Consuming a low-energy-dense soup as a preload led to a significant reduction in test meal intake compared to consuming no soup as a preload. However, the type of soup consumed did not significantly affect test meal intake. Overall, when soup was eaten as a preload, subjects reduced total energy intake at lunch by 20%. This reduction in energy intake was not associated with increased ratings of hunger or decreased fullness at the end of the meal. These results show that consuming low-energy-dense soup in a variety of forms at the start of a meal can reduce subsequent food intake, leading to significant reductions in total energy intake at the meal.
Several studies have established that soup is more satiating than some other types of food. For example, when women and men consumed a preload of soup, subsequent test meal intake was reduced compared to when solid foods, such as cheese and crackers or cantaloupe, were eaten as a preload (Kissileff et al., 1984
; Rolls et al., 1999
; Rolls et al., 1990a
). However, even if consuming a soup preload reduces subsequent test meal intake, it is possible that total energy consumed at the meal (soup and test meal energy combined) may be greater when a soup preload is eaten compared to when no soup is consumed. Few studies have employed a study design that includes a no preload control condition in order to test the effects of consuming soup on total meal energy intake. Of those studies that have, some reported no differences in total meal energy intake with and without a soup preload (Rolls et al., 1997
, Kissileff et al., 1984
), while others, including the present study, reported a decrease in total meal energy intake when a soup preload was consumed compared to when no soup was consumed (Rolls et al., 1999
; Himaya & Louis-Sylvestre, 1998
). These conflicting findings may be the result of variations in preload characteristics, such as volume, energy content, or energy-density, or differences in subject characteristics. In addition, variations in the test meal used to measure subsequent food intake may have affected outcomes; a single-food test meal may be more sensitive to changes in satiety than a mixed-food test meal (Long, Griffiths, Rogers, & Morgan, 2000
In the present study, the effects of soup form were isolated, and results showed that varying the form of soup by changing the way in which identical ingredients were blended did not significantly affect food intake or satiety. In a similar study, Himaya & Louis-Sylvestre (Himaya & Louis-Sylvestre, 1998
) measured satiety in lean and overweight men (N=22) following intake of 300 g (95 kcal; 397 kJ) preloads prepared from identical ingredients: vegetables with a glass of water, chunky vegetable soup, and pureed vegetable soup. In lean men, there were no significant differences between the preloads, while in overweight men, chunky soup significantly reduced lunch intake compared to vegetables with a glass of water. Based on these results, the authors suggested that chunky soup may be the most satiating form of soup. However, it is not clear that simply drinking water along with a food is an appropriate control condition for variations in volume and energy density between preloads. Previous work indicates that water consumed as a beverage has different effects on satiety than water incorporated into a food (Rolls et al., 1999
; Santangelo et al., 1998
). For example, drinking a glass of water alongside a casserole was not as satiating as the same ingredients combined into soup (Rolls et al., 1999
). This difference in satiety may have occurred because water consumed from a glass was perceived as a beverage intended to satisfy thirst, not hunger. In the present study, instead of separating only the water from the vegetables in the soup, subjects consumed a bowl of broth alongside the vegetables. The separate broth and vegetables had similar effects on satiety as identical ingredients combined into soup. Therefore, differences in ingestion method may explain why the present findings differ from those found in overweight men by Himaya & Louis-Sylvestre; the glass of water may have been perceived as a thirst-quenching beverage, while the bowl of broth may have been perceived as a food that satisfies hunger. These perceptions may have had different effects on food intake and satiety. Further research is needed to determine whether ingestion method and perceptions of a liquid as either a food or a beverage influence food intake and satiety.
There are a number of physiological mechanisms that may be involved in the satiating effects of soup. While liquids empty from the stomach at a faster rate than do solids, viscous solutions with particles of varying size may differentially influence gastric distension and gastric emptying rate (Read & Houghton 1989
, Vincent et al., 1995
). Increased gastric distension and decreased rate of gastric emptying have been shown to be associated with enhanced sensations of satiety (Geliebter et al., 1992
). Some studies have shown that consuming pureed tomato soup before a meal can increase gastric distension and decrease the rate of gastric emptying, resulting in increased satiety compared to consuming nothing prior to the meal (Cecil, Francis, & Read, 1998
; Spiegel et al
., 1993). These data suggest that the increased satiety and reduced intake that occurred when the soup preloads were consumed in the present study may have been due to enhanced gastric distension and a decreased rate of gastric emptying.
