Effects on lipid and glucose metabolism of diets with different types of fat and sugar in male fatty Zucker rats

The nutritional problem with regard to fat and sugar consumption in relation to lipid and glucose metabolism, and the ultimate goal of the study are generally outlined in Chapter 1. The obese Zucker rat was chosen as being likely a suitable animal model for a study like this. Chapter 2 is a review of the literature on the Zucker rat strain, of restricted size but aimed to be complete.

In Chapter 3 the design of the study is provided with regard to the grouping and dieting of the rats involved. To six groups of at least 12 obese rats per group, of approximately 6 weeks of age, semi-synthetic diets were given, being either low-fat or high-fat diets; the groups fed on the high-fat diets were provided with either a saturated (consisting of two parts of cocoa butter and one part of palm oil) or a poly-unsaturated (sunflower oil) type of fat. Each of the three types of dietary fat (low-fat, high-saturated and high-poly-unsaturated- fat) was combined with either sucrose or starch. One group of obese and one group of lean Zucker rats fed on a commercial ration served as control groups. Only male rats were used.

Chapter 4 deals with the results of the body weight gains, food consumption and digestion as obtained during the first part of the experiment, at an age of the rats of approximately 6 to 22 weeks. There was a large similarity in body weight gain of the group of obese rats. On the high-saturated-fat diets a steatorrhoea was observed; the food intake of the rats fed on the poly-unsaturated-fat diets had to be somewhat restricted, In the rats fed on the commercial ration a decreased apparent digestibility of protein was found.

In Chapter 5 the results are given for the plasma levels of cholesterol, triglycerides, insulin and the blood glucose, as measured after 2, 4, 9 and 15 weeks of the experiment. The lean rats showed, as expected, by far the lowest plasma cholesterol concentrations. The low-fat diets gave the highest plasma cholesterol levels, even higher than did the high- saturated-fat diets (which latter, however, gave rise to steatorrhoea). The high-poly-unsaturated-fat diets gave lower levels, but the lowest values for obese rats were seen on the commercial ration. The plasma triglyceride concentrations were significantly higher on all high-fat diets than on the low-fat diets. Again, the values observed in the lean controls were by far the lowest. Sucrose in the diets had an elevating effect particularly during the first weeks of dieting, firstly on the triglyceride and somewhat later on the cholesterol levels of the plasma, but this effect gradually disappeared. There was no interaction found between the effects of dietary fats and sugar on the plasma lipid levels.

The high-poly-unsaturated-fat diets gave significantly higher blood glucose concentrations (after the rats had fasted overnight) than did the other diets (p<0.001). The presence of sucrose in the diets had no significant effect on the blood glucose levels. The only tendency to a significant difference between the plasma insulin concentrations of the obese rats was a slightly lower level in the rats of the obese control group; this level, however, was significantly higher than that of the lean controls (p<0.05).

Chapter 6 gives a description of the cholesterol turnover study carried out on half of the rats from most dietary groups, with the help of two cholesterol isotopes provided. This turnover was higher in obese than in lean rats. The obese rats had larger pool masses and higher values for the transfer from the rapidly to the slowly exchangeable pool. The most striking result was the observation that all groups of obese rats, in contrast to the lean rats, appeared to have reached their maximal capacity for excretion of cholesterol. This leads us to the conclusion that the differences in plasma cholesterol concentrations observed between the groups of obese rats are the result of a different distribution of the cholesterol over the tissues.

Further, a positive correlation was found between the mass of the slowly exchangeable pool and the intestinal absorption of cholesterol. Of the significant differences found in the intestinal absorption of cholesterol between the dietary groups (p<0.05), the higher value for this on the low-fat vs. the high-saturated-fat diet and that for the starch vs. the sucrose containing diet must supposedly be ascribed to the concomiting lower cellulose content of the former respective diets ( cf. Table 19 with Table 2 in Chapter 3). The lower cholesterol absorption from the high-saturated-fat diet than from the comparable high-poly-unsaturated- fat diet will be related to the impaired fat absorption from this former diet.

