Introduction

Providing appropriate food forms is crucial to patients with reduced mastication or swallowing function to prevent malnutrition and aspiration1. Foods modified for dysphagia are food forms especially formulated for patients with reduced mastication and swallowing functions in terms of form, thickness, and flexibility to match patients’ swallowing abilities. The International Dysphagia Diet Standardization Initiative (IDDSI) Framework, which provides global standards, was published in 20172, whereas the earlier Japanese Dysphagia Diet domestic framework was developed in 2013 and has since been widely used in Japan3.

Although the Japanese Dysphagia Diet 2013 was revised in 2021 (JDD2021)4, the core principles remain unchanged. This classification includes five stages (codes 0–4) for modified foods and three for modified liquids. The thickening standards align closely with those used internationally5. A unique feature of JDD2021 is code 0, which is the swallowing training diet designed for early-stage swallowing rehabilitation. It further distinguishes between 0j (“j” for jelly) and 0t (“t” for thickened) to accommodate a wider range of adult patients with mid-stage dysphagia. Code 0j refers to a homogeneous, less sticky, cohesive, softer, and low-shrinkage jelly that can be sliced easily. Specifically, it can be scooped with a thin, flat spoon into a mass of 2 cm (length) × 2 cm (width) × 5 mm (thickness) and be directly swallowed without crushing or masticating. It must also be easily mechanically suctioned when residues remain in the mouth6. This type of jelly is now commercially available for rehydration7 and medication8. Similarly, code 0t refers to a moderately thick liquid (150–300 mPa viscosity) with a consistency that flows slowly down a tilted spoon. This thickened liquid is commonly used worldwide for dysphagia therapy. However, some foreign countries consider jelly unsuitable for patients with dysphagia.

Although the JDD2021 is widely used in Japan, no consensus has been reached on the use of 0j and 0t. Current guidance recommends 0t over 0j only when the jelly is aspirated or when it dissolves in the mouth. Therefore, the purpose of this study is to examine the swallowing characteristics of code 0j, which is less commonly used in dysphagia rehabilitation outside Japan, compared to other food forms, and to scientifically validate whether using jelly in swallowing therapy is appropriate.

Methods

This study was approved by the Suiseikai Kajikawa Hospital Ethics Committee (Approval no. 2016-1) and the Hiroshima University Graduate School of Biomedical and Health Sciences Ethics Committee (Approval no. E-1151). All procedures were performed in accordance with institutional guidelines and adhered to the Declaration of Helsinki. Written informed consent was obtained from all participants and/or their legal guardians.

We enrolled consecutive patients with acute stroke who were admitted to the Suiseikai Kajikawa Hospital between August 1, 2016, and March 31, 20209,10. However, since time analysis was only incorporated after the results of Takeda et al.9 were published, the subjects enrolled in this study were those admitted between January 11, 2018, and March 31, 2020.

The inclusion and exclusion criteria were the same as those used by Nakamori et al.10. The inclusion criteria were patients with first-time stroke (ischemic or hemorrhagic) admitted within 1 week of onset, were aged ≥ 20 years, and consented to participate in the study (if the patient was unable to provide his/her consent, it was sought from relatives). We excluded patients with a history of stroke or dementia (using the International Statistical Classification of Diseases and Related Health Problems 10th Revision), those with altered mental state after severe stroke (if the best Glasgow Coma Scale eye response score ≤ 3) and those who had not resumed oral intake. Patients who had more than 4 points on the modified water swallow test (MWST) and were able to swallow ≥ 3 times within 30 s on the Repetitive Saliva Swallowing Test (RSST) were considered eligible11. To further minimize risk, we included those who exhibited the lowest possible risk of dysphagia with a score ≥ 95 on the Modified Mann Assessment of Swallowing Ability (MMASA)12. Because VF involves radiation exposure and is unsuitable for healthy individuals, these criteria were further used to select patients with stroke showing relatively normal swallowing function.

All patients underwent VF within 14 days of the stroke event. Three food forms with different consistencies were tested: 3 mL thin liquid, 3 mL thick liquid containing 3% thickening agent (defined as extremely thick), and 3 g jelly, each containing 50% iodine contrast medium (Oypalomin 370, Fuji Pharma Co. Ltd., Tokyo, Japan). Neo High Toromeal (Food Care Co., Ltd., Tokyo, Japan) was used as the thickening agent, while Aqua Jelly Powder (Food Care Co., Ltd.) was used to prepare the jelly. The tester placed each food form on the patient’s tongue, who were then instructed to swallow the food in one motion. The tests were conducted once or twice for each food form in a fixed sequence: thin liquid, thickened liquid, and jelly. The best replicate based on inter-examiner discussion was used for analysis.

X-ray examinations were conducted with a Philips BV Pulsera X-ray system (Amsterdam, Netherlands) at 30 frames per second, and the data were recorded on a DVD. With the patients in a seated position, images were captured of lateral views encompassing anatomical landmarks anteriorly to the lips, posteriorly to the pharynx wall, superiorly to the nasal cavity, and inferiorly to the upper esophageal sphincter. Two experienced dentists (TN and MY) who specialized in VF evaluation and were blinded to the patients’ clinical data, evaluated the presence of natural light-visible aspiration and quantitatively analyzed indicators commonly used to assess aspiration risk on VF. These parameters, which are useful in assessing aspiration risk by VF, included pharyngeal delay time (PDT), defined as the interval from when the bolus head reached the mandibular ramus–tongue base junction to the onset of laryngeal elevation13,14; pharyngeal transit time (PTT), the interval from when the bolus head reached the mandibular ramus–tongue base junction until the bolus tail passed the cricopharyngeal region or pharyngoesophageal segment13,15; and laryngeal elevation delay time (LEDT), the interval from bolus arrival at the pyriform sinus to the onset of maximum laryngeal elevation16.　Furthermore, to distinguish between bolus passage in the middle pharynx and hypopharynx, PTT was divided into two phases: oropharyngeal transit time (oPTT), measuring the time to bolus epiglottic passage, and hypopharyngeal transit time (hPTT), the interval from bolus epiglottic passage to cricopharyngeal transit.

