Prolonged Exposure to Monosodium Glutamate in Healthy Young Adults Decreases Perceived Umami Taste and Diminishes Appetite for Savory Foods | The Journal of Nutrition | Oxford Academic

Introduction

Experimental and observational studies provide evidence that increased dietary consumption of sweet, salt, or fat is associated with diminished perceived intensity of the stimulus, shifting preference to higher concentrations with prolonged exposure (1–3). Research suggests that adaptive changes occur within the sensory systems with repeated exposure to stimuli, decreasing sensory responses, and ultimately requiring more intense stimulation to elicit the same response (1, 2, 4, 5). Specific to the taste system, supplementation of the diet with highly sweetened beverages for 1 mo was linked to altered sweet taste and preference (3), whereas a low-sugar diet increased perceived sweet intensity after 3 mo (6). A high-salt diet increased the preferred concentration of salt after just 3 wk (2), whereas a low-salt diet increased perceived saltiness and decreased preferred concentrations of salt within 2 mo (7). Likewise, a high-fat diet decreased fat sensitivity, whereas a low-fat diet increased sensitivity after a 4-wk treatment (1), possibly due to altered expression of the putative fat taste sensor transporter CD36 (8).

Although sweet, salt, and fat have been routinely studied, umami is the least-characterized taste, despite being highly relevant to our diet, food choices, and metabolic health. There is limited research on umami taste perception and its connection to diet (9), with epidemiologic studies investigating taste often entirely lacking an assessment of umami (10, 11). Umami taste is thought to signal the ingestion and regulation of protein and amino acids (12–14) and may be linked to body weight maintenance, obesity, and satiation (13–19). Frequently described as savory or meaty, umami taste is strongly elicited by the presence of glutamate or glutamic acid (20, 21). Although glutamates are naturally abundant in many foods (19, 22, 23), a common and powerful stimulus of umami taste in the human diet is monosodium glutamate (MSG). Some evidence suggests that the body may not effectively distinguish added MSG from dietary glutamate (20). Although high-protein foods are naturally high in umami taste (24), gustatory and hedonic responses to MSG have also been linked to dietary protein (12, 25).

We tested the hypothesis that repeated consumption of an umami-rich, MSG-supplemented stimulus in healthy adults would decrease perceived umami intensity and hinder the ability to discriminate low concentrations of umami, and further, would alter hedonics, food preferences, and satiation. We report a randomized controlled study in which participants in the treatment group supplemented their diet for 4 wk with a broth containing the umami-rich stimulus MSG and participants in the control group consumed the same broth, which was sodium-matched but without the added MSG.

Discussion

Perceived umami intensity after a diet high in MSG

Our data show that repeated exposure to umami taste diminishes perceived umami intensity. However, this effect was limited to women in our study. This sex dependence may be partially explained by a lower number of men in our study. Perceived salt taste also tended to decrease across the study period, regardless of treatment group. These results are in line with previous literature suggesting that the appetitive tastes of sweet, salt, and fat may be attenuated, or preferences shifted to more intense stimuli, with a diet high in the respective taste stimuli (1–3). Equivalent associations have been reported for diets low in sugar, salt, and fat (1, 6, 7), suggesting an adaptive relation that is plastic with either high or low exposure to stimuli, although a diet low in umami was not assessed here.

We speculate that our results could be attributed to a downregulation in expression of either the T1R1 (taste receptor type 1 member 1) or T1R3 subunit of the umami-sensing G protein–coupled receptor (43), analogous to that shown for CD36 with repeated dietary exposure to fats in mice (8). In our study, sweet taste intensity (sensed by a T1R2 and T1R3 receptor heterodimer) followed a similar downward trend in those exposed to dietary glutamate compared with controls. This raises the possibility that repeated umami exposure influences the expression or function of the T1R3 subunit, as umami and sweet taste both act partially through this receptor (43). Preliminary research in our group supports the hypothesis of decreased expression of T1Rs with long-term exposure to MSG in mice (44). Similarly, earlier work showed an association between increased consumption of umami-rich foods and impaired umami perception in a free-living human population (45).

Our results show notable sex differences, for which few studies investigating tastant exposure reported testing. Sartor et al. (3) found no differential sex effects on sweet taste after 1 mo of soft drink supplementation. Regardless, sex differences are regularly observed in taste (3, 11, 46, 47), although many studies lack an assessment of umami (10, 11, 48). Circulating sex hormones, such as estrogen, have been hypothesized to differentially influence taste perception between sexes (47), particularly during pregnancy and certain phases of the menstrual cycle (49, 50). Despite this, baseline and post-treatment testing sessions were separated by 28 d, the approximate length of a typical menstrual cycle (51), limiting any effect of menstrual cycle on taste. Sex differences have been previously reported in studies of umami taste (9, 45) and may modify associations between taste and BMI (9) and weight change (45). This may explain some of our results because weight was gained differentially between the sexes across the study period, although any linkage is speculative in nature.

