Fast foods, energy density and obesity: a possible mechanistic link


  • a

    Energy density is defined as the energy content per unit weight of foods, meals or diets (expressed here as kJ 100 g−1). Owing to differing effects on satiation and satiety, it is necessary to consider solid foods and drinks separately. This paper concentrates on the solid components of diets.

  • b

    Data are from 147 women studied by 6-d weighed inventory (A Prentice & LMA Jarjou, personal communication). Diet composition was derived from an in-house software package based on extensive compositional analysis of Gambian foods (26) and energy density was calculated after excluding drinks.

  • c

    Data are from the 1986/87 Survey of British Adults aged 16–64 years (n = 2197) (27); the 1994/95 National Diet and Nutrition Survey of elderly aged > 65 years (n = 1275) (28); and the 1997 National Diet and Nutrition Survey of children aged 4–18 years (n = 1701) (29). Energy density was calculated for the whole diet minus tea, coffee, water and soft drinks.

  • d

    The examples cited here are from Burger King (, Jack in the Box (, KFC ( and McDonald's ( These are generally representative of the market sector as a whole. Data from a fast food outlet that promotes a healthy image for its products were also extracted ( Data were extracted in July 2002.

  • e

    In addition to foods already summarized in Fig. 4, data were collated on a selection of packaged ready meals and their healthy option equivalents from a UK supermarket (

Professor AM Prentice, MRC International Nutrition Group, London School of Hygiene and Tropical Medicine, 49–51 Bedford Square, London WC1B 3DP, UK.


Fast foods are frequently linked to the epidemic of obesity, but there has been very little scientific appraisal of a possible causal role. Here we review a series of studies demonstrating that the energy density of foods is a key determinant of energy intake. These studies show that humans have a weak innate ability to recognise foods with a high energy density and to appropriately down-regulate the bulk of food eaten in order to maintain energy balance. This induces so called ‘passive over-consumption’. Composition data from leading fast food company websites are then used to illustrate that most fast foods have an extremely high energy density. At some typical outlets the average energy density of the entire menus is ∼1100 kJ 100 g–1. This is 65% higher than the average British diet (∼670 kJ 100 g–1) and more than twice the energy density of recommended healthy diets (∼525 kJ 100 g–1). It is 145% higher than traditional African diets (∼450 kJ 100 g–1) that probably represent the levels against which human weight regulatory mechanisms have evolved. We conclude that the high energy densities of many fast foods challenge human appetite control systems with conditions for which they were never designed. Among regular consumers they are likely to result in the accidental consumption of excess energy and hence to promote weight gain and obesity.