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Department of Laboratory Animal Science, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 2, 3584 CM Utrecht, The Netherlands;
* Department of Nutrition, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 12 3584 CM Utrecht, The Netherlands;
Department of Physiology, Faculty of Veterinary Medicine, Yalelaan 1, 3584 CM Utrecht, The Netherlands; and
** Loders Croklaan b.v., Hogeweg 1, 1521 AZ Wormerveer, The Netherlands
2To whom correspondence should be addressed. E-mail: a.h.m. terpstra{at}las.vet.uu.nl.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • conjugated linoleic acid • mice • body composition
A large number of studies have demonstrated that feeding conjugated linoleic acid (CLA)2 to mice considerably lowered the proportion and amount of body fat (1–6). This body fat-lowering effect also occurs in rats (7–9), chickens (10), pigs (11), hamsters (9) and humans (12), but the effect in humans is less striking than that in mice (13).
CLA is produced by isomerization of free linoleic acid; thus it is generated as a free fatty acid (FFA). FFA have an unpleasant taste (14) and reesterification of the FFA of CLA into triacylglycerols (TAG) results in a more palatable product. Most of the studies on CLA in experimental animals, however, were done with CLA as FFA. Only a few studies in humans (15), hamsters (16), diabetic mice (2,3) and rats (7,17) have used CLA in the form of TAG.
The rate and the degree of absorption of fatty acids are affected by the type of fatty acid and the form in which it is administered. In general, short-chain fatty acids are better absorbed than long-chain fatty acids (18) and unsaturated fatty acids better then saturated ones (19) when fed as FFA. Further, the fatty acids in the sn-1 and sn-3 position of the TAG molecule are cleaved by lipases and the remaining 2-monoacylglycerol is efficiently absorbed, independent of the type of the fatty acid (20). The fatty acids released from the sn-1 and sn-3 position are absorbed as FFA and the rate of their absorption is determined by their structure. Lipases will hydrolyze CLA in the form of TAG into FFA and 2-monoacylglycerol, and it is possible that CLA as TAG may be more efficiently absorbed than CLA as FFA and may also be more effective in lowering body fat.
More than 50 years ago, Reiser (21) compared the appearance of CLA in the liver, plasma, organs and depot fat of rats fed CLA in the form of FFA or TAG. After ingestion of CLA as TAG, the maximum CLA levels in these tissues occurred after 16 h, whereas after ingestion of CLA as FFA, the maximum CLA levels occurred after 24 h. In addition, the maximum levels were also greater after ingestion of CLA as TAG. These findings suggest that the rate and possibly also the percentage of absorption of CLA in the form of TAG may be greater than that of CLA in the form of FFA.
To our knowledge, there is only one study in obese, diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats (7) that directly compared the effects of CLA in the form of FFA and TAG on visceral adipose tissue, on enzymes involved in fat oxidation and on various plasma metabolites. The two forms of CLA did not differ except that the FFA significantly increased plasma glucose levels compared with CLA as TAG. Studies in nonobese and nondiabetic animal models have not yet been done. Further, mice appear to be more susceptible to changes in body fat and lipid metabolism due to dietary CLA than rats (9); therefore, the mouse may be a more sensitive model with which to examine and detect possible differences in the effects of the two forms of CLA. In the present study, we compared the effect of CLA fed in the form of FFA and TAG on the proportion of body fat and the energy balance in Balb-C mice. In addition, we compared the effects of these two forms of CLA on plasma insulin, cholesterol, nonesterified fatty acids (NEFA) and glucose concentrations.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Animals.
Male mice (BALB/cOlaHsd: n = 45; 5 wk old) were purchased from Harlan, 5960 AD Horst, The Netherlands and housed in a temperature-controlled (21°C) animal room with a 12-h light:dark cycle (lights on 0600–1800 h). On arrival, the mice were placed in polycarbonate cages with wood chips (9 mice per cage) and fed a pelleted commercial rodent diet (Hope Farms, 3440 AB Woerden, The Netherlands) for 7 d. Subsequently, all of the mice were transferred to individual polycarbonate cages with a wire-mesh bottom and fed a semipurified control diet with 2 g/100 g high oleic sunflower oil (HOSF) for 7 d (Table 1). A polyethylene pipe with a diameter of 5 cm and a length of 14 cm was added to the cages as environmental enrichment. Then, the mice were divided into 5 groups of 9, balanced for body weight. One group was killed to collect preexperimental values on body composition and energy. The remaining 4 groups were used for the 42-d feeding trial. Mice that had been spilling food during the 7-d preexperimental period were allocated to the group that was killed at the beginning of the study. Thus, the experimental groups comprised only mice that did not spill any food.
