ABSTRACT
Studies published in the last decade suggest that chewing (or mastication) and other chewing-like activities such as biting, clenching and bruxing, are involved in regulating bone growth, development and maintenance, reducing stress, increasing attention, contributing to better cognition, stabilizing the temporomandibular joint (TMJ), flagging a possible obstructive sleep apnea (OSA), clearing the oral cavity and allowing the absorption of foods more efficiently. Chewing exerts a considerable force that normally is absorbed by teeth and bones but when it’s disordered, it creates disharmony of the oral structures and functions. Therefore, mastication is an important and complex oral function involving the coordination of muscles and hard tissues such as bones and teeth, as well as nerves and saliva. The evolution of oral health is now intertwined with the devolution of the human face and is linked to reduced chewing in humans, which begins with the modern and Western-like weaning of babies. A new oral health paradigm, including physiologically and epigenetically appropriate weaning and chewing practices, is needed.
How do we usually wean babies? We have available a wide selection of nutritionally balanced jars of baby foods and creative parents can blend their own selection of fruits and veggies. We spoon feed our babies trying to keep one eye on the amount of food consumed and one eye on keeping everything as non-messy as possible. When babies are teething and crying, we offer rubber rings, often cooled to be even more soothing. We have spill-free containers for food on the go, usually breakfast cereals, which become mushy when in the mouth. Then, for the majority of children in the US, it’s mac-and-cheese, frozen snacks, potato chips, pancakes, pizza, fast-food, and maybe some healthy protein-packed smoothies.
Reduced chewing, modern weaning, eating habits, cultural preferences and the economy may have already accelerated the evolution of the human face and head, as described by several authors (Lieberman, 2011, Palmer 1998, Corruccini 1984 and 1990; see also Shaw, 2011), through a process called klinorhynchy. This is a shift of the pattern of facial growth down and back, as opposed to forward, and with the incisors in front of the nose, as in ancestral humans. The klinorhynchy pattern is reducing the optima oral volume for functions. Also, studies by Morquette et al. 2012; Westberger & Kolta, 2011; Lund & Kolta, 2006, indicated that the chewing pattern and the central pattern generator for chewing develop thanks to trigeminal feedback circuits. The trigeminal connections activated during chewing are astounding, not just because they modulate the neuro-occlusal circuit, but because they also stimulate saliva production, bolus control, and trigger the swallowing, with trigeminal (V) in plexus with facial (VII) and glossopharyngeal (IX) cranial nerves, and in predisposing the digestive system by way of trigeminal (V) in plexus with the vagus (X) and the hypoglossus (XII) cranial nerves.
Weaning by baby food may actually slow down the establishment of chewing patterns, as suggested by several studies on animal models. An animal model study, using baby mice, by Suzuki et al, 2007, identified a major component of the muscle fibers, the Myosin Heavy Chains (MHC), as intensifying in the temporalis muscles. This indicates that the chewing muscles get stronger from suckling to chewing a more solid adult diet. Moreover, the supra hyoid chewing muscles, called anterior digastrics, change position and composition as the animal is weaned from milk to solid food (Okubo et al, 2006). Even the muscles of the tongue change between a liquid neonatal diet to a solid diet in mice during weaning, increasing the strength and speed of contraction of the tongue (Maejima et al, 2005). This requires the activation of genes regulating the Myosin Heavy Chains (MHC), which are found in the tongue in higher density than before weaning. Not only do muscles change during weaning to prepare for chewing but salivary glands increase the output as well, as suggested by a study by Elewa et al., 2014, on kid goats’ salivary glands before and after weaning.
Because of the epi-genetic influence of the environment on functions (Kutoba et al. 2003; Moss, 1997; see also Francis, 2011; Hallgrimsson & Hall, 2011), what we chewed during weaning and childhood may have an impact on facial bone growth, development and maintenance. Studies done on mice that were fed soft food, compared with a control group eating hard pellets, showed that the orofacial bone growth is indeed guided by chewing. A study by Mavropoulos et al., 2010, indicates that masticatory changes have been shown to influence the quantity and quality of the alveolar bone in rats during growth. That study was designed to investigate the effect of rehabilitating masticatory function on the morphology and the trabecular architecture of the mandibular alveolar bone, after cessation of growth. Although the rehabilitation of masticatory function led to a significant improvement of alveolar bone architecture in adult rats, the negative effects of hypofunction were not totally reversed during the period under study. Similar results were found by Katsaros et al., 2006. Chewing is also involved in the health and function of the temporomandibular joint (TMJ), as its stability relies on the optimal tone of the chewing muscles masseters and temporalis. Not coincidentally, these are also muscles that provide postural stability to the jaw (Miles, 2007) and the re-conditioning of chewing muscles is frequently included in TMJ rehabilitation and reduction of facial pain (DeFelicio et al., 2011; Tartaglia et al., 2011).
Chewing and swallowing are clearly connected to proper digestion and the optimization of the digestive system, as the digestive system really begins at the mouth, through the mechanical break-down of food and the mixing of the food with saliva, which contains enzymes that break down carbohydrates and other large molecules. Kimura et al., 2006, analyzed the chewing and swallowing of 60 nursing home residents, 30 of them on g-tubes or ng-tubes. The results of the study indicated that there was a functional decrease in the autonomic system of those who were not able to chew and swallow, eventually compromising the functionality of the entire digestive system by decreasing motility and secretions, including saliva. Similar results were found by Lunding et al., 2008, who compared 14 subjects with dyspepsia to 14 healthy subjects. The subjects were asked to eat clear soup followed, or not, by sham chewing of gum and spitting out any saliva produced. Without sham feeding (chewing), patients with functional dyspepsia had a lower motility index than healthy volunteers. In functional dyspepsia patients, vagal stimulation by sham feeding improved antral motility in response to the soup. So, even “fake” (sham) chewing can be beneficial, as chewing is an intrinsic part of eating and bypassing the mouth by delivering food directly to the stomach or intestine seems to “confuse” the brain, which relies on the oral sensory system for feedback.