It is possible that dietary differences between sexes could alter the effect of our intervention on taste. In line with previous accounts (52), men tended to report a higher intake of protein at baseline than women, as well as greater habitual glutamate consumption. However, differences in protein or glutamate intake at baseline did not explain differences in umami taste perception. Due to the small sample size of men in the treatment group (n = 8), we lacked power to assess whether men differed in taste response after prolonged dietary exposure to MSG according to relative protein intake. Even so, we reason that if men regularly consume a diet higher in glutamate, any added exposure via our treatment would have less of an effect on taste than that observed in women. Previous reports highlighted similar phenomena, in which a high-fat diet had no effect on fat sensitivity in a group of individuals who were overweight, unlike with a low-fat diet. Another study showed an association between habitual protein intake and reported pleasantness of MSG stimuli, but only when participants were in a state of protein deprivation (25).

Intake and desire for savory food with repeated exposure to umami taste

Our data suggest that desire for and intake of savory foods are diminished with repeated dietary exposure to MSG. There is mixed evidence detailing a link between umami taste, appetite, and satiation. In 2 studies, preload soups with added MSG/inosine monophosphate (IMP) were rated as having a stronger flavor than soup without additional umami stimuli, with the resultant consumption of the preload with MSG decreasing subsequent intake at a test meal (13, 53). It should be noted that such an effect is not consistently supported in the literature (54). Although one study reported increased appetite after intake of soup with MSG (13), another reported a decrease (54), with a third reporting no influence on the motivation to eat (53). Consistently higher hedonic ratings are given to foods supplemented with umami, which is usually attributed to enhanced flavor (53–55), with heightened positive emotions and satisfaction also reported after consumption (55). On the basis of these results, we initially hypothesized that the treatment group in our study would perceive lower umami taste in the savory course than at baseline, and thus would show a diminished appetite compared with the control group, presumably due to lower perceived palatability in the test meal. However, we observed no group differences for hunger, fullness, or prospective food consumption ratings at any point in the meal in this study, and we have no data on the palatability of the meal. Alternatively, the treatment group could perceive less umami, be less satiated, and be driven to eat more than the treatment group. However, this was not supported in our data. We can also rule out any demand effects on appetite due to varying liking of the 2 groups’ broths, because analyses showed no significant group differences in hedonic ratings of the broth after the 4-wk treatment.

Exploratory data analyses suggest that, irrespective of treatment, attenuated umami taste at lower concentrations may be associated with decreased desire for savory foods. Because women primarily showed decreased perceived umami intensity with repeated exposure to MSG, whereas both sexes reported decreased desire for and intake of savory food, perceived umami intensity may not entirely explain observed behavior associated with appetite. It is possible that the intake of MSG may have postingestive appetite effects beyond the peripheral taste system, as suggested by previous literature (56, 57).

Alternatively, our results could be explained with decreased intake in the test meal attributed to a diminished desire for savory food. Indeed, this is supported in our data, in which a decreased desire for savory food correlates with decreased intake in the savory course of the test meal, which was especially evident before the beginning of the meal. Research has shown that exposure to savory has an especially strong effect on ensuing appetite and food choices (58, 59). We speculate that the treatment group may have become overstimulated with umami taste during the treatment period and were simply less driven to consume savory, which is in line with sensory-specific satiety theory (60).

Desire for high-protein foods with a diet high in MSG

The implicit measures of liking and wanting suggested a slight increase in desire for high-protein foods relative to baseline, with little change in the controls, although this did not reach the statistical threshold between groups. Those consuming the broth with MSG tended to be more likely to choose high-protein foods over low-protein foods in forced-choice measures, and showed greater implicit wanting for high-protein foods after the intervention. Assuming that umami taste simulates amino acid consumption, this result is in contrast to some reports of increased implicit wanting for high-protein foods after a low-protein diet, with no change after a high-protein diet (38). Alternatively, as with our study, previous results have shown that decreased perception of umami is associated with decreased desire for protein (12). Meanwhile, rated liking of real foods in this study did not differ with treatment, which could imply that implicit measures are more susceptible to change with exposure to umami taste than explicit measures.

Limitations and future work

Results from this study are limited to relatively young, normal-weight, nonsmoking, and nonrestrained eaters. Our randomized controlled study design further limits confounding factors on the outcomes. It should be noted, however, that even though treatment groups in our study were randomly assigned and balanced on sex and habitual glutamate consumption, and thus any influence of sex hormones or diet should be considered nondifferential bias, it is possible that our sample was not large enough to truly limit other confounding factors. Furthermore, this study was powered to detect differences between treatment groups in perceived taste intensity, as opposed to other secondary measures. Although it has been suggested that satiation and satiety can be quantified with a single ad libitum meal (30), future studies should duplicate our findings with >1 test meal. Replication in a larger population with adjustment for multiple comparisons would also serve to remedy any concerns with testing for numerous secondary outcome measures, as well as the smaller sample size of men in our study. Although our study contributes to unraveling the relation between diet, umami taste, and health, umami taste remains relatively poorly studied. Further studies that examine umami taste to understand additional environmental or genetic factors that may contribute to variations in perception and food preference, and how sex may modify these relations, would be valuable.

Conclusions

Our results highlight a complex relation between diet, umami taste, food preference, and appetite. Relative to controls, increased dietary exposure to MSG diminished umami taste response (selectively in women) and decreased the desire for, and intake of savory foods at an ad libitum meal. Findings from this research can be applied to the study of food choice, a critical factor in the development and maintenance of diet-related diseases, including obesity, osteoporosis, and kidney disease

Source: Prolonged Exposure to Monosodium Glutamate in Healthy Young Adults Decreases Perceived Umami Taste and Diminishes Appetite for Savory Foods | The Journal of Nutrition | Oxford Academic