Effect of sip size on food intake and on the estimated amount consumed

We hypothesized that ad libitum intake would be higher when subjects consume large sips and that they would underestimate how much they had consumed. The results showed, indeed, that ad libitum intake was higher when consuming large sips, in agreement with previous studies [4], [6], [7], [13]–[18]. Consuming large sips led to underestimation, whereas small sips, led numerically, but not significantly, to overestimation of the amount consumed. This indicates that sip size affects the perceived food intake. Larger sips are by definition associated with fewer sips per gram food. The fact that fewer sips are consumed, may explain the underestimation of food intake in the large-sip condition. This underestimation during consumption may delay satiation, because food intake is influenced by believes about the amount of food intake [25].

Interestingly, when subjects determined their sip size and frequency by themselves (free-sip condition), ad libitum intake was similar to the large-sip condition. Subjects also underestimated how much they had consumed in the free-sip condition. Moreover, subjects consumed soup in the free-sip condition with almost similar sip size than the large-sip condition (∼14 g and 15 g, respectively). Dividing the data of the free-sip condition into two groups (small-sip and large-sip), showed that taking large sips lead to an underestimation whereas smaller sips did not. This indicates that underestimation of consumption also occurs when people take relatively large bites by themselves.

The mean sip size in the free-sip condition of ∼14 g is larger than the sips that are taken when the soup is consumed with spoons: 7–9 g [44]. The sip size was probably influenced by the manner of consumption, which was through a tube. It has been shown that consuming with a straw instead of a spoon increased eating rate, possibly through relatively large sips facilitated by straws [45]. The tube may therefore facilitate large sips compared to spoons.

The reason “I terminated consumption because I did not like the manner of consumption” was more important in the small-sip condition compared to the large-sip and free-sip conditions. It probably contributes to the 30% lower intake in the small-sip condition. The lower intake in the small-sip condition may also explain why subjects felt less full directly after consumption. However, when the statistical model on ad libitum intake was corrected for “manner of consumption”, there is still a strong significant effect of sip size on ad libitum intake.

Subjects felt less full after consumption in the small-sip condition compared to the large-sip and free-sip conditions. However, these differences in hunger and fullness ratings diminished at one to three hours after consumption (Figure 4). No differences in hunger after three hours may indicate that the reduced food intake in the small-sip condition will not be compensated. Two studies [16], [46], that used an oral device to decrease bite sizes, have shown that the device led to a reduction in meal size without changes on rated satiety between meals. Small bites or sips may therefore lead to a reduction in food intake on longer term.

Sip size did not influence sensory characteristics of the soup (Table 1). In addition, the initial pleasantness and the decrease in pleasantness after ad libitum intake were not affected. Therefore, the effect of sip size on ad libitum intake was not mediated via differences in flavor perception or pleasantness of the food.

Larger sips are associated with fewer sips per gram food. Fewer sips means less effort, which facilitates food intake. Effort is related to the ease with which a food can be consumed and has a strong influence on the amount of food intake [47], [48]. For example, a longer distance to a snack product increase the effort to obtain the snack and this was shown to reduce energy intake [49]. We observed that subjects took relatively large sips when they chose the size themselves. Peoples' natural behavior act through the “Law of least effort” [50]. Consuming with large sips or bites means that people chose the path of least effort or resistance.

Effect of sip size on food intake may not only externally regulated but also internally. In a previous study, we showed that large sips decrease the relative oral sensory exposure to the taste of the food (i.e., exposure to taste per gram food) [10]. Oral sensory exposure has been shown to play an important role in the development satiation [3], [4], [51]–[54]. In addition, also a higher number of swallows that is associated with smaller sips or bites, may play a role in the onset of signals of satiation [55].