Explain Fibre and Cardiovascular Disease (CVD)?
The role-of dietary fibre in modulation of blood lipids was demonstrated by Keys and his co-workers in a series of experiments conducted during 1960's. Later Trowell supported the protective effect of dietary fibre against hyperlipidemia and ischemic heart disease. An inverse relationship between CVD and dietary fibre has been shown in many prospective and epidemiological studies and cross-sectional population survey. However, uniform results have not been yielded across studies. Studies in which diets were modified to reduce fat and increase carbohydrate and fibre level have shown favourable impact on the incidence and regression of CVD. Evidence supports a protective effect of dietary fiber for CHD, particularly viscous fibers that occur naturally in foods, which reduce total cholesterol and LDL cholesterol concentrations. Reduced rates of CHD were observed in individuals consuming high fiber diets. These studies used fiber-containing foods; fiber supplements may not have the same effects. The type of fiber is important, oat bran (viscous fiber) significantly reduces total cholesterol, but wheat bran (primarily non-viscous fiber) may not. Viscous fibers are thought to lower serum cholesterol concentrations by interfering with absorption and recirculation of bile acids and cholesterol in the intestine and thus decreasing the concentration of circulating cholesterol. These fibers may also work by delaying absorption of fat and carbohydrate, which could result in increased insulin sensitivity and lower tiiacylglycerol concentrations, Dietary fiber intake has also been shown to be negatively associated with hypertension in men but not women. Fiber intake was shown to have an inverse relationship with systolic and diastolic pressures. Thus, it is important to note that with respect to CVD, only soluble fibres which are also viscous have been shown to reduce serum cholesterol. This effect is not simple but could be due to multiple factors operating simultaneously. Possibly dietary fibre displaces fat from the diet. Also polyunsaturated fatty acids consumed in conjunction with fibre play a role. Some fibres reduce the reabsorption of bile acids in the ileum, thus affecting the enterohepatic circulation. Enterohepatic pool is renewed by increased synthesis of bile acids from cholesterol, which in-turn reduces body cholesterol. Fibres such as oat bran and pectin may decrease absorption of dietary cholesterol by altering the composition of bile acid pool. Since exogenous cholesterol represents only a small proportion of the body's cholesterol, this mechanism may contribute partially to the fibre-induced hypocholestermia.
Data is available from some animal studies, which indicate that endogenous cholesterol synthesis is affected by feeding dietary fibre. HMG CoA reductase, the rate limiting enzyme in cholesterol biosynthetic pathway is inhibited by deoxy cholic acid (DCA) as compared to cholic acid or chenodeoxycholic acid. Administration of certain fibres increases the proportion of DCA in bile acid pool. The importance of this mechanism needs to be studied in humans. All these factors may contribute to the hypocholesterolemic effect of fibre, but the relative importance of each is not well known. Further, many natural plant constituents have been shown to affect lipid metabolism. These components are frequently present in dietary fibre sources and. may confound effects of dietay fibre. A diet that prevents CVD or slows its progression is the one which is low in fat and high in complex carbohydrates. Such diets, which are minimally processed, are high in dietary fibre and may contain other hypocholestermic components like phytoesterogens. So, we have seen the benefits of fibres, as well as, its role in preventing the disease like cancer and CVD. But is there a minimum amount of daily fibre intake or we can consume as much as we like? Let's find out in the next section, what is the desirable level of fibre intake as recommended by the Nutritional Institutes / Associations.