Introduction
Saliva has many crucial roles in promoting health, including protecting the oral cavity and facilitating eating. Within the mouth, saliva hydrates mucosal tissues, removes cell and food debris, buffers oral pH, lubricates the oral cavity aiding mastication and preventing dental wear, forms food boli to assist swallowing, protects against teeth demineralization, has antimicrobial activity and prevents infections, and closes wounds while stimulating healing [1, 2]. Saliva also plays essential roles in food perception and digestion. The exact mechanisms of digestion remain unclear. For taste, the physical and compositional characteristics of saliva facilitate perception. For example, the fact that saliva is an aqueous liquid makes it an ideal vehicle for carrying taste stimuli and nutrients to the taste receptors [3], which are widely distributed on the tongue, soft palate, and pharynx. Unstimulated saliva also presents low levels of taste stimuli, such as salts and glucose, in comparison to plasma, which enables low detection threshold [1, 4]. Taste perception guides dietary choices as well as influences physiological processes pre- and post-absorptively [5, 6]. The anticipatory phase of digestion is labeled the “cephalic phase responses” and serves to prime the body to metabolize ingested nutrients efficiently, making it an important step in food digestion and the prevention of dysglycemia and dyslipidemia.
Additionally, saliva contains a large number of proteins involved in lipase, peptidase, and hydrolase activities. When comparing the saliva and plasma proteomes, it is clear that the distributions of the salivary proteins are geared toward metabolic and catabolic processes. This indicates that saliva has a major physiologic role in food digestion [7]. The most abundant protein in human saliva is the digestive enzyme α-amylase [8]. This enzyme cleaves large starch molecules into dextrin and subsequently into smaller maltooligosaccharides (MOS) containing α-D-(1,4) linkages, isomaltooligosaccharides (IMOS) containing α-D-(1,6) linkages, the trisaccharide maltotriose, and the disaccharide maltose [9]. Glucose will then be generated from maltose via the action of disaccharide enzymes, such as maltase. In the human body, amylase is predominantly produced by the salivary glands and the pancreas. Although salivary and pancreatic amylases are similar, they are encoded by different genes (AMY1 and AMY2, respectively) and show different levels of activity against starches of various origins [10].
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The physiological significance of salivary amylase is still being uncovered and aspects of it are controversial, for example, its normative secretory function in plasma remains a mystery. Salivary amylase has a relatively short active contact time with starch. Once a food bolus is swallowed and infiltrated with gastric juice, its catabolic activity is mostly stopped by low acidic pH. Some activity remains within particles due to the barrier protection provided by partially digested starch on the outside of the particle [11], but the majority of the starch is digested by the abundant pancreatic amylase, which is released into the duodenal portion of the small intestine. Nevertheless, studies have demonstrated that considerable starch hydrolysis occurs within seconds in the oral cavity, transforming the gelatinous texture of starch into a semiliquid [12, 13]. This change of texture might itself influence starch digestion, sensory preferences, and starch intake. Additionally, recent studies have also demonstrated that the small MOS amylolytic products can be detected in the oral cavity via the taste system [14]. These findings strongly support a physiological pre-absorptive role of salivary amylase in starch digestion.
In this review, we will discuss the evolutionary forces that drive the existence of salivary amylase, the benefits of generating higher salivary amylase levels, the possible physiological consequences of early oral starch breakdown, and its roles in protecting blood glucose profile and blood insulin, as well as the disease states of metabolic syndrome, diabetes, and obesity.
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