Gustation is categorized into five primary tastes: sweet, salty, sour, bitter, and umami. Umami is one of most attractive tastes for humans [1,2,3] since umami taste stimulates the amygdala in the brain, leading to food satisfaction [4]. Recently, a clinical study found that the umami taste sensitivity of obese Western women was dull [4]. Thus, the regulation of umami sensitivity may contribute to the control of food intake and thereby to reduced obesity. Here, we studied the correlation between umami taste sensitivity and obesity in Japanese adults. In addition, we investigated the influence of low umami taste sensitivity on physical condition or dietary composition in a prospective study for one year.

We recruited 47 Japanese adults, and diabetic patients and those taking oral anti-hypertension drugs that have been reported to influence gustation were excluded. There were more women (33 subjects) than men (14 subjects), and the subjects were relatively young, with a mean age of 37.4 years. Subjects with a body mass index >25 were classified into the obese group. Fasting blood samples were collected on the examination day. Participants with a current smoking habit were classified into the smoking group, while those with a current alcohol consumption habit were classified into the alcohol consumer group. Insulin resistance was determined by a homeostasis model assessment as an index of insulin resistance (HOMA-IR) level >2.5. Metabolic syndrome (MetS) was diagnosed based on the guidelines established by eight academic societies, including the Japanese Society of Internal Medicine (JSIM).

To examine sweet and salty taste sensitivities, we used a paper-disc method with Taste Disk™ (Sanwa Kagaku Kenkyusho Co., Ltd.) as the reagent [5]. The subjects who were able to taste sweetness/saltiness with the third paper (10% sucrose solution or 5% saline solution) were classified as normal responders (control), while those who could not taste the third paper were classified into the group with a sweet/salty taste disorder. To examine umami taste sensitivity, we used a diluted Ajinomoto (monosodium glutamate, 97.5%) solution. The subjects held 1 ml 0.03% Ajinomoto solution in their mouths, and those who did not sense the taste were classified into the low umami taste sensitivity group. This criterion was consistent with the results of a previous study [6]. To examine taste preference, we used the questionnaire described in our previous study [7]. For the question “Do you like sweet (salty) food?”, subjects were asked to select among the following possible answers: (1) I hate it, (2) I do not like it, (3) It is okay, (4) I like it, and (5) I love it. To eliminate crossover, subjects who selected response number 5 were considered to have a sweet (salty) taste preference, and those who selected response numbers 1, 2, or 3 were defined as controls. We used the 24-h dietary recall method to investigate the content of the meals the subjects were consuming [8]. To estimate daily salt intake, we used Tanaka’s method [9], using a casual urine sample. To investigate the influence of lower umami taste sensitivity on physical condition and the dietary proportion of nutrients, we evaluated the same subjects again at 9–12 months after the first assessment. At the second assessment, the subjects were examined following the same protocol and by the same staff who carried out the 1st assessment. All analyses were performed using SPSS software version 17.0 (IBM Corp., Armonk, NY, USA). For statistics, the chi-squared test, paired t-test, and logistic regression analysis were used; in the latter, age and gender were used as adjustment factors. The significance level was set at p < 0.05. This study was approved by the Ethics Review Board of Tottori University, Faculty of Medicine (approval number G-84), and Sanin Rosai Hospital (2011). This research was financially supported by the Society for Research on Umami Taste (to EM).

The correlation between umami sensitivity and each clinical parameter is shown in Table 1. The results showed that the frequency of obese subjects was significantly higher in the group with low umami taste sensitivity than in the group with normal umami taste sensitivity (36.4% vs. 11.5%, p = 0.011). In addition, the frequency of subjects with hyperuricemia was significantly higher in the group with low umami taste sensitivity than in the control group. Moreover, the sweet tooth frequency was significantly higher among the subjects with low umami taste sensitivity than among the controls (71.4% vs. 33.3%, p = 0.033). Changes observed among the subjects with umami taste disorder after approximately one year are shown in Table 2. The data indicated that the frequency of subjects with increased daily caloric intake was significantly higher in the low umami taste sensitivity group than in the control group (68.2% vs. 36.0%, p = 0.032).

Table 1 Correlation between umami taste sensitivity and clinical parameters
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Table 2 Changes shown by the subjects with umami taste disorder after about one year
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In summary, we found that (1) impaired umami taste sensitivity estimated by the recognition threshold was significantly associated with obesity; (2) low umami taste sensitivity was significantly correlated with sweet taste preference; and (3) subjects with low umami taste sensitivity prospectively consumed more calories than subjects with normal umami taste sensitivity.

First, we postulate that the subjects with low umami taste sensitivity might eat an excessive amount of sweets because they gain satisfaction not by consuming food with umami flavor but by consuming food containing sugar. Both umami and sweet taste stimulate the amygdala in the brain, leading to food satisfaction [4]; therefore, low umami taste sensitivity would enhance the stimulation of the amygdala induced by sweet taste.

Second, it was reported that sucrose sensitivity was significantly attenuated by monosodium glutamate binding to the extracellular domain of the sweet taste receptor [10]. This report supported the idea that the preference for sweets is enhanced by low umami taste sensitivity.

Finally, although the number of subjects was small, this is the first report suggesting that low umami taste sensitivity may be a new predictor of obesity in Japanese individuals. Recently, the number of Japanese subjects with obesity and metabolic syndrome has rapidly increased. Thus, we considered that assessing and preserving umami taste sensitivity and the habit of obtaining food-related satisfaction via umami taste have an important clinical implication for countries with rapidly increasing lifestyle-related diseases due to the Westernization of food.