Glycaemic variability challenges the accuracy and use of the glycaemic index (GI). The purpose of the current study was to determine the role of mastication on GI. Using a randomized, controlled, crossover, non-blind design, 15 healthy young subjects returned on 5 separate days for three glucose and two rice test sessions. At the rice sessions, subjects chewed each mouthful either 15 or 30 times. Rice chewed 15 times produced a total glycaemic response (GR; 155 mmol min/l), peak GR (2.4 mmol/l) and GI (68) significantly lower than when chewed for longer (30 times) (184 mmol min/l, 2.8 mmol/l and 88, respectively). The study shows that the GI of rice is affected by the degree of mastication. Chewing 15 times compared with 30 times significantly attenuates the GI, suggesting that mastication may potentially contribute to the glycaemic variability of rice. While future work must establish the extent and limits to which mastication affects glycaemia, it could also explore the potential of using mastication to reduce the glycaemic load of rice.
Glycemic variability is a persistent observation that challenges both the accuracy and application of the glycaemic index (GI).1 We previously reported that rice elicited the greatest inter-individual variations in the glycaemic response (GR) compared to spaghetti and carbohydrate drinks.2 This suggested that differences in habitual mastication of whole grains may be a contributor to its glycaemic variations. Subsequent studies confirmed that mastication rate varied between individuals (15–40 chews per mouthful), and that it was associated with GR.3, 4 No studies have thus far examined the effect of mastication on GI, which is by definition a property of foods and independent of intrinsic digestion variables. The specific aim of the current study was to, therefore, determine the effect of mastication on the GI of rice. We hypothesized that the GI of rice will be significantly greater when it is chewed extensively.
Subjects and methods
The study was a randomized, controlled, crossover, non-blinded design and was conducted in Singapore, using 15 healthy young Asians (eight males and seven females, mean (±s.d.) age 26±6, body mass index 20.5±4 kg/m2, fasting blood glucose 4.8±0.3 mmol/l). The subjects were limited to healthy young adults to maximize subject homogeneity and thereby minimize variations that may interfere with the observation of mastication effects.5 The protocol used to measure the GR and GI has been previously described.6 Participants returned on 5 non-consecutive days to test a standard 50 g oral bolus of glucose on three occasions, and white rice on two occasions. The rice used was white Thai Jasmine rice (Double FP brand, NTUC Fairprice Co-operative Ltd, Singapore) and was cooked as previously described.3 The rice was served in a bowl along with a standard dessert spoon. On each test day, participants arrived at the Clinical Nutrition Research Centre (CNRC) laboratory following a 10-h fast. Two finger-prick capillary blood samples were taken 5 min apart (between-sample coefficient of variation <4%) to determine baseline blood glucose levels and the reference/test food given to consume within 10 min. Mastication rate was standardized in the rice sessions: participants chewed each mouthful 15 times at one session and 30 times at the other. Mastication rate was quantified using electromyography (EMG)7 that was recorded using bipolar surface electrodes attached lengthwise along the left and right masseter muscles, and connected to a data acquisition unit (DataLOG MWX8, Biometrics Ltd, Newport, UK). Subjects were instructed to take mouthfuls as they usually would and keep chewing until asked to stop and swallow. Using real-time EMG, the researchers counted the number of chews as they masticated and stopped them when they completed either 15 or 30 chews. Further finger-prick blood samples were taken 15, 30, 45, 60, 90 and 120 min from the onset of eating. Blood glucose was measured using a glucose dehydrogenase method (HemoCue AB, Angelholm, Sweden). Blood glucose values after consuming the foods was corrected for baseline concentrations to obtain the absolute increments from baseline. The incremental area under the curve (iAUC) and GI were calculated using these values as previously described.8 Paired t-tests (one-tailed) were carried out on these data using the Statistical Package for the Social Sciences (Version 16, Chicago, IL, USA) to determine significance (P=0.05). The study received ethical approval from the Domain Specific Review Board of the National Health Group, Singapore, and informed consent was obtained from participants prior to taking part.
Post-study EMG data analysis showed that the mastication rate was controlled well at all the sessions for both (15 and 30 times) treatments (mean±s.d. 15.1±0.9 and 30.4±0.8).
All participants completed three reference (glucose) tests. The mean (M) (±s.d.) GR for the three glucose tests was 228.2±92 mmol min/l. In agreement with recommendations,9 between-test coefficient of variation was below 30% for all participants (15.6±9%).
The GR of rice chewed 15 times (M=155; s.e.=19) was significantly lower than when chewed 30 times (M=184; s.e.=18), t(14)=−2.13, P=0.025, r=0.5 (Figure 1). For both rice sessions, the peak GR was observed at 45 min after meal initiation. The peak GR for rice chewed 15 times (M=2.39; s.e.=0.22) was significantly lower compared to 30 times (M=2.81; s.e.=0.19), P=0.014, r=0.55 (Figure 1). Similarly, the GI of rice chewed 15 times (M=68; s.e.=3.4) was significantly lower compared to 30 times (M=88; s.e.=9.5), t(14)=−2.11, P=0.027, r=0.5 (Figure 1). The 30 times GI data set contained one outlying value that was greater than 2 s.d. of the sample mean.
The study showed that masticating 30 times compared to 15 times increased the GI of rice by approximately 29%. The rice tested was Thai Jasmine rice, an aromatic medium-low amylose white rice (13–18%) popularly consumed throughout the world and particularly in the United States of America, China and South-East Asia.10
The present study suggests that modifying the mastication rate alters the GI of rice, and its GI classification (medium GI if chewed less, and high GI if chewed more). The GI is by definition a food property that is independent of intrinsic digestion variables.11 However, this remains true only if the factors affecting the GR to the test and reference foods are the same. This and previous reports3 have shown that mastication affects rice GR. However, it has no impact on the glucose reference that is presented as a liquid. The current study thus demonstrates that mastication may be an intrinsic variable that can differentially impact on the reference and test foods, notably foods relying on mechanical breakdown for carbohydrate release. Differences in mastication may thus contribute to glycaemic variability, and account for the greater between-subject variations seen for rice compared to other solid and liquid foods.12 While this is a preliminary study using one food system, further work must be carried out to determine the effect of mastication on GR of other carbohydrate foods.
Rice is the staple for a vast majority of the world’s population and accounts for over 20% of global calorie intake. It is the predominant staple consumed in Asia, and contributes a significant glycaemic load to the diet due to both the large quantities consumed and high GR. Asia is rapidly becoming home to the largest number of patients with diabetes in the world13 and it is no surprise that rice has been implicated in its aetiology.14 This highlights the importance of devising ways and means of reducing the glycaemic impact of rice. In societies with a tradition of rice consumption, advocacy to reduce intake will remain an untenable goal. Strategies to reduce the GR of rice should therefore become a priority. It is worthwhile to examine the potential of using mastication to attenuate the glycaemic impact of rice. The present study used only two levels of chewing and thus provides only preliminary data. Using dose–response designs future studies must establish the limits and extents to which mastication impacts on glycaemia. As mastication is an involuntary act, the practicalities of consciously altering chewing rate must also be explored before it can be recommended as a practical intervention. The encouraging results justify further work into this interesting research area.
This study was supported by the Singapore Institute for Clinical Sciences. We are grateful to Julieana Binte Ibrahim for her help with carrying out the study. VR conducted the research and wrote the paper. ML and JH facilitated the study, contributed to the discussion and reviewed the manuscript.
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Current Oral Health Reports (2017)