The effects of various sources of dietary fibre on cholesterol metabolism and colonic function in healthy subjects

This thesis deals with the influence of several types of dietary fibre on cholesterol metabolism and colonic function in young healthy subjects. Dietary fibre has been defined as those plant polysaccharides (cellulose, hemicelluloses, pectic substances) and lignin which are resistant to hydrolysis by the digestive enzymes of man.<p/>After a general introduction in Chapter 1, a review of reported experiments with healthy subjects is presented in Chapter 2. The few available data on dietary fibre consumption in Western communities are also summarized, and possible mechanisms for the action of dietary fibre on cholesterol metabolism and colonic function are discussed.<p/>Chapter 3 describes an experiment with 46 young healthy volunteers. The effects of a mixed high-fibre diet were studied, in which half of the dietary fibre was provided by vegetables and fruits, and the rest cam from cereal products. In Chapter 4 the results are presented from a trial with 62 subjects. In this study the effects of isolated citrus pectin were compared with those of the same amount of pectic substances contained naturally in fruits and vegetables, and also with those of a comparable amount of dietary fibre from wheat bran.<p/>Subjects in both studies were under strict dietary control. All foodstuffs except for 200 or 100 kcal (0.8 or 0.4 MJ respectively) per day were individually supplied, taking into account each subject's energy needs. Measurements were made of body weight, food consumption, serum total and high-density lipoprotein (HDL)-cholesterol, faecal mass, the content of steroids, fat, and electrolytes in faeces, frequency of defaecation, intestinal transit-time, and blood pressure.<p/>From these two studies it is concluded that in short-term controlled experiments fibre-rich foodstuffs have only a small (vegetables and fruits) or no (bran) favourable effect on the concentration of serum cholesterol. However, in uncontrolled circumstances, a mixed high-fibre diet may through its low fat and cholesterol content indirectly reduce the level of serum cholesterol.<p/>After 5 weeks of high-fibre diets the amount and type of dietary fibre had no significant effect on the concentration of serum HDL-cholesterol. The effects on serum total cholesterol could be explained only to a small extent by changes in the excretion of faecal steroids.<p/>Although there was a wide interindividual variation in colonic response to increased dietary fibre intake (Chapter 3.2.), in general a high-fibre diet with vegetables and fruits as well as a diet with bran shortened the intestinal transit-time and enhanced faeces production. Thus, it appears that the effects of certain types of dietary fibre on serum cholesterol are unrelated to their effects on colonic function.<p/>Finally, the above results are discussed in Chapter 5. It is concluded that a diet rich in cereals, as well as fruits, vegetables, and legumes may be advisable. However, at the present state of knowledge, it would be premature to make precise recommendations for dietary fibre intake.