The excretion of 3-beta-OH-sterols in the faeces (pooled samples) was somewhat higher in the obese control group than in that of the lean rats. It was also higher than in the groups fed on the semi-synthetic diets, with the exception of the groups fed on the high-saturated-fat diets which gave the highest values for this, probably connected to the steatorrhoea observed in these latter groups. Since the rats fed on this type of fat had shown to have almost identical figures for the cholesterol turnover ( = the production rate) as the obese rats in the other groups, a different synthesis of cholesterol will presumably have compensated for this difference in sterol excretion with the faeces.

Chapter 7 reveals the data obtained on the blood parameters mentioned ( vide supra ) during a period of four days of fasting of the second half of the rats from all groups participating in the study. Body weights decreased similarly in all groups, plasma triglycerides fell almost exponentially, whereas the plasma cholesterol levels showed an initial rise, with highest figures on the second day, and the most pronounced in the groups fed on the low-fat diets, to be followed by a gradual decrease. The blood glucose concentrations tended to increase between the second and the fourth day of fasting, whereas the plasma insulin levels did not change significantly.

Chapter 8 is devoted to the measurements, in the blood plasma of half of the rats from all groups, of the enzyme lecithin : cholesterol acyl-transferase (LCAT). A relative LCAT deficiency might be involved in the development of the increased plasma cholesterol concentrations of obese Zucker rats. Significant differences (p<0.05) were found as to a decreasing effect on the LCAT activity of the high-saturated-fat compared to the low-fat and the high-poly-unsaturated-fat diets, and a decreasing effect of sucrose compared to starch. There was no interaction found between dietary fat and sugar regarding the LCAT activities.

In Chapter 9 the results are presented of the lipid determinations performed in liver and perirenal fat of the same rats as were used in the preceeding latter two chapters, after their sacrifice. The perirenal fat of obese rats contained twice as much cholesterol as that of lean rats, without a dietary effect measured. The livers of the obese rats were very much fatter than those of the lean rats and had a different fatty acid composition. Very pronounced fatty livers were found in the groups fed on the low-fat diets, particularly in the combination with sucrose. There was a statistically very significant interaction (p<0.001) between dietary fat and sugar with regard to the total fat, triglyceride and cholesterol content of the livers. The hepatic fat content of the obese rats was lowest in the groups fed on the high-poly-unsaturated-fat diets. In the high-fat diets, the presence of a proportion of approximately 39 energy % of sucrose had no increasing effect on the liver lipids, contrasting to the steatotic effect on the liver with approximately 70 energy % of sucrose in the low-fat diets. Lipogenesis appeared to correlate negatively with the degree of mono-desaturation of the fatly acids present in the tissue fats.

Chapter 10 on the aortic atheromatosis points to the lack of atherosclerotic processes occurring in our Zucker rats despite a dietary induced hyperlipidaemia during a period regarded as sufficiently extended, of more than seven months. This negative result will be related to the different lipoprotein composition of rats in general, making them not particularly prone to a development of atherosclerosis in their large arteries.

In Chapter 11 the combined aspects of the fat and sugar metabolism as arising from the respective parts of the total investigation are discussed. Although the differences between obese and lean Zucker rats exceeded by far those observed between the dietary groups of obese rats, a number of significant differences was found between these groups, which resulted from dietary treatment. These differences concern plasma cholesterol and triglyceride levels as well as blood glucose concentrations, cholesterol pool masses, LCAT activities of the plasma, hepatic lipid metabolism and fatty acid composition of both liver and perirenal fat. Further, these differences are related to the quantity and the type of dietary fat as well as to the type of carbohydrate used and, in addition, to the degree of purity of the diet.

The differences in dietary composition, however, apparently did not affect the hepatic excretion of cholesterol, which was found to be similar, and nearly maximal, in all groups of obese rats studied.

In the end an investigation was made to determine the significance of these results for human nutrition.