known (27
). In our study, only 25% of avocado fibers are soluble; thus, even at the highest fiber level, their amount (25 g/kg diet) may not have been sufficient to decrease plasma cholesterol. Similarly, pectin (28
) at 30 g/kg diet, failed to lower plasma cholesterol levels, and in contrast, wheat and rice bran (29
) elevated plasma cholesterol in rats. High plasma cholesterol level caused by elevated hepatic cholesterol production is unlikely because diets high in cholesterol suppress hepatic cholesterol synthesis (30
). Thus, the fact that hepatic cholesterol levels were not affected may suggest that avocado interferes with plasma cholesterol clearance to the liver. The majority of blood cholesterol in rats (70–80%) is circulated in the HDL form, in contrast to 20–30% in humans, and blood cholesterol clearance by the liver requires hepatic binding of lipoproteins and is receptor-mediated. Illman et al. (31
) suggested that dietary factors, yet unknown, in wheat bran down-regulate hepatic HDL binding, and the same may apply to avocado. Surprisingly, we noticed that feeding avocado pulp with or without cholesterol, in contrast to cellulose, resulted in less hepatic fat (Fig. 1
, A and B). This suggests that avocado may also interfere with hepatic fat metabolism, perhaps by increasing VLDL secretion and, thus, providing fat to support a higher basal metabolic rate.
In this study we demonstrated that avocado pulp, used as a dietary fiber, affected food consumption, body weight gain and cholesterol and fat metabolism in male rats. The exact mechanisms are not yet understood, and additional studies are required to characterize the metabolic mode of actions.
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FOOTNOTES |
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4 Abbreviation used: DF, dietary fiber.
Manuscript received 10 October 2001. Initial review completed 4 December 2001. Revision accepted 8 April 2002.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Rainey, C., Affleck, M., Bretschger, K. & Alfin-Slater, R. B. (1994) The California avocado, a new look. Nutr. Today 29:23-27.
2. Kadam, S. S. & Salunkhe, D. K. (1995) Avocado. Salunkhe, D.K. Kadam, S.S. eds. Handbook of Fruit Science and Technology, Production, Composition, Storage and Processing 1995:363-375 Marcel Dekker Inc New York, NY. .
3. Willis, R.B.H., Lim, J.S.K. & Greenfield, H. (1986) Composition of Australian foods: tropical and subtropical fruit. Food Technol. Aust. 38:118-123.
4. Bergh, B. (1992) Nutritious Value of Avocado 1992:123-135 California Avocado Society Book CA. .
5. Davidson, M. H. & McDonald, A. (1998) Fiber: forms and functions. Nutr. Res. 18:617-624.
6. Sanchez-Castillo, C. P., Dewey, P.J.S., De Lourdes Solano, M., Finney, S. & James, W.P.T. (1995) The dietary fiber content (nonstarch polysaccharides) of Mexican fruit and vegetables. J. Food Comp. Anal. 8:284-294.
7. Kritchevsky, D. & Bonfield, C. (1995) Dietary Fiber in Health & Disease 1995 Eagan Press St. Paul, MN. .
8. Spiller, R. C. (1996) Cholesterol, fiber and bile acids. Lancet 347:415-416.[Medline]
9. Gray, D. S. (1995) The clinical uses of dietary fiber. Am. Family Phys. 51:419-426.[Medline]
10. Hillman, L. C., Peters, S. G., Fisher, C. A. & Pomare, E. W. (1986) Effects of the fiber components pectin, cellulose, and lignin on bile salts metabolism and biliary lipid composition in man. Gut 27:29-36.
11. Schneeman, B. (1998) Dietary fiber and gastrointestinal function. Nutr. Res. 18:625-632.
12. Spiller, G. A. (1991) Beyond dietary fiber. Am. J. Child. Nutr. 54:615-617.
13. Topping, D. L. (1991) Soluble fiber polysaccharides: effects on plasma cholesterol and colonic fermentation. Nutr. Rev. 49:195-202.[Medline]
14. Association of Analytical Communities (1994) Official Methods of Analysis 15th ed. 1994 Association of Analytical Communities .
15. Association of Analytical Communities (1992) 15th ed. Official Methods of Analysis 3rd suppl. Association of Analytical Communities .
16. Association of Analytical Communities (1994) 15th ed. Official Methods of Analysis 5th suppl. Association of Analytical Communities .
17. Association of Analytical Communities (1984) Official Methods of Analysis 14th ed. 1984 Association of Analytical Communities .
18. Association of Analytical Communities (1984) Official Methods of Analysis 14th ed. 1984 Association of Analytical Communities .
19. Pomerantz, Y. & Meloan, C. E. (1994) Food Analysis: Theory & Practice 1994 Chapman & Hall New York, NY. .
20. Schneeman, B. (1986) Dietary fiber: physical & chemical properties, methods of analysis and physiological effects. Food Technol. 40:104-110.
21. Reeves, P. G., Nielsen, F. H. & Fahey, G. C. (1993) AIN-93 purified diets for laboratory rodents: final report of the American institute of nutrition ad hoc writing committee on the reformulation of the AIN-76A rodent diet. J. Nutr. 123:1939-1951.
22. Seary, R. L. & Bergquist, L. M. (1960) A new color reaction for the quantification of serum cholesterol. Clin. Chem. Acta 5:192-199.[Medline]
23. Prophet, E. B., Mills, B., Arrington, J. B. & Sobin, L. H. (1992) Laboratory Methods in Histotechnology 1992 American Registry of Pathology Washington, DC. .
24. Anderson, G. H., Li, E.T.S. & Glanville, N. T. (1984) Brain mechanisms and the quantitative and qualitative aspects of food intake. Brain Res. Bull. 12:167-173.[Medline]
25. Nishina, P. M. & Freedland, R. A. (1990) The effects of dietary fiber feeding on cholesterol metabolism in rats. J. Nutr. 120:800-805.
26. Jackson, C. D., Weis, C., Poirier, L. A. & Bechtel, D. H. (1996) Interaction of varying levels of dietary fat, carbohydrate and fiber on food consumption and utilization, weight gain and fecal fat content in female Sprague Dawley rats. Nutr. Res. 16:1735-1747.
27. Lazarov, K. & Werman, M. J. (1996) Hypocholesterolemic effect of potato peels as a dietary fibre source. Med. Sci. Res. 24:581-582.
28. Tinker, L. F., Davis, P. A. & Schneeman, B. (1994) Prune fiber or pectin compared with cellulose lowers plasma and liver lipids in rats with diet-induced hyperlipidemia. J. Nutr. 124:31-40.
29. Anderson, J. W., Jones, A. E. & Riddell-Mason, S. (1994) Ten different dietary fibers have significantly different effects on serum & liver lipids of cholesterol-fed rats. J. Nutr. 124:78-83.
30. Arjmandi, B. H., Ahn, J., Nathani, S. & Reeves, R. D. (1992) Dietary soluble fiber and cholesterol affect serum cholesterol concentration, hepatic portal venous short chain fatty acid concentrations and fecal sterol excretion in rats. J. Nutr. 122:246-253.
31. Illman, R. J., Topping, D. L., Dowling, K., Trimble, R. P., Russell, G. R. & Storer, G. B. (1991) Effects of solvent extraction on the hypocholesterolemic action of oat bran in the rat. Br. J. Nutr. 65:435-443.[Medline]
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