This experiment demonstrated that varying the timing of serving a salad, as well as whether its consumption was compulsory or ad libitum, affected both salad intake and meal energy intake. Ad libitum salad intake was increased 23% by serving the salad before rather than with the main course, and compulsory salad intake was even greater than ad libitum intake. Energy intake at the meal was reduced by 11% by adding a fixed amount of salad to the meal, similar to the findings of a previous study; ad libitum consumption of salad did not significantly affect meal energy intake. Across all participants, the timing of salad consumption did not affect meal energy intake; the effect of timing, however, depended on participant scores for flexible dietary restraint. Strategies that maximize consumption of low-energy-dense salad for a given individual are likely to be the most effective approach for reducing meal energy intake.
Most previous research on the influence of the timing of consumption on satiety has focused on variations in the interval between eating occasions. Studies that tested delays ranging between 15 and 180 min after consumption of preloads found that meal energy intake increased with the increasing interval (Rolls et al., 1991
; de Graaf, de Jong, & Lambers, 1999
). Gray et al. conducted two studies using the same soup preloads but with different intervals before the test meal (Gray, French, Robinson, & Yeomans, 2002
; Gray, French, Robinson, & Yeomans, 2003
). Although no statistical test was reported, the researchers noted that participants with an interval of 10 min consumed 27% less energy at the meal than those with an interval of 30 min. Across all participants in the present study, there was no difference in energy intake between consuming the salad 20 min before the main course and together with the main course.
The response of participants to the timing of serving the salad differed according to their degree of flexible control of dietary restraint. Research on dietary restraint has not always exhibited the expected relationships with either food intake (Stice, Sysko, Roberto, & Allison, 2010
) or body weight (Cappelleri et al., 2009
; Teixeira et al., 2010
). In large studies, the subscale for flexible control of dietary restraint has been more consistently related to body mass or weight loss (Williamson et al., 1995
; Westenhoefer, Stunkard, & Pudel, 1999
; Provencher, Drapeau, Tremblay, Després, & Lemieux, 2003
; Texeira et al., 2010
), although not always (McGuire, Jeffery, French, & Hannan, 2001
). In the present experiment, women with low scores for flexible control showed the hypothesized effect on meal intake. For these individuals, eating a low-energy-dense salad as a first course displaced intake of the more energy-dense main course, leading to a reduction in meal energy intake compared to eating the foods simultaneously. Women with high scores for flexible control, however, had the lowest energy intake when the salad and main course were served together. This finding could have several explanations. Controlled feeding studies have shown that among individuals with high dietary restraint, providing information about the energy content of foods can affect meal intake (Kral, Roe, & Rolls 2002
; Miller, Castellanos, Shide, Peters, & Rolls, 1998
). The present results suggest that flexible control of intake may be more effective when the cognitive and sensory cues from different foods at a meal are experienced simultaneously. Alternatively, it may be that in individuals with high flexible control, being presented with a larger amount and variety of food activates a greater degree of self-monitoring. Additional studies are required on the potential for characteristics such as flexible dietary restraint to moderate the response to satiety cues in the meal environment.
The comparison of fixed and ad libitum consumption of salad highlights several issues. Although the women in this experiment liked the salad and many preferred it to the main course, under ad libitum conditions they ate only about 60 to 70% of the amount they consumed in the compulsory conditions. The previous studies comparing fixed and ad libitum intake served a greater amount in the ad libitum conditions (Rolls & McDermott, 1991
; de Graaf, de Jong, & Lambers, 1999
). These studies showed that ad libitum intake can be made comparable to fixed intake if the serving is increased sufficiently; however, serving different portion sizes may confound the interpretation of other outcomes. Examples of cognitive factors that may influence the comparison between fixed and ad libitum intakes are perceptions of the food and the motivations for eating it (Finkelstein & Fishbach, 2010
). In the present study, requiring the consumption of a fixed amount led to decreased hunger ratings and reduced meal energy intakes compared to ad libitum intake; these differences were consistent with the differences in the amount of salad consumed, but the contribution of motivational factors cannot be independently evaluated. To determine how perceptions of food affect intake, it would be of interest to test the effect of the fixed and ad libitum conditions on satiety ratings and intake when portion sizes were manipulated so that salad intakes were equal.
A main outcome of this study is that ad libitum intake of salad was increased 23% by consuming it before rather than with the main course, but this increase was insufficient to reduce energy intake of the main course and of the meal. This result is consistent with that of a previous study, which found that increasing the amount of cooked vegetables served at a meal increased ad libitum vegetable consumption, but did not significantly change intake of the other foods or energy intake at the meal (Rolls, Roe, & Meengs, 2010
). Since in both studies the vegetables were low in energy density, adding them to the meal had the dual effects of increasing food portion size and reducing meal energy density; these two effects had opposite consequences on energy intake, resulting in no net difference. Although it is beneficial that including a low-energy-dense salad in the meal added a nutrient-rich food without increasing energy intake, for individuals managing their weight, it is preferable for meal energy intake to be decreased. In the present study, this result was seen only when salad intake was compulsory and thus a large amount was consumed. The salad served in this experiment was similar to one of the preloads in a previous study that found effects of salad on satiety (Rolls, Roe, & Meengs, 2004
). The results of both studies showed that adding a fixed amount of low-energy-dense salad to a meal decreased energy intake in women by about 11%, despite an increase in the amount and variety of available food.
There are some aspects of the experiment that limit the generalizability of the findings. The sample included only women who reported that they were not actively dieting, thus the outcomes may not apply to men or to women who are dieting. There were no differences in energy intake between participants who chose to eat the with-meal salad before the pasta and those who ate the salad and pasta alternately; however, the statistical power for this between-subject comparison was low. It may be that consciously consuming low-energy-dense foods first can reduce meal intake without the need for separate courses, and this possibility should be investigated further. Finally, the effects on intake were only examined in the short term, and it is possible that over the longer term learning occurs and there is accommodation to the effects of timing of food consumption.
The results of this experiment confirm previous findings that adding a low-energy-dense food such as salad to a meal can reduce meal energy intake. This study extends these findings by providing information on the timing of consumption, which has implications for strategies to modify food intake at meals. If the goal is to increase vegetable intake, serving salad or soup as a first course in the absence of other foods is the most effective approach. For many people, eating a low-energy-dense first course also reduces meal energy intake (Rolls, Bell, & Thorwart 1999
; Rolls, Roe, & Meengs, 2004
; Flood & Rolls 2007
; Flood-Obbagy & Rolls 2009
). Restrained eaters who are attentive to their food intake, however, may adjust intake more readily when the low-energy-dense food is served together with other foods rather than in a separate course. Consuming a low-energy-dense salad can be used as a strategy to reduce meal energy intake, but the structure of the meal may be less important than an individualized approach that maximizes salad consumption.