![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
Institute of Nutritional Sciences, Martin-Luther-University of Halle-Wittenberg, D-06108 Halle/Saale, Germany
1To whom correspondence should be addressed. E-mail: eder{at}landw.uni-halle.de.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
-tocopherol (P < 0.05) in liver and plasma than rats fed lard diets. The iron concentration of the diet did not influence the concentrations of TBARS, the activities of superoxide dismutase and glutathione peroxidase or the concentration of -tocopherol in plasma or liver. The activity of catalase (P < 0.01) and the concentrations of total, oxidized and reduced glutathione (P < 0.05) in liver were slightly but significantly higher in rats fed high iron diets than in rats fed adequate iron diets, irrespective of the dietary fat. Rats fed the high iron diets with salmon oil, moreover, had higher concentrations of various COP in the liver (P < 0.001) than rats fed adequate iron diets with salmon oil. These results suggest that feeding a high iron diet does not generally affect the antioxidant status of rats but enhances the formation of COP, particularly if the diet is rich in polyunsaturated fatty acids.
KEY WORDS: • iron • lard • salmon oil • antioxidant status • cholesterol oxidation products • rats
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
), but there are some instances in which iron overload is considered a health issue as well. Iron overload occurs primarily in individuals with inherited disorders such as hereditary hemochromatosis, juvenile hemachromatosis, atransferrinemia and Friedreich’s ataxia (2
). A high dietary intake of iron through meat or nutritional supplements is also a potential cause of iron overload. Iron promotes the generation of oxygen radicals (especially hydroxyl radicals via Fenton chemistry), which may cause oxidative damage such as degradation of proteins and nucleic acids and peroxidation of polyunsaturated fatty acids (PUFA)2
(3
,4
). There are several animal and epidemiologic studies suggesting that a high iron status may increase the risk of heart disease (5
,6
) and cancer, especially colon carcinoma (7
,8
), whereas others did not confirm this (1
, 9
). One reason for the discrepancy between different feeding studies could be the fat used in the diet, because the effect of high dietary iron concentrations on the antioxidant system might be dependent on the type of fat. PUFA are a source of oxidative stress. The effect of high dietary iron is therefore likely to be strongest when diets with high concentrations of PUFA are used. Little published information is available to date about the interactions between high dietary iron concentrations and different dietary fats. The present study was therefore carried out to determine the effect of high iron diets on the antioxidant status of rats fed two different dietary fats, either lard or salmon oil.
Several studies have investigated the effect of iron on lipid peroxidation (10
–12
). In many of these studies, the generation of thiobarbituric acid-reactive substances (TBARS) is a preferred method for estimation of lipid peroxidation. But in addition to PUFA, cholesterol may also undergo oxidative modifications. Cholesterol oxidation products (COP) have been demonstrated to possess a wide variety of biological effects and are especially implicated in the pathogenesis of atherosclerosis via oxides in LDL (13
). The formation of COP may occur through enzymatic or nonenzymatic oxidation. Nonenzymatic processes include autoxidations involving several active oxygen species. It is believed that transition metal ions such as iron act as catalysts in tissue. Iron accumulation in tissues can therefore be expected to enhance the formation of COP. Studies about the relationship between the iron supply of animals and the formation of COP are lacking. Miyajima et al. (14
) described enhanced concentrations of 7ß-hydroxycholesterol and 7-ketocholesterol in brains and visceral organs of patients with aceruloplasminemia, a phenomenon characterized by excessive neurovisceral iron accumulation. The concentrations of these COP correlated with the amount of iron accumulated in various organs. The authors concluded that lipid peroxidation induced by the intracellular accumulation of iron is involved in the pathogenesis of aceruloplasminemia. It seems possible that raised iron levels in tissues as a result of dietary overload may also lead to enhanced lipid peroxidation and enhanced generation of COP. Our study therefore included the measurement of COP in the liver.
