Development of an umami taste sensitivity test and its clinical use.

There is a close relationship between perception of umami, which has become recognized as the fifth taste, and the human physical condition. We have developed a clinical test for umami taste sensitivity using a filter paper disc with a range of six monosodium glutamate (MSG) concentrations. We recruited 28 patients with taste disorders (45-78 years) and 184 controls with no taste disorders (102 young [18-25 years] and 82 older [65-89 years] participants). Filter paper discs (5 mm dia.) were soaked in aqueous MSG solutions (1, 5, 10, 50, 100 and 200 mM), then placed on three oral sites innervated by different taste nerves. The lowest concentration participants correctly identified was defined as the recognition threshold (RT) for MSG. This test showed good reproducibility for inter- and intra-observer variability. We concluded that: (1) The RT of healthy controls differed at measurement sites innervated by different taste nerves; that is, the RT of the anterior tongue was higher than that of either the posterior tongue or the soft palate in both young and older individuals. (2) No significant difference in RT was found between young adults and older individuals at any measurement site. (3) The RT of patients with taste disorders was higher before treatment than that of the healthy controls at any measurement site. (4) The RT after treatment in these patients improved to the same level as that of the healthy controls. (5) The cutoff values of RT, showing the highest diagnostic accuracy (true positives + true negatives), were 200 mM MSG for AT and 50 mM MSG for PT and SP. The diagnostic accuracy at these cutoff values was 0.92, 0.87 and 0.86 for AT, PT and SP, respectively. Consequently, this umami taste sensitivity test is useful for discriminating between normal and abnormal umami taste sensations.

Gustatory sensation of (L)- and (D)-amino acids in humans.

Amino acids are known to elicit complex taste, but most human psychophysical studies on the taste of amino acids have focused on a single basic taste, such as umami (savory) taste, sweetness, or bitterness. In this study, we addressed the potential relationship between the structure and the taste properties of amino acids by measuring the human gustatory intensity and quality in response to aqueous solutions of proteogenic amino acids in comparison to D-enantiomers. Trained subjects tasted aqueous solution of each amino acid and evaluated the intensities of total taste and each basic taste using a category-ratio scale. Each basic taste of amino acids showed the dependency on its hydrophobicity, size, charge, functional groups on the side chain, and chirality of the alpha carbon. In addition, the overall taste of amino acid was found to be the combination of basic tastes according to the partial structure. For example, hydrophilic non-charged middle-sized amino acids elicited sweetness, and L-enantiomeric hydrophilic middle-sized structure was necessary for umami taste. For example, L-serine had mainly sweet and minor umami taste, and D-serine was sweet. We further applied Stevens' psychophysical function to relate the total-taste intensity and the concentration, and found that the slope values depended on the major quality of taste (e.g., bitter large, sour small).

Application of umami taste stimulation to remedy hypogeusia based on reflex salivation.

Enjoying taste should be one of the greatest pleasures in human life. However, aging is sometimes associated with decreased taste sensitivity, also known as hypogeusia. The loss of taste not only affects quality of life, but can also cause weight loss and health problems in the elderly. Our recent study has shown that 37% of test subjects over 65 years of age exhibited hypogeusia. Further, whole saliva secretion, including minor salivary secretion, was significantly decreased in elderly patients with gustatory impairment, but was normal in all elderly subjects with normal taste thresholds. These data indicate that hyposalivation is closely related to hypogeusia. Moreover, clinical studies have shown that treatment of hyposalivation diminishes hypogeusia, indicating that salivation is essential to maintain normal taste function. However, many medications for relief of dry mouth, such as parasympathomimetic (cholinomimetic) drugs, have serious adverse effects. Palpitation, sweating, nausea, diarrhea and dizziness have all been observed in elderly patients taking parasympathomimetic drugs. To circumvent this problem, glutamate, which produces umami taste, was demonstrated to increase salivary secretion and thereby improve hypogeusia by enhancing the gustatory-salivary reflex. Our data suggests that umami is an effective tool for the relief of hypogeusia without the side effects of parasympathomimetic drugs.

Contribution of umami taste substances in human salivation during meal.

The oral gustatory perception during a meal has very important physiological roles such as inducing appetite, smoothing mastication and swallowing, promoting digestion and each nutrient availability. One hundred years ago, L-glutamate was discovered as a new taste substance in Japan. Since then, Japanese taste physiologists have lead the research to establish L-glutamate as the prototype molecule for the fifth basic taste (umami taste), in addition to saltiness, sweetness, bitterness and sourness. Meanwhile, various lines of evidence demonstrated that taste perception is linked to taste stimuli-oral/pharyngeal reflexes. In this review, we focus on the efficacy of L-glutamate for human salivation and discuss the possible application of umami taste simulation to the nutritional management for the elderly due to amelioration of their quality of life (QOL).