The effect of dietary vitamin E supply and a moderately oxidized oil on activities of hepatic lipogenic enzymes in rats |
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Authors: | Klaus Eder Manfred Kirchgessner |
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Affiliation: | 1. Institut für Ern?hrungsphysiologie der Technischen Universit?t München-Weihenstephan, Driverstra?e 22, 85350, Freising, Germany
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Abstract: | Diets rich in polyunsaturated fatty acids (PUFA) are well known to suppress hepatic lipogenic enzymes compared to fat-free
diets or diets rich in saturated fatty acids. However, the mechanism underlying suppression of lipogenic enzymes is not quite
clear. The present study was undertaken to investigate whether lipid peroxidation products are involved in suppression of
lipogenic enzymes. Therefore, an experiment with growing male rats assigned to six groups over a period of 40 d was carried
out. Rats received semisynthetic diets containing 9.5% coconut oil and 0.5% fresh soybean oil (coconut oil diet, peroxide
value 5.1 meq O2/kg oil), 10% fresh soybean oil (fresh soybean oil diet, peroxide value 0.5 meq O2/kg oil), or 10% thermally treated soybean oil (oxidized soybean oil diet, peroxide value 74 meq O2/kg oil). To modify the antioxidant state of the rats, we varied the vitamin E supply (11 and 511 mg α-tocopherol equivalents
per kg of diet) according to a bi-factorial design. Food intake and body weight gain were not influenced by dietary fat and
vitamin E supply. Activities of hepatic lipogenic enzymes were markedly influenced by the dietary fat. Feeding either fresh
or oxidized soybean oil diets markedly reduced activities of fatty acid synthase, (FAS), acetyl CoA-carboxylase, (AcCX), glucose-6-phosphate
dehydrogenase, (G6PDH), 6-phosphogluconate dehydrogenase, and ATP citrate lyase (ACL) relative to feeding the coconut oil
diet. Moreover, feeding oxidized soybean oil slightly, but significantly, lowered activities of FAS, AcCX, and ACL compared
to feeding fresh soybean oil. Activities of hepatic lipogenic enzymes were reflected by concentrations of triglycerides in
liver and plasma. Rats fed the coconut oil diet had markedly higher triglyceride concentrations in liver and plasma than rats
consuming fresh or oxidized soybean oil diets, and rats fed oxidized soybean oil had lower concentrations than rats fed fresh
soybean oil. The vitamin E supply of the rats markedly influenced concentrations of thiobarbituric acid-reactive substances
in liver, but it did not influence activities of hepatic lipogenic enzymes. Because the vitamin E supply had no effect, and
ingestion of an oxidized oil had only a minor effect, on activities of hepatic lipogenic enzymes, it is strongly suggested
that neither exogenous nor endogenous lipid peroxidation products play a significant role in the suppression of hepatic lipogenic
enzymes by diets rich in PUFA. Therefore, we assumed that dietary PUFA themselves are involved in regulatio of hepatic lipogenic
enzymes. Nevertheless, the study shows that ingestion of oxidized oils, regardless of the vitamin E supply, also affects hepatic
lipogenesis, and hence influences triglyceride levels in liver and plasma. |
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