In general, animal studies allow standardized treatments, although many different regimens have been described. Iron overload can be achieved with different iron sources and different methods of administration. A feeding trial would be the method of choice for testing the effects of iron overload through supplementation. It is important to consider the possibility that high levels of iron may promote oxidation of the diet even before it has been fed to the animals. In preliminary studies, we prepared diets with a high iron concentration (500 mg/kg diet) and PUFA-rich salmon oil (100 g/kg diet). After 7 d storage at 4°C, the diets had peroxide values > 1200 mEq O2/kg oil. Even when salmon oil was replaced with lard, the diets had peroxide values of 
260 mEq O2/kg oil. Feeding diets with high concentrations of lipid peroxidation products has profound effects on the antioxidant system (15
). Therefore, if diets with high concentrations of lipid peroxidation products are used, it is impossible to determine which effects are caused by high dietary iron concentrations per se and which are caused by dietary lipid peroxidation products. In the present study, such methodological problems were avoided by providing iron and fat in two separate dietary components.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Adult male Sprague-Dawley rats (n = 40) with an average initial body weight of 244 ± 7.7 g (mean ± SD), obtained from Charles River GmbH (Sulzfeld, Germany), were randomly assigned to one of four groups of 10 rats each. They were housed individually in Macrolon cages in a room controlled for temperature (22 ± 2°C), humidity and light (12-h light:dark cycle). All animal procedures described followed established guidelines for the care and handling of laboratory animals and were approved by the regional council of Saxony-Anhalt.
Diets.
According to a bifactorial experimental design, four diets were used, differing in their iron supplement and their fat (Table 1
). Iron was supplemented at a concentration of either 50 mg/kg diet ("adequate iron diets") or 500 mg/kg diet ("high iron diets"). Ferrous sulfate was used as the iron source because it is highly bioavailable and often used in iron supplements and for food fortification. The basal iron concentration of the diet was 10 mg/kg. The total iron concentrations of the diets were 60 and 510 mg/kg in the iron-adequate and the high iron diets, respectively. The dietary fat was either lard (obtained from O. Stiegele Schmalzsiederei, Dresden, Germany) or salmon oil (obtained from Caelo GmbH, Hilden, Germany), at a concentration of 100 g/kg diet. The fatty acid composition and the tocopherol concentrations of the fats are shown in Table 2
. We equalized the tocopherol concentrations of the fats by supplementing them individually with all-rac--tocopheryl acetate to a final concentration of 400 mg -tocopherol equivalents/kg oil [allowing for the fact that the biopotency of all-rac--tocopheryl acetate in rats is 67% of that of -tocopherol (16
,17
)]. Both types of diet contained 40 mg -tocopherol equivalents per kg. To avoid autoxidation of dietary PUFA, fat and iron were administered in two separate diet portions, which were administered in identical amounts. The two portions combined yielded the whole diet of the rats.
|
|
Diets were administered in restricted amounts to standardize the feed intake. Feeding took place once daily at 0800 h. The amount of food offered daily was 18 g. Water was freely available from nipple drinkers. The diets were fed for 12 wk.
Sample preparation.
Feces of five rats per group were collected from wk 8 to 12 to determine the digestibility of the iron. At the end of the feeding period, the rats were deprived of food overnight, anesthetized with diethyl ether and killed by decapitation. Blood was collected into heparinized polyethylene tubes and EDTA-treated tubes. Plasma was prepared by centrifuging the heparinized blood (1100 x g, 10 min), and the remaining red blood cells (RBC) were washed three times with 9 g/L sodium chloride solution. Liver and duodenum were removed. Identical sections of the duodenum were cut open lengthwise and mucosa was scraped from duodenal tissue with a clean microscope slide. All samples were immediately shock-frozen in liquid nitrogen and stored at -20°C pending analysis.
Hematological variables.
Whole EDTA blood was used immediately for hematocrit and hemoglobin determination as well as counting of different cell type numbers using "Buffy Coat" profiles with the VetAutoRead hematology system (Becton Dickinson, Wörrstadt, Germany). After uptake of the fluorescent dye acridine orange, different cell types were identified by their different fluorescent properties.
Iron analysis.
Atomic absorption spectrometry (model # 3300, Perkin Elmer, Rodgau-Jügesheim, Germany) was used to determine the iron concentrations in diets, liver and feces. Aliquots of freeze-dried liver were dissolved in nitric acid and ashed under pressure at 170°C for 6 h. The ashes were dissolved in deionized water. Aliquots of diet and dried fecal samples were ashed at 550°C for 16 h. The ashes were dissolved in hydrochloric acid at boiling temperature. All samples were filtered, measured at 248.3 nm, and calculated with reference to standards.
Antioxidant status.