Statistical analysis

The VF characteristics of patients who aspirated during thin liquid swallowing were compared with those who did not. Basic information and VF time measurements were compared using the χ2 test and Mann–Whitney U test, respectively. Among those who did not aspirate during thin liquid swallowing, PTT, PDT, LEDT, oPTT, and hPTT of jelly, thickened liquid, and thin liquid during VF were compared using the Friedman test and Scheffe post hoc test. All analyses were conducted using IBM SPSS ver. 23 (IBM, Tokyo, Japan), and results were reported with 95% confidence intervals.

Results

Table 1 presents patient data on aspiration during thin liquid swallowing. Out of 175 patients (104 men and 71 women, median age: 70 years) included in the study, 24 (13.7%) experienced aspiration. Although no significant differences in age or underlying diseases were observed between those who aspirated when swallowing thin liquid and those who did not, VF time analysis revealed that LEDT was significantly prolonged in those who aspirated (p < 0.05). Half of those who aspirated had an LEDT > 0.35 s, which is the cutoff value12.

Table 1 Relationship between thin liquid swallowing aspiration findings in videofluoroscopy and various variables.
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Swallowing dynamics were quantitatively compared for each food form, excluding patients who aspirated during thin liquid swallowing (Table 2). All three food forms were swallowed all at once, as instructed, and no residue remained after swallowing. The results indicated no significant difference in PTT among the three food forms. However, oPTT was significantly prolonged and hPTT significantly shortened in swallowing jelly (p < 0.05). Furthermore, PDT was significantly delayed in swallowing jelly compared with thickened and thin liquid. In contrast, LEDT was significantly delayed during swallowing of thin liquid compared with jelly and thickened liquid (p < 0.05).

Table 2 Results of Temporal videofluoroscopic analysis of three test materials in patients who did not aspirate during thin liquid swallowing.
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Discussion

In this study, we compared the swallowing dynamics of jelly and thickened liquid, the food forms most commonly used for dysphagia therapy in Japan, in stroke patients who were unlikely to have dysphagia based on the screening test. Normally, this study would have been conducted on healthy elderly subjects, but due to the radiation exposure requirement17, only selected stroke patients who exhibited near-normal swallowing function with screening tests. No gold standard has been established for dysphagia screening; thus, a multimodal assessment is desirable18. In this study, we employed a combination of MWST and RSST, which have been well-validated and are commonly used in Japan11. Moreover, the MMASA was incorporated to identify patients with low aspiration risk19. Despite this multimodal approach, approximately 10% of aspirations were missed during the screening tests. Similarly, in the present study, aspiration during thin liquid swallowing was observed in 13.7% of the patients. Patients who aspirated exhibited significantly prolonged LEDT, which indicates reduced sensitivity to peripheral nerve input16. This aligns with the study by Jafari et al.20, which reported that afferent signals from the superior laryngeal nerve help facilitate laryngeal closure during swallowing. These results imply that reduced pharyngolaryngeal sensory feedback during swallowing may cause aspiration when swallowing thin liquids. This study compared the swallowing dynamics of jelly and thickened water in patients with near-normal swallowing function, excluding those who aspirated thin liquids since they were considered to have dysphagia.

We conducted VF temporal analysis of the swallowing dynamics, specifically the pharyngeal phase of swallowing, using jelly, thickened liquid, and thin liquid. All food forms demonstrated a PTT of approximately 1 s, which is comparable to previous findings on healthy elderly subjects10,21 and contrasts with the prolonged PTT observed in patients with dysphagia22. However, pharyngeal passage patterns differed among the food forms. Significant differences in LEDT were observed between thin and thickened liquid, indicating that the thickening agent could potentially reduce aspiration risk by slightly delaying bolus arrival at the piriform sinus. The jelly induced a swallowing reflex slightly deeper than the faucial isthmus, specifically upon bolus arrival at the epiglottis. This indicates that the jelly settled briefly in the vallecula before eliciting the swallowing reflex. These findings provide empirical evidence of existing clinical practices, indicating that jelly should be avoided in cases where premature pharyngeal entry may occur, such as in cases of impaired consciousness or attention disorders.

This study has some limitations. First, we did not evaluate patients who aspirated with jelly or thickened liquid. Therefore, we could not provide definitive comparisons of the swallowing dynamics of patients who actually aspirated with these food forms. Further confirmatory studies are thus needed. Second, the study did not examine the movement and range of motion of the hyoid bone and larynx, which may be involved in aspiration, because our screening test may have excluded individuals with such swallowing impairments. Furthermore, although the IDDSI is currently being promoted as an international standard, its classification system is based on clinical practice and must be objectively investigated to determine their effects on swallowing dynamics and function. Nevertheless, this study provides empirical support for the JDD2021 diet selection criteria and establishes a framework for selecting food forms.

Conclusion

This study reports important findings regarding swallowing dynamics. First, thin liquids reach the piriform sinus before maximal laryngeal elevation, posing a risk of penetration and aspiration before complete laryngeal closure. Second, thickening agents may reduce aspiration risk by slightly delaying bolus arrival at the piriform sinus. Third, jelly may reduce aspiration risk by delaying oropharyngeal passage, triggering the swallowing reflex in the epiglottis, and preventing hypopharyngeal passage after complete laryngeal closure. These findings provide scientific validation of existing clinical practices and highlight the importance of tailoring food consistency to swallowing function in patients.