Validation of preferred salt concentration in soup based on a randomized blinded experiment in multiple regions in Japan—influence of umami (L-glutamate) on saltiness and palatability of low-salt solutions

Reducing dietary salt intake is essential for preventing the onset and aggravation of hypertension and improving healthy longevity. Elevated blood pressure remains the largest single contributor to global disease burden and mortality, and hypertension prevalence is expected to rise in coming years. In Japan, despite guideline targets recommending salt intake below 6 g/day, average intakes remain substantially higher (approximately 10.8 g/day in men and 9.1 g/day in women). Global and national initiatives aim to reduce population-level salt consumption, including WHO recommendations of less than 5 g/day and a 30% relative reduction by 2025. Randomized trials and meta-analyses show a positive correlation between sodium intake and systolic blood pressure, underscoring the need to lower salt in the food supply, including processed foods and restaurant meals. However, salt reduction often diminishes palatability, hindering adherence to low-salt diets. Prior work suggests that umami compounds such as monosodium L-glutamate (MSG) or calcium diglutamate can enhance palatability and allow reductions in NaCl without compromising taste in various broths and soups. Given soup’s ubiquity in diets, controlling its NaCl concentration is an important strategy for reducing daily salt intake. Previous studies had limitations related to age and number of panelists and used complex broth matrices. This study evaluated the influence of an umami substance on the saltiness and palatability of low-salt aqueous solutions and sought to clarify preferred NaCl and MSG concentrations using a randomized double-blind experiment across multiple regions in Japan.

The paper reviews evidence linking higher sodium intake with increased blood pressure and the global public health imperative for salt reduction, including WHO and national targets. It highlights challenges in real-world adherence due to palatability loss from salt reduction and notes reports of difficulty maintaining low-salt diets even after education. It references studies indicating that impaired salty taste recognition may be associated with masked hypertension. Prior research has shown that adding umami substances (MSG or calcium diglutamate) can maintain or enhance pleasantness, saltiness, and taste intensity while lowering NaCl in soups and broths (e.g., dried bonito, chicken, vegetable, and spicy chicken broths). Given soup’s widespread consumption, these findings motivate exploring umami’s role in facilitating salt reduction. The current study extends this literature by using distilled water solutions to isolate MSG’s effects and by employing a large, diverse sample across multiple regions.

Design: Multicenter, randomized, double-blind sensory evaluation conducted in Japan (2017–2018). Participants tasted six aqueous solutions in randomized order and rated saltiness and palatability using a visual analog scale (VAS). After a first pass (A→F), participants completed questionnaires, then repeated evaluations in reverse order (F→A). The average of the two evaluations was used for analysis. Ethics: Conducted per the Declaration of Helsinki with written informed consent. Approvals: Fukuoka Women’s University Research Ethics Committee (2016-31) for students and Nara Women’s University Research Ethics Review Committee (18-02) for general adults. Registered at UMIN (UMIN000035280; UMIN000035289). Participants: Initially 651 participants (259 university students; 392 adult seminar attendees). Exclusions: taste disorder (n=20), no answer on taste disorder (n=8), incomplete sensory evaluation (n=39). Final analytic sample: 584 men and women from 19 regions. Settings: Evaluations conducted at 8 universities (Aomori, Chiba, Kanagawa, Shizuoka, Hiroshima, Fukuoka, Kagoshima, Nara) and 11 health seminars (Miyagi, Saitama, Tokyo, Shizuoka, Nara, Hiroshima, Kitakyushu, Munakata, Fukuoka City, Nagasaki, Okinawa). Procedures: Standardized materials and instructions provided; room and water temperatures recorded. Mouth rinse with clean water before each tasting; participants spat out samples. VAS: 100-mm line anchored from lowest to highest intensity (not at all salty/extremely unpalatable to extremely salty/palatable). Test samples: Six aqueous solutions with three NaCl concentrations (0.3%, 0.6%, 0.9%) each with or without 0.3% MSG (NaCl >99%, Salt Industry Center of Japan; MSG >99%, Ajinomoto Co., Inc.; distilled water by Kyoei Pharmaceutical). Ten mL aliquots; codes A–F randomized. Rationale for concentrations: Prior internal sensory work found 0.6% NaCl and 0.3% MSG most palatable, and 0.9% NaCl reflects typical soup salt concentration in Japan. Statistics: First and second ratings averaged. Repeated measures ANOVA assessed effects; Tukey HSD for pairwise comparisons. Significance at p<0.05. Software: JMP 14.2 (SAS Institute).

- Saltiness increased with higher NaCl concentrations (0.3%, 0.6%, 0.9%). - MSG enhanced perceived saltiness at low NaCl: 0.3% NaCl with 0.3% MSG vs without MSG: 39.1 ± 0.7 vs 23.5 ± 0.6 VAS mm, p<0.0001. - At 0.6% NaCl, MSG produced a smaller but significant increase in saltiness: 58.2 ± 0.7 vs 55.8 ± 0.7 VAS mm, p=0.0070. - At 0.9% NaCl, MSG did not significantly change saltiness: 72.3 ± 0.7 vs 73.8 ± 0.6 VAS mm, p=0.3013. - Palatability ratings were higher with MSG than without MSG at all NaCl concentrations. The 0.3% NaCl + 0.3% MSG solution’s palatability was approximately double that of 0.3% NaCl without MSG and significantly higher than all other five test solutions. - Subgroup analyses showed similar patterns across sex, age, region, smoking status, fasting time, and medication use. Enhancement of saltiness by MSG at 0.6% NaCl varied by subgroup (e.g., more evident among women, <30 years, nonsmokers, and nonusers of medication). - Environmental conditions during testing were controlled and similar across sites (mean room temperature 25.5 °C ±1.26 SD; water temperature 23.7 °C ±1.92 SD).

The study addressed whether umami (MSG) can enhance saltiness and palatability in low-salt solutions and identified preferred salt concentrations for soup. Findings show that adding 0.3% MSG substantially enhances both saltiness and palatability at low NaCl levels, particularly at 0.3% NaCl, and improves palatability at all tested salt concentrations. At typical soup salt levels (0.9% NaCl), MSG does not further increase perceived saltiness but maintains a palatability advantage. These patterns were consistent across diverse demographic subgroups and regions, suggesting broad applicability. Collectively, the results indicate that MSG can compensate for palatability loss from salt reduction, potentially enabling substantial NaCl reduction in soups (e.g., from 0.9% to 0.3%) without compromising consumer acceptability.

This multicenter randomized double-blind study demonstrates that the addition of 0.3% MSG enhances perceived saltiness and significantly improves palatability of low-salt aqueous solutions. In particular, 0.3% NaCl combined with 0.3% MSG achieved the highest palatability and outperformed higher-NaCl solutions without MSG, suggesting practical potential to reduce soup salt content from 0.9% to 0.3% without sacrificing palatability. Future work could evaluate real-world soup and food matrices, long-term consumer acceptance and behavior change, optimal MSG levels across cuisines, and implications for blood pressure and health outcomes in intervention trials.