Concentrations of -tocopherol in plasma, liver, and dietary fats were determined by HPLC (HP 1100, Hewlett Packard, Waldbronn, Germany) (18
). Samples (200 µL plasma, 50 mg liver) were mixed with 1 mL of 0.1 g/L pyrogallol solution (ethanol, absolute) and 150 µL of saturated sodium hydroxide solution. This mixture was heated for 30 min at 70°C, and tocopherols were extracted with n-hexane. Dietary fat samples were diluted 1:100 with n-hexane. Individual tocopherols of the extracts were separated isocratically using a mixture of n-hexane and 1,4 dioxane (96:4, v/v) as the mobile phase and a LiChrosorb Si-60 column (5-µm particle size, 250 mm length, 4 mm i.d., Merck, Darmstadt, Germany), and detected by fluorescence (excitation wavelength, 295 nm; emission wavelength, 330 nm).
Catalase activity was determined in liver homogenates at 25°C using hydrogen peroxide as the substrate by the method of Beers and Sizer (19
). One unit of catalase activity is defined as the amount consuming 1 µmol hydrogen peroxide/min. Total superoxide dismutase (SOD) activities of duodenal mucosa homogenate and RBC were determined using the method of Marklund and Marklund (20
) with pyrogallol as the substrate. One unit of SOD activity is defined as the amount of enzyme required to inhibit the autoxidation of pyrogallol by 50%. Glutathione peroxidase (GSH-Px) activity in plasma was measured with t-butyl hydroperoxide at 25°C according to the method of Paglia and Valentine (21
) as modified by Levander et al. (22
). One unit of GSH-Px activity is defined as 1 µmol NADPH oxidized/min. Total glutathione (GSH) concentration was determined in protein-free liver homogenates according to Griffith (23
) with glutathione reductase and Ellman’s reagent. Calibration was performed using a standard curve. Oxidized glutathione (GSSG) was detected after derivatization of GSH with 2-vinylpyridine. The concentration of reduced GSH was calculated as total GSH - 2 x GSSG. Sample protein content was determined according to the method of Lowry et al. (24
). Enzyme activities of RBC, liver, and mucosa were expressed per milligram or gram protein.
Lipid analysis.
The fatty acid composition of the dietary fats was determined by gas chromatography. Fats were methylated with trimethylsulfonium hydroxide (25
). Fatty acid methyl esters (FAME) were separated by gas chromatography, using a system (HP 5890, Hewlett-Packard GmbH, Böblingen, Germany) equipped with an automatic on-column injector, a polar capillary column (30 m FFAP, 0.53 mm i.d., Macherey and Nagel, Düren, Germany), and a flame ionization detector. Helium was used as the carrier gas with a flow rate of 5.4 mL/min. FAME were identified by comparing their retention times with those of individually purified standards. Concentrations of total cholesterol and triglycerides in plasma were determined using commercially available enzymatic reagent kits (Merck; Cat. Nos. 1.14830, 1.14856).
Thiobarbituric acid reactive substances.
TBARS were measured in plasma, liver, and mucosal homogenates using a modified version of the TBARS assay (26
). Sample aliquots were mixed with thiobarbituric acid reagent (8 g/L thiobarbituric acid with 7% perchloric acid, 2:1,v/v) and heated for 60 min at 95°C. TBARS were extracted with n-butanol, and absorption was measured at 532 nm. Concentrations were calculated via a standard curve with 1,1,3,3,-tetraethoxypropan.
Cholesterol oxidation products.
COP were determined in liver by gas chromatography/mass spectrometry in selected ion monitoring mode according to Mori et al. (27
) with modifications. Total lipids of liver were extracted with a hexane/isopropanol mixture (3:2, v/v) (28
). Aliquots of the extracts were mixed with 20 µg of the internal standard 5-cholestane. The probes were dried under nitrogen, mixed with 2 mL of 1 mol/L methanolic potassium hydroxide, and incubated for 18 h at room temperature. Cholesterol oxides were extracted with 2 mL diethyl ether into the nonsaponifiable fraction, separated, dried under nitrogen, and dissolved in 100 µL pyridine. After derivatization of the oxides with 100 µL 0.1 g/L trimethylchlorosilane solution [bis(trimethylsilyl)-trifluoroacetamide], samples were separated using a nonpolar capillary column (DB-5, 30 m x 0.25 mm i.d., J&W Scientific, Folsom, CA) and detected by single ion monitoring. Helium was used as the carrier gas at a flow rate of 1.0 mL/min. The cholesterol oxides 7-ß-hydroxycholesterol, 7-ketocholesterol, cholesterol-5,6-epoxide, cholesterol-5ß,6ß-epoxide, cholestanetriol, and 25-hydroxycholesterol were identified by comparing their retention times with those of authentic standards and quantified with the internal standard.