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We used high fat semipurified diets (Table 1) containing 0.5 g (1.78 mmol) CLA/100 g diet. As a control fat, we used HOSF. The semipurified diet without the 2 g HOSF/100 g was prepared by Research Diets Services, Hoge Maat 10, 3961 NC Wijk bij Duurstede, The Netherlands. We fed CLA in the form of TAG and FFA; therefore, we had a control group with HOSF in the form of TAG and a control group with HOSF in the form of FFA (Table 2). The CLA preparations (Clarinol G-80 containing CLA as TAG and Clarinol A-80 containing CLA as FFA) and HSOF (TAG and FFA) preparations were provided by Loders Croklaan B.V. 1521 AZ Wormerveer, The Netherlands. The Clarinol A-80 preparation (FFA) contained 80.9 g (287.1 mmol) total CLA/100 g. The G-80 preparation (TAG) contained 79.4 g (269.7 mmol) total CLA/100 g (Table 2). CLA was added at the expense of the HOSF oil and 0.66 g of the Clarinol G-80 (1.78 mmol CLA) and 0.62 g of the Clarinol A-80 (1.78 mmol CLA) preparation were added per 100 g of diet. As a control group for the CLA in the form of FFA, a group was fed a diet containing 0.62 g of hydrolyzed HOSF/100 g diet at the expense of the HOSF. The CLA diets contained 122 mg trans-10, cis-12 CLA/MJ metabolizable energy
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Carcass analysis.
At the end of the study, the mice were weighed after the bladder had been emptied and killed between 0900 and 1000 h by cervical dislocation. The mice had not been deprived of food overnight; mice have a very high metabolic rate (13) and other studies have indicated that depriving mice of food overnight (16 h) reduced body weight 12% and liver weight 20% (24). The livers were removed and weighed and the carcasses were cut in pieces. Carcasses and livers were dried in a forced-hot air oven at 60°C for 3 d. The dried carcasses and livers were weighed to calculate the percentage of water, homogenized in a coffee grinder and stored in air-tight glass containers. Excreta were dried, homogenized and stored in the same way. Body composition, the retention of water, protein, fat and ash during the 42-d feeding period, and the amount of fat in the excreta were measured exactly as described previously (4).
Bomb calorimetry.
The gross energy content in the dried, homogenized carcasses, excreta and diet was determined with a bomb calorimeter (IKA Calorimeter C4000 Adiabatic, IKA Analysetechnik, Grißheimerweg 5, D-79423, Heitersheim, Germany). Benzoic acid (BHD Limited, Poole, UK) was used as a thermochemical standard. The total amount of energy that was lost as heat (heat production or energy expenditure) was calculated with the formula:
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Energy stored in the body was determined as total energy at the end of the 42-d feeding period minus the energy in the body at the beginning of the experimental period. Total body energy at the beginning of the experiment was calculated from a regression line describing the correlation between body weight and total body energy in the 9 mice that had been killed at the beginning of the study.
Analytical methods.
We also measured the concentrations of plasma total cholesterol, NEFA and insulin after the 42-d feeding period in subgroups of 6 mice each from the groups fed CLA in the form of either TAG or FFA and the group fed the HOSF oil. Blood was collected by heart puncture into tubes containing EDTA from mice under light anesthesia with diethyl ether. Insulin was measured with a kit supplied by Linco Research, St. Charles, MO (rat insulin RIA, catalog # RI-13K) and purified rat insulin was used as a standard. Plasma cholesterol (catalog # 1 489 232) and nonesterified fatty acids (catalog # 1383175) were measured with kits supplied by Roche Diagnostics GmbH, D-68298 Mannheim, Germany, and glucose (HK125, catalog # A11A00116) was measured with a kit supplied by ABX, Montpellier, France. Cholesterol, NEFA and glucose were measured on a COBAS BIO autoanalyzer, Roche, CH-4002 Basel, Switzerland.
Statistical methods.
