Dissociation of the effects of preload volume and energy content on subjective appetite and food intake
Introduction
One of the most influential laboratory techniques used in the study of human ingestive behaviour has been the manipulation of food or drink intake before exposure to an ad libitum test meal. This ‘preloading’ method has formed the basis for countless studies of ingestion in humans and animal models (see [1], [2] for reviews). However, relatively few studies have examined the effects of manipulating the mass and volume of preloads without any change in total energy content, for example, by increasing their bulk with water. This may be an important question in the context of human obesity, given the potential for low-energy-density (LED) foods to decrease passive overeating (see Ref. [3]). LED foods have been reported to have a greater satiating efficiency than high-energy-density (HED) foods [4], [3]. Of three examples of studies, in which subjects were given drinking water to supplement preloads, one showed no effect of 100 g water plus methyl cellulose on appetite ratings after an hour in obese patients [5], one showed no effect of 356 g water on lunch intake after a 17-min interval in healthy women [6], and one found that, when a meal was supplemented with 400 ml water, women rated subjective appetite lower during the meal [7].
Incorporating the extra water directly into a preload has been more effective. In two studies with liquid preloads, one showed an effect of bulk (preloads total 250, 500, 750 g) on periodic appetite ratings, and intake after 2 h, in men and women [8], and the other showed similar effects with a test lunch after 30 min (preloads 300, 450, 600 ml) in men [9]. A related study comparing a chicken casserole preload with the same preload made up as soup by adding 356 g water also produced a reduction in both intake and appetite ratings in women [6]. Most recently, an alternative design manipulated preload volumes (300, 450, 600 ml) by whipping air into liquid preloads, with no changes in energy or mass [10]. This, too, was effective in reducing short-term intake and subjective appetite in men.
In the present study, hot soup preloads were manipulated in mass and volume by the addition of water, while being matched for sensory properties. This manipulation was combined with the addition of maltodextrin to enhance energy content. Previously, we have reported consistent reductions in food intake in response to maltodextrin preloads [11], [12]. The direct comparison of volume and energy manipulations in the same context has only been reported once previously [8], in a quite different design. In addition to combining volume and energy manipulations, a novel aspect of this study was the use of the Universal Eating Monitor (UEM [13]) apparatus from some of our previous energy preload studies [11], [12], [14]. Monitoring intake constantly during a test meal while taking a battery of subjective appetite measures has enabled us to dissociate treatment effects on initial appetite, from effects on the hedonic enhancement of appetite early in the meal (the ‘appetiser effect’), and on the process of satiation throughout (see Ref. [15] for a review). The inclusion of this method was also designed to see whether the previously reported effects of preload volume and energy on intake were reflected in similar or distinctive changes in subjective appetite during the test meal.
Previous research suggests additive effects of volume and energy in terms of intake suppression. A study of rats with pyloric cuffs [16] showed that the gastric signal to inhibit short-term intake (30-min ad libitum trial) depended on volume irrespective of nutrients (e.g. saline). With the cuffs open, permitting intestinal stimulation, the nutrients then became more effective. In human studies, Geliebter has done considerable work with water-filled gastric balloons in relation to distension, discomfort, spontaneous food intake, and the treatment of obesity and bulimia (e.g. [17], [18], [19]). Although gastric capacity tends to be greater in the clinical groups, preloads of up to 450 ml used in the present study would constitute greater than half the capacity of most healthy adults [18]. In addition, a 400-ml balloon was sufficient to reduce food intake significantly in a group of lean and obese subjects [17]. More recently, Feinle et al. [20] reported that a duodenal infusion of maltodextrin transformed balloon-induced gastric pressure sensations into sensations more characteristic of having consumed a meal. This finding in particular suggests that we may see qualitative and/or quantitative differences in the pattern of changes in appetite ratings during meals consumed following preloads that vary in volume and energy.
In the present study, we predicted that preload volume and energy in soup-based preloads would act to reduce subjective appetite, and subsequent food intake during a test lunch, and that these effects would be additive. The use of sensory-matched preloads also allowed us to examine whether the results of some previous studies (including [8], [10]) may have been confounded by sensory differences between preloads. This approach included a low-energy, sensory-matched condition (A) as a positive control, in place of the no-preload [9], [10] and no-energy preload (tap water [8]) conditions used elsewhere.
Section snippets
Subjects
Twenty young men (aged 24.3±1.1 years) were recruited from a volunteer database of students and staff at the University of Sussex. Standard exclusion criteria included: diabetes; food allergies; regular smoking; weight-loss dieting; prescription medication; eating disorders; and drug abuse problems. Subjects were also required to score seven points or less for dietary restraint on the Three Factor Eating Questionnaire [21]. Their mean restraint score was 2.9±0.5, and their mean BMI was 22.6±0.7
Food intake
Intake of the pasta test meal (Table 2) varied between preload conditions [F(3,57)=6.28, P<.001]. Conditions A, B, and C did not differ, but all three differed significantly from D in post hoc comparisons (P=.001). Relative energy compensation (difference in test meal intake divided by difference in preload energy) by subjects receiving Condition D was 37.1%, 56.1%, and 50.0% compared with A, B and C, respectively. Analysis of the food diaries revealed no significant effects of preload on
Discussion
Trebling the volume of a preload using water reduced subjective appetite in the period between preload and lunch, whereas trebling the energy content of this large preload using carbohydrate had no significant additional effect on ratings. However, this energy manipulation reduced intake at lunch by about one-fifth relative to the other conditions, while the corresponding volume manipulation did not affect intake in the present study.
Measures of subjective appetite at the critical times
Acknowledgements
This research has been wholly funded by a grant from the Biotechnology and Biological Sciences Research Council, UK. We are also grateful to Cerestar UK for donating materials, to Drs. Michelle D. Lee and Tamzin L. Ripley for computer programming, and to Lucy Day for data processing.
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