Statistics.
Treatment effects were analyzed by two-way ANOVA using the Minitab statistical software (Release 13, Minitab, State College, PA). Classification factors were iron level and fat type, as well as their interaction. Means of the four treatment groups were compared by Fisher’s multiple range test. Differences were considered significant if P < 0.05. Values in the text are means ± SD.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Body weight gains during the experimental period were slightly, but significantly higher in rats fed the high iron diets than in the rats fed the iron-adequate diets (n = 20; adequate iron diets, 212 ± 20.4 g; high iron diets, 227 ± 16.8 g; P < 0.05). Rats fed salmon oil diets had higher body weight gains than rats fed lard diets (n = 20; salmon oil diets, 231 ± 16.1 g; lard diets, 208 ± 17.4 g; P < 0.05).
Iron status.
Liver iron concentration was significantly influenced by dietary iron concentration and dietary fat (Table 3
). It was significantly higher in rats fed high iron diets than in rats fed adequate iron diets. The difference between rats fed the high iron diets and rats fed the adequate iron diets was 20%. Rats fed the lard diet had significantly higher hepatic iron concentrations than rats fed the salmon oil diet. Fecal iron concentration, iron excretion, and apparent iron digestibility were significantly affected by the dietary iron concentration, but not by the dietary fat type. The iron concentration of the feces and the amount of iron excreted with the feces were approximately four times higher in rats fed the high iron diets than in rats fed the adequate iron diets. In the rats fed the adequate iron diets, 25% of the iron was apparently absorbed; in the rats fed the high iron diets, the apparent digestibility of iron was slightly negative, indicating that in a given period, they excreted more iron than they consumed.
|
Hematological variables did not differ among the four groups (overall means ± SD, n = 40; hematocrit, 0.44 ± 0.05; hemoglobin, 14.5 ± 1.93 g/L; reticulocytes, 2.84 ± 1.08%; leukocyte count, 7.46 ± 3.71 x 109/L; granulocyte count, 5.18 ± 2.88 x 109/L). The numbers of lymphocytes and monocytes, however, were higher in the rats fed salmon oil (2.82 ± 1.49 x 109/L) than in the rats fed lard (1.73 ± 0.58 x 109/L, P < 0.05).
Antioxidant status.
The concentrations of TBARS in plasma, liver, and mucosa were not influenced by the dietary iron concentration (Table 4
), but the dietary fat affected the concentrations of TBARS in plasma and liver. Rats fed salmon oil diets had significantly higher concentrations of TBARS in plasma and liver than rats fed lard diets. The concentrations of TBARS in the mucosa did not differ among the four treatment groups.
|
). The concentrations of all COP measured were one- to threefold higher in rats fed salmon oil diets than in rats fed lard diets. The effect of the dietary iron on the concentrations of some COP depended on the type of dietary fat. Rats fed the salmon oil diet with high iron concentration had significantly higher concentrations of 7-ß-hydroxycholesterol, 7-ketocholesterol, and cholestanetriol in the liver than rats fed the salmon oil diet with adequate iron. In contrast, in rats fed the lard diet with high iron concentration, only the concentration of cholestanetriol was significantly higher than in rats fed the lard diet with adequate iron; the concentrations of 7-ß-hydroxycholesterol and 7-ketocholesterol did not differ between these two groups. The concentrations of cholesterol epoxides were not affected by the dietary iron concentration. There was a significant interaction between iron and fat on the concentrations of 7-ß-hydroxycholesterol and 7-ketocholesterol.
|
). Rats fed high iron diets had significantly greater activity than rats fed adequate iron diets. Dietary fat had no effect on hepatic catalase activity. The activities of GSH-Px in plasma as well as of SOD in RBC and mucosa were not affected by either dietary iron or dietary fat (overall means ± SD, n = 40; GSH-Px, 2.51 ± 1.01 kU/L; RBC SOD, 9.97 ± 2.13 U/mg protein; mucosa SOD, 7.62 ± 2.23 U/mg protein).
|
). Rats fed the high iron diets had significantly greater concentrations of total GSH, reduced GSH, and GSSG in the liver than rats fed adequate iron diets. Rats fed salmon oil diets had significantly higher concentrations of total GSH as well as GSSG than rats fed lard diets; the concentration of reduced GSH was not affected by dietary fat.
