Bran yesterday . . . bran tomorrow?

Dietary fibre rose to prominence in the late 1960s. For a decade it held the enthusiastic attention of many doctors, nutritionists, and the public at a time of increasing awareness of the part played by diet in health. Dietary fibre had been considered previously to be an inert component of plant foods; because it was not digestible by human intestinal enzymes it was thought to have no important effects on nutrition. Only when Cleave, Burkitt, Trowell, Painter, and others propounded what became known as the fibre hypothesis did its possible part in the prevention and treatment of disease receive much attention.'4 The fibre hypothesis took the epidemiological observation that many diseases of the civilised world are rare in less developed countries, where an unrefined diet rich in fibre is eaten, and postulated that deficiency of dietary fibre contributed to the development of these diseases in the West. The list is considerable, ranging from dental caries through ischaemic heart disease, diabetes, obesity, and gall stones to diverticu-lar disease and colonic cancer. Fifteen years later is the hypothesis proved, or still relevant? What is the real value of dietary fibre now that the dust has settled? Dietary fibre comprises an enormous range of substances of widely different chemical composition and physical properties. All are structural or storage components of plant cells, though some authors also like to include indigestible components of animal foods.5 Their different physical properties of bulking, water holding, gel formation, and viscosity determine their effects on gastro-intestinal function. The great diversity of forms of fibre makes it nonsensical to equate the unrefined, high fibre diet of the rural African in the epidemiological study with a Western diet with added bran. Fibre content may be similar, but the type of fibre and its properties and effects are almost certainly not. The protein and fat contents of these diets are also not comparable, and this may be equally important. Experimental confirmation of the epidemiologically observed association between many diseases and a diet deficient in fibre is still lacking. Long term controlled experiments are difficult or impossible, and short term studies do not answer the crucial questions. The many published experimental reports require critical scrutiny and cautious interpretation.6 Three major reviews in the past four years have helped to restore perspective.79 The gastrointestinal conditions in which some proved benefits come from treatment with dietary fibre are constipation ,'° irritable bowel syndrome in …