The data of the four dietary groups were analyzed statistically using a two-way ANOVA with diet (HOSF and CLA) and form of the FFA (TAG and FFA) as independent variables. When ANOVA indicated a significant effect, the following groups were compared pairwise with correction for multiple comparisons (t test with the Bonferroni adaptation): 1) HOSF diet vs. CLA diet within each form of fatty acids and 2) TAG vs. FFA within the CLA-containing or CLA-free diets. Thus, each group was used for two comparisons; therefore, the level of significance for these multiple comparisons was preset at P < 0.025 (=0.05/2) according to the Bonferroni adaptation. Plasma lipids, glucose and insulin were measured only in the HOSF-TAG, CLA-FFA and CLA TAG groups and these three groups were compared with a one-way ANOVA. The SigmaStat statistical software package (Version 2.0, Jandel, San Rafael, CA) was used for all the statistical analyses. Values are presented as means ± SD.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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There was no effect of CLA or the form of CLA (FFA vs. TAG) on food intake, but CLA did reduce final body weight (Table 1). Absolute and relative liver weights were greater in the CLA-fed mice, but the form of CLA did not have an effect.
Mice fed CLA as both FFA and TAG had 50% less body fat and 7% more body water than those fed the respective HOSF diets (Table 3). The fatty acid form (FFA vs. TAG) did not affect the amount or proportion of body fat or water. The amount of body protein was not affected by CLA, but the proportion of body protein was greater in the CLA-fed mice than in those fed the respective HOSF diets. There was no effect of the fatty acid form (FFA vs. TAG) of CLA on body protein. CLA did not affect the amount and retention of body ash, but the proportion of body ash was greater in mice fed CLA only when CLA was fed as TAG. The form of CLA did not affect body ash; the amount, proportion and retention of body ash did not differ in the mice fed CLA as FFA or as TAG. Thus, CLA affected body composition and fat and water retention, but the form in which CLA was administered had little influence.
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98% of the weight of the carcasses (recovery in Table 3). This 2% difference may partially represent losses, but may also be due to glycogen in the body, which we did not measure. Energy balance.
Total body energy was 31% lower in the mice fed CLA as FFA and 32% lower in those fed CLA as TAG than in the respective groups fed HOSF (Table 3). The percentage of energy in the food that was stored in the body was greater in mice fed CLA than in those fed HOSF and was not affected by the fatty acid form. The percentage of energy in the food that was expended as heat was greater in mice fed CLA than in those fed HOSF; the form of the lipid did not have an effect. There was no effect of CLA and the lipid form on the percentage of energy in the food that was lost in the excreta and the apparent gross energy and apparent fat digestibility
Plasma lipids and glucose.
CLA did not affect plasma cholesterol (4.35 ± 0.39 mmol/L in the HOSF-TAG group and 4.25 ± 0.25 and 4.38 ± 0.17 mmol/L in the CLA-TAG and CLA FFA groups, respectively), NEFA (0.98 ± 0.12 mmol/L in the HOSF-TAG group and 1.00 ± 0.05 and 0.94 ± 0.15 mmol/L in the CLA-TAG and CLA FFA groups, respectively) or glucose concentrations (12.32 ± 0.93 mmol/L in the HOSF-TAG group and 13.80 ± 1.07 and 13.15 ± 1.48 mmol/L in the CLA-TAG and CLA FFA groups, respectively). Similarly, plasma insulin concentrations were not different in the groups fed the HOSF-TAG (163 ± 88 pmol/L) and CLA (330 ± 321 and 265 ± 105 pmol/L in de CLA-TAG and CLA-FFA groups, respectively).
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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We fed the mice 0.5 g CLA/100 g diet. In a previous study (4) we fed 1 g CLA/100 g diet and found that the percentage of body fat was 16% in the control group and 4% in the CLA group. In other unpublished studies, we observed that feeding mice 4 g CLA/100 g diet also resulted in 4% body fat. Thus, it appears that 4% body fat is the minimum proportion of body fat that can be achieved by feeding CLA, and this amount of fat may represent the structural fat in the body required for minimal functioning of the body. The objective of our study was to examine whether there were possible differences in the body fat-lowering and energy expenditure-increasing effects of CLA in the form of FFA or TAG. Adding 1 g CLA to 100 g of diet would possibly result in the minimum amount of 4% body fat with both forms of CLA, and differences in effect on body fat would then be difficult to detect. Therefore, we added 0.5 g CLA to 100 g diet, which resulted in a proportion of 9% body fat. Thus, there was enough room left for a lower proportion of body fat if the CLA preparation in the form of TAG had been more effective than the CLA preparation in the form of FFA.