|
-tocopherol in plasma and liver were not influenced by dietary iron. However, rats fed the lard diets had higher concentrations of -tocopherol in liver and plasma than rats fed the salmon oil diets (means ± SD, n = 20; liver, 1.71 ± 0.54 vs. 0.71 ± 0.21 µmol/mmol triglycerides plus cholesterol, P < 0.001; plasma, 6.73 ± 1.64 vs. 5.08 ± 1.56 µmol/mmol triglycerides plus cholesterol, P < 0.05). |
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Surprisingly, feeding the high iron diets (500 mg iron supplemented/kg) increased the hepatic iron concentration only 20% compared with feeding the iron-adequate diets (50 mg iron supplemented/kg). The hepatic iron concentrations in rats fed the high iron diets were lower than those observed in similar studies in which rats were fed high iron diets (9
, 10
). The difference between those studies and our results could be attributable to the age of the rats used; in our study, adult rats were used whereas, Bristow-Craig et al. (10
) and Soyars and Fisher (9
) studied growing rats. The moderate accumulation of iron in the livers of rats fed the high iron diets could also be attributed to a marked reduction in the apparent digestibility of iron induced by a high iron intake. A net loss of iron was observed in rats fed the high iron diets, suggesting a mechanism for preventing iron overload via increased excretion. The apparent digestibility of iron depends on the body’s iron stores (1
). In a study by Reddy and Cook (29
), feeding a diet with 500 mg iron/kg lowered the rate of iron digestibility from 50 to 27% within only 1 wk; in a study by Rimbach and Pallauf (30
), increasing the dietary iron concentration from 30 to 300 mg/kg reduced the apparent digestibility of iron from 25 to 8%. The observation that rats fed salmon oil had lower concentrations of hepatic iron than rats fed lard is consistent with other published studies that demonstrated that fish oil reduces the absorption of iron (31
,32
). The amounts of iron excreted with the feces in wk 8 and 12 of the experiment, however, were not different between rats fed lard and those fed salmon oil. The differences in the absorption of iron may have occurred during the earlier phase of the experiment as a result of the dietary fat used, which would explain the different hepatic iron concentrations.
As expected, feeding the salmon oil diets caused a marked oxidative stress compared with feeding the lard diets. This was shown by greater concentrations of TBARS in liver and plasma, reduced concentrations of -tocopherol in liver and plasma and greater concentrations of total and oxidized glutathione in the liver. This finding agrees with many other studies that also reported oxidative stress in animals fed diets containing marine oils (12
, 33
). The present study, moreover, showed for the first time that dietary salmon oil also enhances the formation of COP in the liver. Cholesterol is susceptible to oxidation under a variety of conditions (13
). In contact with air, cholesterol may autoxidize via various active oxygen species, whereas in cell membranes, cholesterol can be oxidized either enzymatically or via free radical–mediated peroxidation processes (13
,34
). We assume that the oxidation of cholesterol was initiated by the autoxidation of highly unsaturated fatty acids, which are very susceptible to oxidation. Feeding fish oil enriches cell membranes in highly unsaturated fatty acids such as eicosapentaenoic acid or docosahexaenoic acid (35
).
The high iron diet did not dramatically stress the antioxidant system, irrespective of the dietary fat as shown by minor or no changes in TBARS, GSH-Px, SOD, GSH or -tocopherol. This agrees with some other studies performed with rats that also did not find effects of a moderate dietary iron excess (
400 mg iron/kg diet) on activities of antioxidant enzymes or concentrations of -tocopherol, glutathione, and malondialdehyde in the liver (10
,12
). In contrast to the other antioxidant enzymes measured, the activity of catalase was increased by feeding high iron diets. This may have been because of increased iron concentrations in the liver rather than to induction by oxidative stress because the activity of catalase depends on iron status.
The finding that the concentrations of TBARS in intestinal mucosa were not elevated by feeding high iron diets suggests that the high iron concentrations in the chyme did not enhance lipid peroxidation in the intestine. Some other studies (36
,37
) suggest that high iron concentrations in the intestine enhance the formation of lipid peroxides in intestinal mucosa.
