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The Journal of Nutrition Vol. 128 No. 5 May 1998,
pp. 875-880
, 5
* Department of Nutritional Sciences, Faculty of Medicine, University of Toronto, Toronto, Ontario M5S 3E2, Canada and Kunin-Lunenfeld Clinical Research Unit, Baycrest Centre for Geriatric Care, Toronto, Ontario M6A 2E1, Canada
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ABSTRACT |
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Abstract
Introduction
Methods
Results
Discussion
References
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The digestibility and absorption of dietary triacylglycerols are dependent on a number of factors including their fatty acid profile. Data demonstrating poor bioavailability of dietary stearic acid would suggest that hydrogenated oil sources would have lower digestibility coefficients compared with their native oils. To test this hypothesis, postweanling rats were fed one of four diets, formulated to contain 40% of energy as fat (assuming complete bioavailability), for 14 d. The diets only differed by fat type, containing soybean oil (SBO), fully hydrogenated soybean oil (HSB), medium-chain triglyceride oil (MCT), or hydrogenated coconut oil (HCO). Rats fed HSB consumed more food during the last 6 d (155.2 ± 2.7 g) than those in each of the other groups (MCT: 118.9 ± 2.2 g; HCO: 124.7 ± 3.2 g; SBO: 123.8 ± 2.3 g), yet, they did not gain more weight. Two-day fecal excretion was almost three times greater in HSB-fed rats than in rats fed any other diet (P < 0.0001) because HSB was very poorly available. The digestibility coefficients (a measure of bioavailability) of the four fats were: HSB (30.9 ± 1.3%) < HCO (94.5 ± 0.4%) < SBO (97.0 ± 0.4%) < MCT (98.7 ± 0.2%) (P < 0.0007). All rats compensated for the incomplete availability of the fats, as apparent absorbable energy consumed did not differ among diet groups. The present data suggest that HSB only contributes 11.6 kJ/g (most fats contribute ~37.7 kJ/g) and that not only manufactured fat substitutes, such as olestra, but also more conventional fats are incompletely available to the body. Foods that currently contain HSB may contribute much less utilizable fat and energy than presently realized.
KEY WORDS: dietary fat · digestibility · fat substitute · stearic acid · rats
The current nutrition recommendation for fat consumption in both Canada (Health and Welfare Canada 1990) and the United States (National Research Council 1989) is that the diet should contain no more than 30% of energy as fat and no more than 10% as saturated fat. In an attempt to help North American consumers meet these recommendations and decrease their risk of developing various chronic diseases, without compromising palatability, several manufacturers have developed fat-based fat substitutes that mimic the flavor and texture of conventional fats but that are inefficiently available to the body and therefore provide reduced energy. [The availability or bioavailability of a fat or fat substitute refers to the portion that has been digested, absorbed and metabolized such that it can be utilized by the body (U.S. Department of Health and Human Services 1996)].
In addition to these fat substitutes, certain conventional fats (fats that do not require extensive processing), such as cocoa butter, also have been shown to be incompletely available in rats (Apgar et al. 1987 Another potential source of tristearin is in highly hydrogenated fat sources. The hydrogenation of natural fats and oils that contain a high content of 18-carbon unsaturated fatty acids will result in an increased content of stearic acid and tristearin, presumably making these fats less available. Fully hydrogenated soybean oil (HSB),6 a fat source that currently is consumed in North America as a component of liquid shortening, would fall into this category. This oil is soybean oil (SBO) that essentially has been hydrogenated to completion (>99% saturated).
The purpose of the present investigation was to determine the bioavailability of HSB compared with three other fat sources and to determine the influence of these fats on body weight and food intake.
Rats and diets.
Postweanling male Wistar rats (Charles River, St. Constant, Quebec), initially weighing 75-100 g, were housed singly in stainless-steel wire-mesh cages (necessary to measure food spillage and fecal excretion) in a temperature-controlled environment (22 ± 1°C). They were maintained on a 12-h light:dark cycle and were given free access to all diets and water. Diets were fed from food cups that were secured with a spring to minimize spillage. The protocol was approved by the University of Toronto Animal Care Committee.
Experimental design.
Rats were assigned to one of four dietary treatment groups (n = 10 or 11 per group) such that initial body weights did not differ among groups. All rats consumed an experimental diet, containing either MCT, HCO, HSB or SBO, for 14 d. The total concentrations of saturated fatty acids were similar among the MCT, HCO and HSB diets (Table 2).
Fatty acid analysis.
After methylation with boron trifluoride in methanol, samples were dried under a stream of nitrogen and redissolved in saline and hexane (Morrison and Smith 1964 Fecal analysis.
Total 2-d freeze-dried fecal excretion from each rat was ground with a mortar and pestle into a homogenous mixture. Fecal lipid was extracted using a modification of the method of Folch et al. (1957) Statistical analysis.