Reiser (21) fed rats CLA in the form of TAG and FFA and compared the appearance of CLA in various tissues at various time points. The results of this study indicated that the rate and possibly also the percentage of absorption of CLA in the form of TAG was greater than that of CLA as FFA. Other studies in rats also suggest that the absorption of CLA as FFA may be slower than that of CLA as TAG. Sugano et al. (26) found that the lymphatic recovery of CLA fed as FFA had reached 30%, whereas Martin et al. (27) reported that the lymphatic recovery of CLA fed as TAG was 57% after 6 h. Our study indicates that there was no difference in effects on body fat and energy expenditure of the two forms of CLA. These findings suggest that the degree of absorption of the two forms of CLA was probably the same, but that the rate of absorption of CLA as FFA could be slower, as suggested by the studies of Reiser (21), Sugano et al. (26) and Martin et al. (27).
Rahman et al. (7) found in obese diabetic OLETF rats that feeding CLA as either FFA or TAG for 28 d resulted in similar amounts of visceral adipose tissue. Further, in that study there were no differential effects of the two forms of CLA on plasma leptin, insulin, NEFA, cholesterol and phospholipid concentrations. CLA as TAG compared with CLA as FFA tended to have a greater activity of carnitine palmitoyltransferase in liver and muscles. Plasma glucose levels, however, were significantly greater when CLA was fed as FFA, whereas CLA as TAG did not affect plasma glucose levels. In mice, we did not find a difference in plasma glucose concentrations between mice fed the two forms of CLA, but diabetic and nondiabetic rats and mice may respond differently to CLA. For example, CLA decreased plasma glucose concentrations in diabetic fatty rats (28,29), but increased it in nondiabetic rats (30). Further, CLA improved glucose tolerance in diabetic mice (3), but worsened it in nondiabetic mice (5).
In a previous study (4), we also examined the effects of CLA on body composition and energy balance in mice, but only CLA in the form of FFA was fed. In that study, 75% of the lower amount of body fat was due to a greater energy expenditure and 25% to a greater excretion of energy in the excreta. In the present study, we found that the lower amount of body energy in the mice fed CLA was due to a greater energy expenditure without differences in energy excretion. There is no clear explanation for this discrepancy in results between the two studies, but differences in the dose of CLA administered may play a role. In the present study, we had added 0.5 g CLA to 100 g of diet, whereas in the previous study, a dose of 1 g CLA/100 g diet was fed. It is possible that the effect of CLA on the excretion of energy in the feces becomes more pronounced when the dose of CLA is increased.
Feeding CLA increased relative and absolute liver weights. Similar results were reported in other studies with mice (4,5) and hamsters (16). Feeding CLA to rats (31) and pigs (32), however, did not affect liver weights. Thus, it appears that the effect of CLA on liver weight may be species dependent and it is not clear what the effects may be in humans.
Preparing CLA in the form of TAG is more time consuming and expensive, but makes the CLA preparation more palatable. CLA as TAG is more suitable for adding to various food stuffs, whereas CLA as FFA can be consumed only in the form of capsules because of the unpleasant taste. The results of our study indicate that administering CLA in the form of FFA or TAG to mice had similar effects on body fat and energy expenditure; thus, the forms of CLA can be used interchangeably, at least in mice.
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FOOTNOTES |
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3 Abbreviations used: CLA, conjugated linoleic acid; FFA, free fatty acid; HOSF, high oleic sunflower oil; NEFA, nonesterified fatty acid; OLETF rats, Otsuka Long-Evans Tokushima Fatty rats; TAG, triacylglycerol.
Manuscript received 22 May 2003. Initial review completed 27 June 2003. Revision accepted 20 July 2003.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Hamura, M. & Kudo, S. (2001) Effects of conjugated linoleic acid (CLA) on abdominal fat accumulation: comparison with other dietary oils. J. Oleo Sci. 50:129-132.
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3. Hamura, M., Yamatoya, H. & Kudo, S. (2001) Glycerides rich in conjugated linoleic acid (CLA) improve blood glucose control in diabetic C57BLKS-Leprdb/leprdb mice. J. Oleo Sci. 50:889-894.
4. Terpstra, A.H.M., Beynen, A, C., Everts, H., Kocsis, S., Katan, M. B. & Zock, P. L. (2002) The decrease in body fat in mice fed conjugated linoleic acid is due to increases in energy expenditure and energy loss in the excreta. J. Nutr. 132:940-945.
5. Tsuboyama-Kasaoka, N., Takahashi, M., Tanemura, K., Kim, H.-J., Tange, T., Okuyama, H., Kasai, M., Ikemoto, S. S. & Ezaki, O. (2000) Conjugated linoleic acid supplementation reduces adipose tissue by apoptosis and develops lipodystrophy in mice. Diabetes 49:1534-1542.[Abstract]
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