Electromyography analysis of natural mastication behavior using varying mouthful quantities of two types of gels
Graphical abstract
Introduction
Food texture plays an important role in food palatability and may thus modify natural eating behaviors [1], [2]. Texture is defined as the mechanical, geometrical, and surface properties that are perceptible using human receptors [1], and food textures perceived during oral processing are greatly influenced by the mechanical properties and sizes of the food items [1], [2], [3]. Electromyography (EMG) of masticatory muscles has been widely used to quantify mastication behaviors [2], [4], [5], [6], [7]. Harder foods generally require greater EMG activities of the jaw-closing muscles, and longer mastication times with a greater number of chews have been reported in association with these [4], [5], [6], [7], [8], [9]. Moreover, previous EMG studies examining the quantity of each mouthful showed that larger masses or volumes of food required longer oral processing times [4], [10], [11], [12], [13], [14], [15], [16], [17], [18].
Forde et al. [19] observed free-eating behaviors and reported that softer foods were generally consumed with larger bites (mouthful masses), leading to faster eating rates. Moreover, smaller bites of semi-solid foods increased oral processing times and decreased food intake [20], whereas harder foods resulted in reduced energy intake that could be sustained till the next meal [21]. Other studies that examined young subjects who ate quickly showed a significantly lower number of chews, shorter durations of chewing, and greater bite size were seen to be associated with a smaller number of bites in subjects who ate rice balls [22]. Finally, a meta-analysis showed that slow eating rates were associated with reduced energy intake [23].
In preceding studies [18], [24], EMGs were recorded during natural chewing of model food gels. These experiments showed that mastication times, number of chews, and number of swallows required for gels with various textures were approximately proportional to 0.7 times to the power of gel masses ranging from 3 to 24 g, and this relationship could be used to make predictions. Studies using fixed masses of foods with different shapes reported that the geometrical characteristics of gels did not influence eating behaviors [14]. Specifically, blocks, small pieces, and thin slices of apples with a fixed mass did not showed significant differences in EMG variables during natural mastication [25].
Forced mastication with a fixed number of chews has also been tested by multiple researchers. In general, more chews per mouthful of food were seen to be associated with reduced eating rates [26], [27], [28], [29]. Recently, Smit et al. [27] confirmed “Fletcherism,” an idea initially propagated by Fletcher which suggests that chewing one mouthful of food many times reduces the food intake and the eating rate despite faster chewing cycles. Increased number of chews also reduced the energy intake of one meal and modulated the plasma gut hormones, insulin and glucose [26], [28]. Slow eating with a greater number of chews reportedly increased diet-induced thermogenesis, postprandial blood flow [29], and postprandial glucose responses in glycemic indices measurements [30], [31]. However, if fixed modes of chewing were imposed, subjects could control chewing forces and rates of eating that were unnatural.
Eating small masses of gel-type foods appeared to be effective in preventing overeating, which often leads to obesity and other health problems [7]. Moreover, to avoid risk of suffocation in children, preparation of gels in cup volumes greater than the size of their mouth [32] and serving in small spoonful masses were recommended.
The objectives of this study were to quantify the effects of gel masses on natural mastication behaviors using portions of gel foods. We hypothesized that smaller mouthful masses and greater number of the mouthfuls increase mastication efforts and reduce rates of eating fixed amounts of food portions. In addition, the chewing side may change more frequently with food items that are more difficult to masticate. The subjects performed natural mastication without being instructed to control chewing forces so as to allow investigation of habitual eating behaviors instead of under controlled conditions such as fixed number of chews and side of chewing.
Two types of gel foods with similar fracture loads (agar gel and konjac jelly) were prepared as the present food samples. Previous studies have reported that the number of chews and mastication times were alike for the gels with similar masses [18], [33]. Accordingly, hydrocolloid gels provide models of solid or semi-solid foods consumed by mastication using the back teeth as the mechanical properties and sizes of these gels can be easily controlled [34], [35], [36], [37]. Moreover, gel alternatives for food items such as cooked rice are extensively consumed as staple foods, and jellies are often served to dysphagic patients and elderly people with mastication difficulties [7], [35], [38], [39]. The chemical components of these hydrogels do not change during oral processing, as salivary enzymes do not decompose non-starch ingredients. In addition, these hydrogels contain sufficient water to minimize the amount of saliva required for bolus formation and lubrication. Hence, size reductions with respect to structural threshold [40] are the main requirement for oral processing, and reduction of mouthful masses is easy and does not require changes in the method of food preparation. Finally, EMG recordings provide quantitative evidence of the effects of serving sizes, and also allow simultaneous evaluation of how the mechanical properties of foods influence mastication efforts and rates.
Section snippets
Participants
This study design was approved by the National Food Research Institute Ethics Committee. Eleven women volunteers (mean age, 36.5; range, 22–49 years) participated in this study were common in our previous study [18]. Women were selected as there may be gender differences in mastication behavior [18]. Their heights and body mass indices ranged from 150 to 170 cm and 18 to 23 kg/m2, respectively. The subjects were free of any masticatory or swallowing dysfunctions and did not use removable denture
Dominant and non-dominant sides of masticatory EMGs
Both masseter EMGs were read and the variables were calculated separately. The results showed that the side of the first chew appeared to be fixed for many subjects, as shown in Table 2. Subjects S1 and S2 started chewing on the right side, and the mean amplitudes and total muscle activities calculated for stage T1 were greater on the right side for all samples (Fig. 3). Subject S2 preferred this side over the left for the entire time for oral processing. During stage T1, the LM (non-dominant
Gel properties and natural mastication behaviors
The present study examined two types of gels (A and K) that had similar fracture loads. Both types of gels were expected to easily fracture during the first chew under a similar force. However, other mechanical properties (Table 1) significantly differed between the two gels (p < 0.001). Specifically, fracture strain and work of elastic K gel were nearly seven times greater than those of brittle A gel, and fracture during the human chew-cycle of K gel would occur later than that of A gel.
Conclusions
In this study, we clarified the effects of mouthful masses of cups of gels (24 g) on natural eating behaviors using EMG. Differences in EMG variables for two types of gels (A and K) with similar fracture loads were used to adjust times for oral processing and were analyzed during the entire oral processing period, and before and after the first swallow. EMG observations suggest that the number of chews, time for oral processing, and total masseter muscle activities are likely to be determined by
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