It is often surprising—and hugely unfair—how few calories are needed for weight gain. Even a seemingly negligible change in caloric intake of 3-4% can result in a one kilogram increase in weight per year in adults. Exploring the factors that contribute to short-term caloric intake allows for improved understanding into the drivers of appetite, practical strategies for reducing energy consumption, and obesity prevention.
One way to contribute to caloric reduction in the long run is by reducing caloric intake in the short term. Since it can be challenging to make an accurate assessment of caloric intake, examining eating episodes in a laboratory setting is an important strategy for gaining a greater understanding of the subtle factors that can contribute to caloric intake. The types of foods eaten before the main meal can broadly be described as a ‘preload’. A preload is a fixed amount of a predefined food, eaten before consumption of an actual meal. The impact the preload has on a subsequent meal (or snack) is measured, therefore allowing greater insight into the drivers of short-term caloric intake. One thing which could impact the influence of a preload on the following meal is its energy density.
The concept of energy density is illustrated in Figure 1. It’s defined as the energy (calories) each gram of food delivers. Energy density can be an indication of food quality. Fat, which delivers nine kcal per gram, has the most potent impact on energy density compared to carbohydrate or protein (which each deliver about four kcal per gram). Foods high in fat are considered to have high energy density (HED). Contrary to the idea that energy density encourages satiety, the opposite may be true: given that HED foods often taste pleasant, it has been suggested that palatability disrupts satiety signaling, therefore promoting overeating.
The energy density of any food—whether a preload or meal—is an important factor to consider, as diets high in energy density have been shown to contribute to both weight gain and obesity. Yet, whether energy density is linked with satiety is not entirely clear. There is not a linear relationship between calories consumed and degree of satiety experienced, and for this reason satiety signaling’s relationship with the energy density of foods has become an urgent area of focus within obesity research.
Typically, foods that are low energy density (LED) contain higher amounts of water. This is the case with fruits and vegetables. Dietitians and medical professionals often recommend incorporating a higher ratio of LED foods, such as fruits, vegetables, and soups, not only as a way to improve diet quality, but also a means to reduce caloric intake. This is because the higher water content in these foods contributes to both increased satiety and slower gastric emptying, thereby leading towards an overall reduction in caloric intake.
In order to gain the clearest understanding of the ways these kinds of foods promote satiety, and therefore control of caloric intake, scientists have investigated the consequences of HED vs. LED preloads. The authors of the study under review sought to methodically review and meta-analyze the literature to date to determine the effect that preloads, and their HED vs. LED energy density, have on subsequent food intake.
The use of low energy density (LED) foods and preloads have been suggested as a means to reduce overall energy intake. However, studies to date show conflicting results in regard to whether LED preloads reduce subsequent energy intake and the role that preload energy density plays in satiety. The study under review is a meta-analysis of clinical trials assessing the effect of preload energy density on energy intake at a subsequent meal.