A comparison of various cholesterol lowering diets in young healthy volunteers; effects on serum lipoproteins and on other risk indicators for cardiovascular diseases

This thesis deals with the effect of type and amount of dietary fat on the concentration and composition of serum lipoproteins, colonic function, plasma glucose and serum insulin levels and blood pressure in healthy human volunteers.Two experiments were carried out. In the first experiment with 60 volunteers a moderate fat diet with a high proportion of polyunsaturated fatty acids (as recommended by several advisory bodies), was compared with three other diets during a test period of 5 weeks. One diet was low in total fat with a low proportion of polyunsaturated fatty acids, one diet was high in total fat with a high proportion of polyunsaturated fatty acids and one was high in total fat with a low proportion of polyunsaturated fatty acids. In the second experiment with 35 volunteers the moderate fat diet rich in polyunsaturated fatty acids and the low fat, low polyunsaturated fatty acid diet were compared again, but this time the test period lasted 13 weeks. The diets were composed of regular foodstuffs and differed in carbohydrate and fat content or fatty acid composition only. There were only minor differences in intake of dietary fiber and other nutrients known to affect cholesterol metabolism. Subjects in both studies were under strict dietary control. All foodstuffs, except for 100 kcal (0,4 MJ) per day were supplied individually according to each person's energy need. Actual food intake was measured by food records and analysis of double portions. The fatty acid composition of cholesterol esters in serum was analysed in the second experiment in order to check adherence to the diets.The serum lipoprotein composition and concentration observed during the experiments are given in Chapters 2 and 3. In serum, total cholesterol , triglycerides, apolipoprotein-A I and -B were measured; in high-density-lipoprotein (HDL), cholesterol was measured; in low-density-lipoprotein (LDL), cholesterol and triglycerides were measured and in very-low-density-lipoprotein (VLDL), triglycerides, apolipoprotein-B and the apolipoprotein-C II /C III ratio were measured. From these experiments two conclusions can be drawn. Firstly, that both a low fat diet, low in polyunsaturated fatty acids and a moderate fat diet, high in polyunsaturated fatty acids lower total serum cholesterol levels when compared with the habitual diet of affluent communities. Secondly, that a low fat diet causes lower HDL-cholesterol and higher fasting VLDL-triglyceride levels than a moderate-fat diet, high in polyunsaturated fat.It is by no means certain that changes in the concentration of HDL and VLDL really result in changes in the risk of death from cardiovascular diseases. In long-term intervention trials, such a hypothesis has not been tested; only the effect of changes in serum total cholesterol has been studied in intervention trials.Chapter 4 deals with effects on colonic function. No changes in mean transit time through the gut, fecal wet and dry weight, frequency of stools and concentration of fecal steroids were found. This shows that the intake of dietary fiber had been roughly equal in all diet groups within each experiment. Because there were no consistent short- or long-term changes in fecal bile acid or neutral steroid excretion, it is concluded that changes at the intestinal level do not explain the changes in total serum cholesterol concentration.Chapter 5 gives the results of measurements of fasting and postprandial serum insulin and glucose concentrations. The results show, that neither the amount nor the type of dietary fat had a strong influence on these variables in healthy subjects.Chapter 6 describes the effect of type and amount of dietary fat on blood pressure as well as the effect of dietary fiber from various sources and type of protein. None of these dietary components had a demonstrable effect on blood pressure in young normotensive subjects.The results of these experiments do not call for changes in the dietary recommendations of the Netherlands Nutrition Council, as far as the risk of death from cardiovascular diseases is concerned.