The present study showed for the first time that dietary iron excess could enhance the formation of COP in the liver, particularly in rats fed a diet rich in PUFA. The fact that the effect of high iron diets on the formation of COP was stronger in rats fed salmon oil than in rats fed lard also suggests that the formation of COP is a consequence of oxidation of PUFA. Interestingly, increased concentrations of 7-hydroxycholesterol and 7-ketocholesterol were also found in brains and visceral organs of aceruloplasminemic patients (14
) who have extremely high concentrations of iron in brain and visceral organs, which are strongly prooxidative and enhance the formation of lipid peroxidation products. Our study shows that even a modest increase in dietary iron enhances the formation of COP in the liver when the antioxidant system is stressed by other factors, such as dietary highly unsaturated fatty acids.
Overall, the results of our research indicate that diets with high concentrations of highly unsaturated fatty acids stimulate the formation of COP in the liver and that simultaneous dietary iron excess amplifies this effect. This is critical given that several unfavorable effects of COP have been described. They are associated with mutagenic and carcinogenic events (38
–40
) and are involved in the pathogenesis of atherosclerosis (41
,42
). COP are transported as constituents of lipoproteins (43
) and may therefore leave the liver. Once they have entered the bloodstream, they can exert their toxic effects on various cell types, as described in in vitro studies of endothelial cells (44
) and smooth muscle cells (45
). Studies investigating the effects of PUFA and of high dietary iron concentrations should therefore include an investigation of hepatic cholesterol oxides.
|
FOOTNOTES |
|---|
Manuscript received 1 February 2002. Initial review completed 4 March 2002. Revision accepted 30 April 2002.
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
1. Yip, R. & Dallman, P. R. (1996) Iron. Ziegler, E. E. Filer, L. J., Jr. eds. Present Knowledge in Nutrition 1996:277-292 ILSI Press Washington, DC .
2. Sheth, S. & Brittenham, G. M. (2000) Genetic disorders affecting proteins of iron metabolism: clinical implications. Annu. Rev. Med. 51:443-464.[Medline]
3. Halliwell, B. & Gutteridge, J. M. (1986) Oxygen free radicals and iron in relation to biology and medicine: some problems and concepts. Arch. Biochem. Biophys. 246:501-514.[Medline]
4. Toyokuni, S. (1996) Iron-induced carcinogenesis: the role of redox regulation. Free Radic. Biol. Med. 20:553-566.[Medline]
5. de Valk, B. & Marx, J. J. (1999) Iron, atherosclerosis, and ischemic heart disease. Arch. Intern. Med. 159:1542-1548.
6. Ascherio, A., Willett, W. C., Rimm, E. B., Giovannucci, E. L. & Stampfer, M. J. (1994) Dietary iron intake and risk of coronary disease among men. Circulation 89:969-974.
7. Nelson, R. L. (1992) Dietary iron and colorectal cancer risk. Free Radic. Biol. Med. 12:161-168.[Medline]
8. Wurzelmann, J. I., Silver, A., Schreinemachers, D. M., Sandler, R. S. & Everson, R. B. (1996) Iron intake and the risk of colorectal cancer. Cancer Epidemiol. Biomark. Prev. 5:503-507.[Abstract]
9. Soyars, K. E. & Fischer, J. G. (1998) Iron supplementation does not affect cell proliferation or aberrant crypt foci development in the colon of Sprague-Dawley rats. J. Nutr. 128:764-770.
10. Bristow-Craig, H. E., Strain, J. J. & Welch, R. W. (1994) Iron status, blood lipids and endogenous antioxidants in response to dietary iron levels in male and female rats. Int. J. Vitam. Nutr. Res. 64:324-329.[Medline]
11. Dabbagh, A. J., Mannion, T., Lynch, S. M. & Frei, B. (1994) The effect of iron overload on rat plasma and liver oxidant status in vivo. Biochem. J. 300:799-803.
12. Ibrahim, W., Lee, U. S., Yeh, C. C., Szabo, J., Bruckner, G. & Chow, C. K. (1997) Oxidative stress and antioxidant status in mouse liver: effects of dietary lipid, vitamin E and iron. J. Nutr. 127:1401-1406.
13. Peng, S.-K. Morin, R.-J. eds. Biological Effects of Cholesterol Oxides 1992 CRC Press London, UK. .
14. Miyajima, H., Adachi, J., Kohno, S., Takahashi, Y., Ueno, Y. & Naito, T. (2001) Increased oxysterols associated with iron accumulation in the brains and visceral organs of acaeruloplasminaemia patients. Q. J. Med. 94:417-422.
