Benefit of chewing: Current evidence and future direction

Oral stimuli, such as flavour, taste and mastication, have been suggested to impact both oral and systemic health. Chewing gum, as a non-nutritive source of such oral stimuli, may serve as a food surrogate and help control adverse health conditions related to food intake, such as caries and obesity. Recent studies have shown potentials as well as challenges in impacting human physiology and psychology by non-nutritive mastication.

Oral food processing is an inevitable step in food digestion and thus intake of nutrients. While there is no direct contribution for nutrient intake by masticating elastic substances, there is a relatively long history of humans chewing natural gums, such as chicle in Central America and mastic in Greece. Chewing gum typically consists of three main ingredients: gum base to provide texture, sweeteners to provide taste, and flavours to provide a pleasant note in addition to taste. Thus, while gum chewing has little impact on intake of nutrients, there are multiple oral stimuli provided by chewing gum. Indeed, gum chewing provides a unique experience in which the consumer can enjoy oronasal stimulation as well as exercising their facial muscles without ingestion of food. These combinations of orofacial sensory stimulation without significant caloric and nutrient intake provide chewing gum a unique position in the food industry.

Research has suggested that there are several reasons that consumers chew gum beyond merely seeking pleasurable experiences. Such reasons include “to freshen my breath”, “help keep my mouth healthy”, “help me feel more relaxed”, and “I can chew gum instead of eating a higher-calorie snack”.

The Wrigley Science Institute was established in 2005 with scope to investigate sound scientific support for such anecdotal use of gum chewing. Concurrently, there is emerging research around mastication and its impact on human homeostasis. The four major areas of research related to mastication / gum chewing are oral health, weight management, cognition and stress management.

Oral health

One of the best elucidated physiological impacts of chewing gum is the increase of salivary secretion elicited by chewing. Saliva, consisting of 99 per cent water and one per cent electrolytes and proteins, plays an important role in the maintenance of oral health. Its functions include clearance of sugar and acid, buffering and providing sources for tooth re-mineralisation. These functions contribute to protecting the mouth from dental caries, erosion, attrition, abrasion, lesions and bacterial overgrowth.

Several studies have shown that chewing gum increases salivary flow1. Salivary proteins, such as mucin, immunoglobulin A and alpha-amylase protect the oral cavity from infections of harmful bacteria2. In addition, saliva has an inherent buffer capacity to maintain the pH neutrality of the mouth against dietary and intrinsic acid insults (stomach acid regurgitation), as well as acid production by caries causing bacteria, such as Streptococcus mutans. Independent studies showed a marked buffer capacity difference in un-stimulated saliva and saliva stimulated by chewing gum3. Chewing gum increases salivary flow rate instantly, peaking within two minutes, but persisting as long as mastication continued. The salivary flow rate gradually fell to plateau values over 40 minutes after starting to chew, remaining significantly higher than the initial un-stimulated flow rate (Figure 1, page 42).

When acidogenic oral bacteria, such as S. mutans, incorporate sugar as an energy source, it produces a variety of acids, including lactic and acetic, as a product. This acid-producing reaction contributes to reducing the pH of the dental plaque, resulting in dissolution of the enamel and dentine, thus promoting caries. Sugar-free chewing gum promotes clearance of acids in oral cavities via stimulating salivary secretion, while inhibiting the acidogenic metabolism by providing sugar alcohol which bacteria cannot easily digest into acids. As a result, when stimulated by chewing gum, the salivary pH remained significantly higher than that of un-stimulated saliva for up to two hours2,4.

A review summarising seven clinical studies which evaluated the impact of chewing gum on caries incidence concluded that there is con sistent evidence that chewing sugar-free gum after meals results in a significant decrease in the incidence of dental caries4. The existing evidence strongly suggests this effect is mediated by salivary flow stimulation in addition to the effect of non-acidogenic sweetener.

Weight management

Anecdotally, chewing gum helps suppress the urge to munch high calorie snacks and hence helps losing weight5. While there is no clinical evidence directly supporting weight loss by chewing gum, there is research to show that chewing, whether gum or other food, influences energy balance. Energy balance, or calorie homeostasis, consists of two parts, namely energy intake and energy expenditure, with the former being traditionally associated with food intake and the latter with exercise.

Multiple studies from independent researchers have shown that chewing gum before snacking can help reduce hunger, diminish cravings for sweets and decrease snack intake, for example by 36 kcal6 (Figure 2, page 42). When participants chewed gum, hunger, desire to eat and sweet snack cravings were significantly suppressed between lunch and an afternoon snack as compared to when they did not chew gum. Also when participants chewed gum, they reported that their energy levels were maintained between lunch and an afternoon snack, and they were significantly less drowsy as compared to when they did not chew gum. The results repeatedly suggested that subjects found that chewing gum helped achieve the same degree of satiety with fewer calories. While the fundamental mechanism of how gum chewing produces satiety is still under investigation, there is some evidence that non-nutritive mastication may impact digestion, hence satiation and food intake behaviour.

One study reported that gastric emptying was stimulated when food was orally processed compared to the same amount of food fed via tube7. The increased gastric emptying suggested higher activity of digestion and increased stimulation in sensors in digestive organs. Other studies also showed sham feeding, mastication of food followed by expectoration, affected the circulating plasma level of ghrelin, a hormone that induces hunger8,9. However, while further research is required to elucidate the hunger state of subjects at the time of study and other factors, both studies support the idea that mastication influences hormonal regulation in the gastrointestinal tract.

