Dietary fibre and lipid metabolism

The influence of dietary fibre on lipid metabolism can be assessed at several levels. In animal studies it is possible to investigate the effects of fibre on serum and tissue lipid levels, on lipid absorption and excretion and, in specific instances, on experimental atherosclerosis. In man we are limited to examination of blood lipid and lipoprotein levels and on patterns of lipid excretion. Ershoff and his co-workers (eg) were among the first to demonstrate beneficial effects of fibre on serum and liver cholesterol levels in cholesterol-fed rats. They also showed that not all that we call fibre is hypocholesterolaemic. Cellulose, for instance, caused increases in liver cholesterol levels in rats. Cellulose also leads to increased levels of carcass cholesterol. Experiments in chickens, rabbits and monkeys have shown cellulose to exert the least beneficial effect on atherosclerosis or aortic sudanophilia. In man the insoluble fibres such as bran or cellulose are not hypocholesterolaemic, whereas soluble fibres such as guar gum or pectin are. The effects are also reflected in levels of high and low density lipoproteins. Studies relating to mechanism(s) of fibre action have shown that fibre may increase cholesterol and bile acid excretion and, in some cases, may affect faecal bile acid composition. Fibres have been shown to bind bile acids in vitro, the extent of binding being a function of both the type of fibre used and the type of bile acid or bile salt being studied. Fibre has also been shown to bind other lipids. Fibre also influences the rate of cholesterol absorption. To date most results can be classified according to the type of fibre used, ie soluble or insoluble; ionic or non-ionic. Our added understanding of modes of action of fibre will depend on better ability to relate structure to function.