ENERGY DENSITY

Energy density is a critical component in the regulation of human appetite and plays an important role in determining total energy intake. In one of the most robust experimental studies Stubbs et al showed that lean, young healthy men, allowed to eat ad libitum, consumed significantly more energy as the fat content of the food was increased (Stubbs et al., 1995a, b). Careful measurements of fat balance over one week in a whole body indirect calorimeter showed that body fat decreased by 0.86 ± 0.61 kg on the 20 per cent fat diet, while increasing by 0.39 ± 0.59 kg and 2.24 ± 0.94 kg on the 40 and 60 per cent fat diets respectively. These studies provided no evidence of any physiological compensation or cognitive ‘learning’ associated with sustained consumption of foods of differing fat content even after a week or more of sustained over-consumption.

Importantly, when the energy density of the food was equalized, through careful experimental manipulation of the recipes, the high fat hyperphagia was abolished (Stubbs et al., 1996) (Figure 3.1). This strongly suggests that excess energy was consumed by a process of ‘passive over-consumption’, in which changes in food quality, not quantity, were the driving force beyond the disruption in the previously accurate regulation of body weight. This phenomenon implies that the bulk of food consumed is an important determinant of energy intake.


In the ‘real world’ energy-dense diets are frequently high in fat, since fat (37 kJ/g) contains more than twice as much energy gram-for-gram as protein (17 kJ/g) or carbohydrate (16 kJ/g). Many low-fat foods, especially dairy products, contain substantially less energy than their full-fat equivalent, allowing consumers to maintain the bulk of food in the diet, while constraining energy intake. However, recent advances in food technology have resulted in some food ranges that are low in fat but where the energy content is similar to traditional equivalents. These foods, such as biscuits, cakes and desserts often contain large quantities of added sugars and might be expected to lead to similar passive over-consumption as high-fat foods of similar energy density.

Foods served in most ‘fast-food’ chains such as burger and chicken outlets are characterized by a particularly high energy density. These foods are frequently high in fat and have a low water content. A recent analysis has shown that the energy density of foods offered in a selection of these outlets has an energy density of over 1000 kJ/100 g relative to the typical energy density of the diet of a woman in the UK of 670 kJ/100 g (Prentice and Jebb, 2003). This implies that for regular consumers the total quantity of food which can be consumed without exceeding energy needs must be constrained to accommodate this increase in the energy density of the diet in regular consumers. The high energy density of these foods provides a plausible biological explanation for the epidemiological associations between ‘fast-food’ consumption and obesity. However this is likely to be compounded by large portion size and specific marketing strategies to encourage further consumption of these products.

(Written By: Susan A. Jebb and Jeremy Krebs in The Book Of Obesity And Diabets)