Energy expenditure is as important as energy intake in maintenance of calorie homeostasis. As epidemiological studies have suggested a connection between urban lifestyle and obesity, a concept of non-exercise activity thermogenesis (NEAT) has been attracting attention as a factor for physiology of weight change. NEAT is defined as ‘the energy expended for everything that is not sleeping, eating or sports-like exercise. It includes the energy expended by walking to work, typing, performing yard work, undertaking agricultural tasks and fidgeting’10. Chewing gum, involving the use of masseter and temporalis muscles, certainly is a NEAT activity with a mean increase in energy expenditure of 11 kcal per hour11.

While overall, existing results suggest the degree of impact for weight management by chewing gum is mostly on self-perception and has minimum impact on energy balance, these results are consistent in showing the potential role of chewing gum for maintaining energy homeostasis by providing small and significant differences. Another potential of chewing gum for weight management is to raise selfawareness of conscious energy intake which may also contribute to increases in physical activity in order to reduce excessive weight gain12.

Stress and cognition

Studies on the impact of mastication on psychological end-points, including stress relief and cognitive improvement, date back to 1939 when a series of studies into mood and performance were assessed with sugar coated chicle13. The history of scientific research in evaluating connections between mastication and mental states has been supported by a consistent belief that chewing helps con – centration and in relieving stress, as consumer research showed that chewing gum helped reducing consumers’ stress level at least a little. As knowledge in human psychology and physiology has advanced, there is more interest in how brain function is affected by peripheral modulation. It has been suggested that mastication has an influence in the hippocampus, a brain area important for learning and memory. Older individuals, with a reduced ability to masticate effectively, due to loss of teeth and diminished orofacial muscular tonicity, may face a greater risk for the development of dementia, since reduced mastication is linked with deterioration of spatial memory which is associated with morphology and function of hippocampal neurons. Active mastication further improves the performance of sustained cognitive tasks by increasing the activation of the hippocampus and the prefrontal cortex, the brain regions that are essential for cognitive processing. In fact, a study showed that chewing gum, compared to mimicking the motion of chewing, showed greater activation in parts of prefrontal and posterior parietal cortex areas monitored by fMRI14. In addition to psychological studies, physiological research has shown a negative correlation between mastication and circulating corticosteroids, biomarkers for stress produced in the pituitary and adrenal glands. This correlation has raised the hypothesis that the contribution of mastication to cognitive function is, in part, suppression of the hypothalamic-pituitaryadrenal axis15. Other links in improvement of cognition and mastication come from studies that showed an increase in heart rate and blood flow to the head.

Several studies have evaluated how these psychological and physiological insights can affect the end outcomes in mental performance and perception. Particular attention has been paid to:

• self rated perception on mood; e.g. anxiety, alertness and stress
• memory
• intellectual performance, such as repeat of calculation (series of three)
• focus and attention

The results of the studies are consistent in all four points listed above16-19. Among the studies which employed different psychological and physiological stressors, gum chewing increased self-rated alertness, while decreasing anxiety and stress levels. Furthermore, chewing gum was associated with greater alertness and a more positive mood. In contrast, less consistent effects of chewing gum were observed in the memory tasks. One study suggested that there is a trend in improvement of memory when subjects chew gum in both memorising and recalling occasions.

As for the effect of chewing on focus and attention, reaction times to the given tasks were quicker when chewing gum and this effect became bigger as the task became more difficult. Chewing gum also improves selective and sustained attention. For example, one study showed chewing gum improved response time in a measure of selective attention which showed increased focusing of attention17. The investigator further tested gum chewing effect on performance using a caffeinated gum, which has been shown to improve performance efficiency and mood, and showed the profile of caffeine gum effects to be in agreement with the existing caffeine literature. In the same study, chewing of non-caffeinated gum was also associated with a positive mood to a lesser degree, and thus also confirmed previous findings18. Physiological measurements, such as heart rate and cortisol levels, were higher when chewing, which confirmed the alerting effect of chewing gum. The results suggested that chewing gum produced a number of benefits that are generally observed and not context-dependent.

While accumulating evidence is helping illustrate in what specific benefit areas chewing gum may provide greater impact, we have yet to elucidate what the degrees of contribution from individual components in chewing gum are, such as gum base and flavour. In other words, the presence of possible synergies among gum ingredients is one of the areas to be explored. The researchers in the area suggest the mechanisms underlying these effects are still unknown but may involve improved cerebral blood flow and / or effects secondary to per formance improvement during gum chewing.

Conclusion

The results of studies suggest that chewing impacts both oral and systemic health. There are physiological responses such as increases in salivary flow, heart rate and blood flow. Psychological studies showed mastication impacts self perception, particularly in satiety and alertness. In parallel with these studies, there have been reports that psychological perceptions link physiological changes. While it is clear that non-nutritive mastication makes much less impact in overall health compared to other interventions such as oral hygiene practices, diet restriction and exercise, this data provides a lead in understanding how oral stimuli impacts human homeostasis. Further research warrants identification of key mechanisms of action in benefit of chewing.

References

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8. Arosio, M., Ronchi, C. L., Beck-Peccoz, P., Gebbia, C., Giavoli, C., Cappiello, V., Conte, D. and Peracchi, M. (2004), Effects of modified sham feeding on ghrelin levels in healthy human subjects. J Clin Endocrinol Metab 89: 5101-5104
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About the author

Taichi Inui

Taichi received his PhD in the field of pharmacognosy. After joining Wrigley in 2008, he has worked on oral and systemic health focuses. He is currently working in The Wrigley Science Institute, an independent research organisation that explores the benefits of mastication.