15. Yoshida, H. & Kajimoto, G. (1989) Effect of dietary vitamin E on the toxicity of autoxidized oil to rats. Ann. Nutr. Metab. 33:153-161.[Medline]
16. Horwitt, M. K. (1976) Vitamin E: a reexamination. Am. J. Clin. Nutr. 29:569-578.
17. Weiser, H. & Vecchi, M. (1982) Stereoisomers of alpha-tocopheryl acetate. II. Biopotencies of all eight stereoisomers, individually or in mixtures, as determined by rat resorption-gestation tests. Int. J. Vitam. Nutr. Res. 52:351-370.[Medline]
18. Balz, M. K., Schulte, E. & Thier, H. P. (1993) Simultaneous determination of -tocopherol acetate, tocopherols and tocotrienols by HPLC with fluorescence detection in foods. Fat Sci. Technol. 95:215-220.
19. Beer, R. F. & Sizer, I. W. (1952) A spectrophotometric method for measuring the breakdown of hydrogen by catalase. J. Biol. Chem. 195:133-140.
20. Marklund, S. L. & Marklund, G. (1974) Involvement of superoxide anion radical in the autoxidation of pyrogallol and a convenient assay of superoxide dismutase. Eur. J. Biochem. 47:469-474.[Medline]
21. Paglia, D. E. & Valentine, W. N. (1967) Studies on the quantitative and qualitative characterization of erythrocyte glutathione peroxidase. J. Lab. Clin. Med. 70:158-169.[Medline]
22. Levander, O. A., DeLoach, D. P., Morris, V. C. & Moser, P. B. (1983) Platelet glutathione peroxidase activity as an index of selenium status in rats. J. Nutr. 113:55-63.
23. Griffith, O. W. (1986) Glutathione and glutathione disulfide. Bergmeyer, H. U. eds. Methods of Enzymatic Analysis 3rd ed. 1986:521-529 VHC Verlagsgesellschaft mbH Weinheim, Germany. .
24. Lowry, O. H., Rosebrough, N. J., Farr, A. L. & Randall, R. J. (1951) Protein measurement with the Folin phenol reagent. J. Biol. Chem. 293:265-275.
25. Butte, W. (1983) Rapid method for the determination of fatty acid profiles from fats and oils using trimethylsulfonium hydroxide for transesterification. J. Chromatogr. 261:142-145.
26. Halliwell, B. Gutteridge, J. M. C. eds. Free Radicals in Biology and Medicine 1989 Clarendon Press Oxford, UK. .
27. Mori, T. A., Croft, K. D., Puddey, I. B. & Beilin, L. J. (1996) Analysis of native and oxidized low-density lipoprotein oxysterols using gas chromatography-mass spectrometry with selective ion monitoring. Redox Rep 2:25-34.
28. Hara, A. & Radin, S. N. (1978) Lipid extraction of tissues with a low-toxicity solvent. Anal. Biochem. 90:420-426.[Medline]
29. Reddy, M. B. & Cook, J. D. (1991) Assessment of dietary determinants of nonheme-iron absorption in humans and rats. Am. J. Clin. Nutr. 54:723-728.
30. Rimbach, G. & Pallauf, J. (1998) Phytic acid inhibits free radical formation in vitro but does not affect liver oxidant or antioxidant status in growing rats. J. Nutr. 128:1950-1955.
31. Araujo, J. A., Romano, E. L., Brito, B. E., Parthe, V., Romano, M., Bracho, M., Montano, R. F. & Cardier, J. (1995) Iron overload augments the development of atherosclerotic lesions in rabbits. Arterioscler. Thromb. Vasc. Biol 15:1172-1180.
32. Sullivan, J. L. (1989) The iron paradigm of ischemic heart disease. Am. Heart J. 117:1177-1188.[Medline]
33. Mouri, K., Ikesu, H., Esaka, T. & Igarashi, O. (1984) The influence of marine oil intake upon levels of lipids, -tocopherol and lipid peroxidation in serum and liver of rats. J. Nutr. Sci. Vitaminol. 30:307-318.
34. Schroepfer, G. J. (2000) Oxysterols: modulators of cholesterol metabolism and other processes. Physiol. Rev. 80:361-554.