Statistical analyses were conducted with SAS 6.08 (SAS Institute, Inc., Cary, NC) for the microcomputer. For all analyses, the acceptable level of significance (Type I error) was P A summary of the body weight, food intake, fecal excretion and bioavailability data for each diet and for each fat source are shown in Table 4. Because an adjustment period to the diets may have been necessary, food intake and body weights were analyzed during both the entire 14-d feeding period and during the last 6 d of feeding.
The results reported here suggest that HSB (a fat that is currently consumed in North America) is poorly digested and absorbed by rats and therefore contributes very little energy. These findings are consistent with the notion that various conventional fat sources are available to the body to different extents, and do not actually provide similar amounts of fat and energy.
The authors wish to thank Mary Ann Ryan and Stephen C. Cunnane for their advice with the fatty acid and fecal analyses and for the use of their equipment.
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
, Chen et al. 1989, Monsma et al. 1996) and to a lesser extent in humans (Mitchell et al. 1989). A number of studies suggest that the poor availability of conventional fats such as cocoa butter is related to their content of stearic acid (18:0) and specifically tristearin (Bergstedt et al. 1990, Kamei et al. 1995, Mattson 1959). Generally, the higher the content of stearic acid and tristearin in a fat, the poorer the digestion and absorption (for review see Kritchevsky 1994). Other evidence also suggests that the presence of long-chain saturated fatty acids at the 1 and 3 positions of the triacylglycerol are partly responsible for the poor absorption of cocoa butter (Bracco 1994).
MATERIALS AND METHODS
Abstract
Introduction
Methods
Results
Discussion
References
-linolenic acid (Bourre et al. 1990, 1993) and similar linoleic:-linolenic acid ratios. This was accomplished by adding a safflower-flaxseed oil (Baldwin's Natural Foods, Toronto, Ontario) mixture to each diet, except for the SBO diet. The four diets contained either medium-chain triglyceride oil (MCT; provided by Mead Johnson Canada, Toronto, Ontario), hydrogenated coconut oil (HCO; Teklad, Madison, WI), fully hydrogenated soybean oil (HSB; provided by Thomas J. Lipton Inc., Toronto, Ontario) or soybean oil (SBO; Noah's Natural Foods, Toronto, Ontario). Diet composition is shown in Table 1.
View this table:
Table 1.
Diet composition of all four diets
View this table:
Table 2.
Fatty acid composition of diets1
View this table:
Table 3.
Fatty acid composition of fat sources1
, Mitchell et al. 1989, Monsma et al. 1996)]. The final 2 d of feeding and of fecal excretion were used for this calculation. The experiment was performed in two identical phases (5 or 6 rats per group per phase). The second phase started 1 wk after the first phase started.
). Fatty acid composition of each dietary fat source was analyzed on a gas chromatograph (Hewlett Packard 5890A GC with 7673A autosampler and 3393A integrator) using a DB-23 fused silica capillary column (30 m × 0.25 mm × 0.25 µ) (J & W Scientific, Folsom, CA) and flame ionization detection. Retention times were verified with purified standards (NuChek Prep Inc., Elysian, MN). Results are expressed as percent of total fatty acids.
. A small amount of water (0.5 ml) was added to 0.5 g of freeze-dried feces. Methanol (5 ml) followed by chloroform (10 ml) was added to each tube, which was shaken and heated in a hot-water bath (60°C, 1 h) (Kamei et al. 1995). Heating was necessary to completely extract HSB because this fat does not readily dissolve in the normal chloroform-methanol Folch solution. The sample was centrifuged and reheated to dissolve any particles that may have precipitated during cooling in the centrifuge. The solution was transferred to a second test tube. No filtering was used to avoid any cooling of the solution, which would allow HSB to solidify and remain on the filter. The remaining solid residue was reextracted with methanol and chloroform and heated (60°C, 1 h). Potassium chloride in water (8.8 g/L) was added to the combined filtrates in a proportion of one-quarter of the total volume. The tube was shaken and centrifuged at 265 × g for 5 min to obtain two distinct layers. The bottom layer containing the purified lipid was transferred to a preweighed test tube and dried under nitrogen until only a solid lipid residue remained. Finally, the tube was freeze-dried overnight to ensure complete drying of the sample. The weight of total lipid was determined by difference.
) incompletely extracted HSB and was therefore unsuitable for use in this experiment.
0.05. Bartlett's test for homogeneity of variance (Rosner 1990) was used to compare group variances. When all variances were equal, comparisons of group means were made by analysis of variance followed by Student-Newman-Keuls' test for multiple comparisons (Steel and Torrie 1960). When at least two group variances were unequal, Student's t tests for samples with equal or unequal variances were used to compare each pair of means separately. Where body weights differed among groups, covariate analyses were performed to remove these differences as confounding variables.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
View this table:
Table 4.
Body weight, food intake, fecal excretion and bioavailability data for rats fed one of four diets differing in fat source for 14 d1
).
