Purpose - The aim of this paper to explain the difficulties associated with applying the science of nutrition in formulating advice to protect public health, using the example of dietary fibre. Design/methodology/approach - Based on this, review of relevant literature published between 1953 and 2006, the first section traces the history of attempts at constructing useful definitions of dietary fibre. Then the problems of measurement of the fibre content of foods, measurement of fibre intake in individuals and with testing for correlations with disease, are outlined. Findings - The equivocal nature of current definitions of dietary fibre, and the lack of accuracy, validity or reliability of methods of determination, have been major limitations to developing an understanding its role in protecting human health. Perhaps more than for any other dietary constituent, the elucidation of the science describing the role of dietary fibre in human nutrition has been confounded by both known and unidentified variables. Practical implications - This paper reflects an increasing scientific awareness that developments in nutrition are subject to the limitations of methods of analysis and research design. It explains why the processes of scientific investigation are often slow to reveal important relationships between dietary factors and health. This presents some difficulties for authorities charged with promoting health protective eating behaviours. Originality/value - Although it has not been possible to unequivocally quantify dietary fibre or its effects on human metabolism, the research provides strong support for choosing a wide range of fresh or minimally intact plant food sources to protect health, as recommended in the FBDG's promoted in many countries around the world. The health protective effects may be achieved with moderation of the information disseminated about dietary fibre, particularly via the marketing of manufactured foods supplemented with fibre isolates.