Jennifer S. Bruce

The effect of different dietary fatty acids on lipoprotein metabolism: concentration-dependent effects of diets enriched in oleic, myristic, palmitic and stearic acids

While it is well established that the fatty acid composition of dietary fat is important in determining plasma lipoprotein cholesterol concentrations, the effects of changing the absolute quantities of the individual fatty acids are less clear. In the present study Golden Syrian hamsters were fed on isoenergetic, low cholesterol (0.05 g/kg) diets containing 100, 150 or 200 g added fat/kg. This consisted of triolein (TO) alone, or equal proportions of TO and either trimyristin (TM), tripalmitin (TP) or tristearin (TS). Each trial also included a control group fed on a diet containing 50 g TO/kg. As the mass of TO in the diet increased, plasma VLDL-cholesterol concentrations rose. The TM-rich diets produced a concentration-dependent increase in total plasma cholesterol which was a result of significant increases in both VLDL and HDL levels. The TP-rich diets increased plasma LDL- and HDL-cholesterol levels in a concentration-dependent manner. TS-containing diets did not increase the cholesterol content of any of the major lipoprotein fractions. Hepatic LDL-receptor mRNA concentrations were significantly decreased in animals fed on TP, while apolipoprotein B mRNA concentrations were significantly increased. Thus, on a low-cholesterol diet, increasing the absolute amount of dietary palmitic acid increases LDL-cholesterol more than either myristic or stearic acid. These effects on lipoprotein metabolism may be exerted through specific modulation of the expression of the LDL receptor and apolipoprotein B genes.

Hepatic microsomal triglyceride transfer protein messenger RNA concentrations are increased by dietary cholesterol in hamsters

In hamsters fed high fat diets enriched in trimyristin, tripalmitin or tristearin, increased dietary cholesterol content was associated with increased plasma concentrations of very low density lipoprotein (VLDL) cholesterol and triacylglycerol (p < 0.0001 and p = 0.0017, respectively). Hepatic microsomal triglyceride transfer protein (MTP) mRNA concentration also increased (p < 0.0001), independent of the nature of dietary fat, and was significantly correlated with the plasma VLDL lipid concentrations (p = 0.0002 and p = 0.0106 for cholesterol and triacylglycerol, respectively) and hepatic cholesterol concentrations. Increased expression of the MTP gene may be part of a coordinated response to hepatic cholesterol accumulation leading to increased VLDL lipid secretion.

Interactive effects of dietary cholesterol and different saturated fatty acids on lipoprotein metabolism in the hamster

The present study examines the interactive effects of three fatty acids: myristic, palmitic and stearic acids, with dietary cholesterol, on lipoprotein metabolism in the hamster. Each saturated fatty acid was fed at a concentration of 100 g pure synthetic triacylglycerol/kg in the presence of 100 g triolein/kg and was fed in the presence of 0.05, 1.2 or 2.4 g dietary cholesterol/kg. Dietary cholesterol increased the concentration of cholesterol in each of the major plasma lipoprotein fractions. The largest effects on VLDL and LDL were seen in the presence of tripalmitin where the increase between the lowest and highest dietary cholesterol groups were 129% and 38% respectively. In contrast, HDL showed the greatest change in the tristearin group when the equivalent increase was 59%. No interactive effects of dietary cholesterol and fat were seen on hepatic mRNA concentrations for the LDL receptor, hydroxymethylglutaryl-CoA reductase or the microsomal triacylglycerol transfer protein. As the amount of cholesterol in the diet increased, large differences were seen in the storage of hepatic cholesterol ester. At the highest dietary cholesterol intake the amount of hepatic cholesterol ester was 1.7-fold higher in the animals fed trimyristin compared with those fed tripalmitin. These results suggest that, as the amount of cholesterol in the diet is increased, palmitic acid becomes more hypercholesterolaemic. This is associated with a reduced ability to store cholesterol ester in the liver.

Use of mutagenically separated PCR for the detection of the mutation associated with porcine stress syndrome

A point mutation in the Ryr1 gene encoding the ryanodine receptor in porcine skeletal muscle is associated with enhanced growth characteristics and leanness but also with porcine stress syndrome and pale, soft exudative meat in some animals. The current diagnostic test for the mutation is based on the polymerase chain reaction (PCR), followed by a restriction enzyme digestion step, prior to agarose gel electrophoresis. Using a technique known as mutagenically separated PCR (MS-PCR), a one-step procedure for the identification of the point mutation associated with porcine stress syndrome has been developed. This removes the requirement of the current PCR-based test for restriction enzyme digestion, is consequently quicker to perform, and may lend itself more readily to automation. DNA from blood samples from a series of animals were genotyped using both the conventional test and MS-PCR, and complete agreement between the two methods was obtained.