-linolenic acid deficiency, the digestibility coefficients for the respective diets do not show the bioavailability of each actual fat source. Therefore, the specific digestibility coefficient for each fat component, assuming that the flaxseed-safflower oil mixture was absorbed to the same extent as SBO also is shown in Table 4. This assumption is reasonable because flaxseed oil, safflower oil and SBO all contain similar concentrations of saturated (~9, 9, and 14%, respectively) and unsaturated fatty acids (U.S. Department of Agriculture 1979). These data show that ~70% of the HSB component of the HSB diet was not available (30.9% digestibility coefficient) to the rats that were fed this fat source. The MCT, SBO and HCO fat components were all much more available than the HSB component. The digestibility coefficient was significantly different for all four fat components; MCT had the highest coefficient, followed by SBO and HCO (P < 0.0007).
). The adjusted absorbed energy densities of each diet were determined to be (kJ/g diet): MCT, 17.8; HCO, 17.5; HSB, 13.7; and SBO, 17.7.
View this table:
Table 5.
Apparent energy absorbed over the final 6 d of feeding by rats fed one of four diets differing in fat source1
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
). Thus a longer adjustment period to MCT may be required before rats fed this diet can consume the proper amount of food to grow at a normal rate.
). Thus it appears that this fat is poorly absorbed at both high and low fat levels and when it is blended with other oils.
, 1991, Kamei et al. 1995, Mattson 1959). Because 84% of HSB is composed of stearic acid, a large component of this fat must be made up of tristearin [the highly hydrogenated soybean oil used by Kamei et al. (1995) contained 57% tristearin]. Furthermore, in the present study, the higher the content of stearic acid in a fat source, the poorer was the availability of the fat. Although MCT, HCO and HSB all contain >98% saturated fatty acids, MCT had the highest availability, followed by HCO and HSB, which mirrors the relative contents of stearic acid in these fats (Table 3). The availability of SBO, which was in between that of MCT and HCO, also reflects the intermediate content of stearic acid in this oil. In fact, the coefficient of determination between the stearic acid content of the four diets and the digestibility coefficient of the diets was very strong (r2 = 0.99, P < 0.003, determined from group means).
found that the digestibility coefficient of a fat was inversely proportional to its content of tristearin and not to stearic acid. In fact, the digestibility coefficient for hydrogenated linseed oil (91% tristearin) when fed to rats as the only fat source was found to be 15.4%.
, Chen et al. 1989, Mitchell et al. 1989, Monsma et al. 1996), raises the issue of whether nutrition labels and health claims on food products containing such fats should reflect this information. Currently, the U.S. Food and Drug Administration (FDA) requires that nutrition labels and nutrient content claims on all products containing fat must reflect total fat content based on "all fatty acids obtainable from a total lipid extraction" (U.S. Department of Health and Human Services 1996). The one exception to this rule is that the manufactured fat-based fat substitute, olestra [a nonabsorbed sucrose polyester composed of six to eight fatty acids bound to sucrose by ester bonds (Bergholz 1992)], does not have to be reported as a source of fat or energy. In addition to olestra, however, other fat-based fat substitutes (e.g., salatrim and caprenin) have been developed that are also incompletely available. As a result, the U.S. FDA is currently considering a proposal that would allow manufacturers of such fat substitutes to report the fat and energy content of foods containing these products based on their availability rather than on the actual fat content of the foods (U.S. Department of Health and Human Services 1996). The rationale behind implementing such a change in the regulations is based on the assumption that the benefits of foods with less available fat are similar to the benefits of foods with less total fat.
, Chen et al. 1989, Monsma et al. 1996), whereas one study has suggested that the coefficient is 89% when consumed by humans (Mitchell et al. 1989). Hence, much more testing would be necessary to determine a proper coefficient, and if a value close to 89% is accurate, then the benefits of using availability rather than actual fat content may indeed be too small to warrant a change in nutrition labeling regulations.
for review]. However, this is not a concern with completely hydrogenated fats because they are virtually devoid of trans fatty acids. Another concern with HSB is its high content of saturated fatty acids, which is often linked to high cholesterol levels. However, most of HSB contains stearic acid, which has been shown in numerous studies to be less cholesterolemic and less atherogenic than other saturated fatty acids (for review see Kritchevsky 1994). Nevertheless, it is possible that the absorbed portion of HSB may contain more cholesterolemic fatty acids, such as palmitate. However, it has been reported that a highly hydrogenated soybean oil prevented an expected dietary cholesterol-induced rise in plasma total cholesterol and reduction in HDL-cholesterol (Kamei et al. 1995). Finally, there is some concern that a poorly digestible fat such as HSB could potentially decrease the absorption of fat-soluble vitamins. If this is the case, foods containing a high concentration of HSB could be supplemented with vitamins as are products containing olestra.
ACKNOWLEDGMENTS
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FOOTNOTES |
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Manuscript received 27 March 1997. Initial reviews completed 9 June 1997. Revision accepted 8 January 1998.
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LITERATURE CITED |
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Abstract
Introduction
Methods
Results
Discussion
References
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