35. Kaasgaard, S. G., Holmer, G., Hoy, C. E., Behrens, W. A. & Beare-Rogers, J. L. (1992) Effects of dietary linseed oil and marine oil on lipid peroxidation in monkey liver in vivo and in vitro. Lipids 27:740-745.[Medline]
36. Lund, E. K., Fairweather-Tait, S. J., Wharf, S. G. & Johnson, I. T. (2001) Chronic exposure to high levels of dietary iron fortification increases lipid peroxidation in the mucosa of the rat large intestine. J. Nutr. 131:2928-2931.
37. Rimbach, G., Markant, A., Most, E. & Pallauf, J. (1997) Liver and colon oxidant status in growing rats fed increasing levels of dietary iron. J. Trace Elem. Med. Biol. 11:99-104.[Medline]
38. Morin, R. J., Hu, B., Peng, S. K. & Sevanian, A. (1991) Cholesterol oxides and carcinogenesis. J. Clin. Lab. Anal. 5:219-225.[Medline]
39. Sevanian, A. & Peterson, A. R. (1986) The cytotoxic and mutagenic properties of cholesterol oxidation products. Food Chem. Toxicol. 24:1103-1110.[Medline]
40. Gruenke, L. D., Wrensch, M. R., Petrakis, N. L., Miike, R., Ernster, V. L. & Craig, J. C. (1987) Breast fluid cholesterol and cholesterol epoxides: relationship to breast cancer risk factors and other characteristics. Cancer Res. 47:5483-5487.
41. Brown, A. J. & Jessup, W. (1999) Oxysterols and atherosclerosis. Atherosclerosis 142:1-28.[Medline]
42. Peng, S. K., Hu, B. & Morin, R. J. (1991) Angiotoxicity and atherogenicity of cholesterol oxides. J. Clin. Lab. Anal. 5:144-152.[Medline]
43. Peng, S. K., Phillips, G. A., Xia, G. Z. & Morin, R. J. (1987) Transport of cholesterol autoxidation products in rabbit lipoproteins. Atherosclerosis 64:1-6.[Medline]
44. Sevanian, A., Berliner, J. & Peterson, H. (1991) Uptake, metabolism, and cytotoxicity of isomeric cholesterol-5,6-epoxides in rabbit aortic endothelial cells. J. Lipid Res. 32:147-155.[Abstract]
45. Hughes, H., Mathews, B., Lenz, M. L. & Guyton, J. R. (1994) Cytotoxicity of oxidized LDL to porcine aortic smooth muscle cells is associated with the oxysterols 7-ketocholesterol and 7-hydroxycholesterol. Arterioscler. Thromb. 14:1177-1185.
This article has been cited by other articles:
|
|
 |
|
  A. Koch, B. Konig, J. Spielmann, A. Leitner, G. I. Stangl, and K. Eder Thermally Oxidized Oil Increases the Expression of Insulin-Induced Genes and Inhibits Activation of Sterol Regulatory Element-Binding Protein-2 in Rat Liver J. Nutr., September 1, 2007; 137(9): 2018 - 2023. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. Konig, H. Kluge, K. Haase, C. Brandsch, G. I. Stangl, and K. Eder Effects of Clofibrate Treatment in Laying Hens Poult. Sci., June 1, 2007; 86(6): 1187 - 1195. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. Ringseis, D. Saal, A. Muller, H. Steinhart, and K. Eder Dietary Conjugated Linoleic Acids Lower the Triacylglycerol Concentration in the Milk of Lactating Rats and Impair the Growth and Increase the Mortality of their Suckling Pups J. Nutr., December 1, 2004; 134(12): 3327 - 3334. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Brandsch, N. Nass, and K. Eder A Thermally Oxidized Dietary Oil Does Not Lower the Activities of Lipogenic Enzymes in Mammary Glands of Lactating Rats but Reduces the Milk Triglyceride Concentration J. Nutr., March 1, 2004; 134(3): 631 - 636. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Eder, U. Keller, F. Hirche, and C. Brandsch Thermally Oxidized Dietary Fats Increase the Susceptibility of Rat LDL to Lipid Peroxidation but Not Their Uptake by Macrophages J. Nutr., September 1, 2003; 133(9): 2830 - 2837. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. Ringseis and K. Eder Insufficient Dietary Vitamin E Increases the Concentration of 7{beta}-Hydroxycholesterol in Tissues of Rats Fed Salmon Oil J. Nutr., December 1, 2002; 132(12): 3732 - 3735. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