The effect of the form of soup on physiological responses has also been investigated. One study tested the effects of chunky and pureed soups, prepared from identical ingredients (591g; 499 kcal, 2090 kJ), in men (N=12), and found that while there were no differences in satiety, the pureed soup increased insulin response and energy expenditure compared to the chunky soup (Laboure, Van Wymelbeke, Fantino, & Nicolaidis, 2002
). In another set of studies, a pureed vegetable soup (660 g; 615 kcal, 2573 kJ) was found to be more satiating than the same ingredients served as vegetables with a glass of water (360 g + 300ml water; 615 kcal, 2573 kJ) (Peracchi et al., 2000
; Santangelo et al., 1998
). The pureed vegetable soup decreased gastric emptying time, increased insulin response, and increased diet-induced thermogenesis compared to the vegetables with a glass of water. Therefore, consuming soup can have gastrointestinal, endocrine, and metabolic consequences that may influence food intake and satiety, and the form of soup may influence these responses. Future work should focus on determining how these various mechanisms combine to reduce food intake and increase satiety.
Increased viscosity has been shown to be associated with increased sensations of satiety and reduced food intake (Hoad et al., 2004
; Marciani et al., 2000
; Marciani et al., 2001
; Mattes & Rothacker, 2001
). However, the relationship between viscosity and satiety has been studied primarily using beverages, and few data exist to suggest that varying the viscosity of soup can influence satiety and food intake. The liquid components of the soups used in the present study differed in viscosity, and participants noticed these differences; thickness ratings for the pureed soup were highest, followed by the chunky-pureed soup, chunky soup, and broth and vegetables. Subjects also perceived the thicker soups to be higher in calories and more filling than the less viscous soups. However, neither the variations in viscosity, nor the differences in ratings of thickness, calorie content, or how filling the soups would be, led to systematic differences in subsequent food intake. In particular, the most viscous soup (the pureed soup) did not decrease subsequent intake compared to the less viscous soups. Therefore, our hypothesis that increased viscosity would enhance satiety and decrease total meal intake was not supported. Because the presence of solid vegetable chunks varied along with the viscosity, this study did not isolate soup viscosity as an experimental variable. Interpretation of these findings is also limited due to the likelihood that the effects of increasing viscosity on satiety depend on a number of post-ingestive factors, in addition to the oro-sensory characteristics. Future studies should systematically test how variations in both oro-sensory viscosity and post-ingestive viscosity influence food intake and satiety.
The results from the present study offer additional support for recommending the inclusion of low-energy-dense soup in the diet as a strategy for controlling energy intake for weight management. In this study, subjects ate significantly less total energy and consumed a significantly greater total weight of food at lunch when soup was consumed at the start of a meal. Several studies that have tested the effects of consuming soup on a regular basis for several months found that routinely eating soup can reduce energy intake, enhance satiety, and promote weight loss (Foreyt et al., 1986
; Jordan et al., 1981
; Rolls et al., 2005
). In one dietary intervention, meals that contained soup had fewer total calories than meals without soup, and total daily energy intake was lower when soup had been consumed (Jordan et al., 1981
). Taken together, these data suggest that consuming low-energy-dense soup is a strategy that can be used to decrease energy intake and enhance weight loss, while allowing individuals to consume satisfying amounts of food.
The findings from this study confirm previous reports that consuming soup as a preload can significantly reduce subsequent entrée intake, as well as total energy intake at the meal. The present study expanded upon prior investigations to show that varying the form and viscosity of soup, by changing the way in which identical ingredients were blended, did not significantly affect energy intake or satiety. Therefore, consuming a preload of low-energy-dense soup, in a variety of forms, is one strategy that can be used to moderate energy intake in adults.