The timing and frequency of meals are established factors influencing diabetes prevalence and blood glucose control. However, the role of food temperature remains largely unexplored, particularly in type 2 diabetes mellitus (T2DM). Recent studies in Asian populations suggest that food temperature might influence glycemic responses to rice and potatoes. Animal studies have also shown that food temperature affects early-phase insulin release. Increased food temperature could potentially activate the nervous system via heat-activated ion channels, such as TRPV1. Gastric emptying, known to correlate with blood glucose levels and influence GLP-1 release, is also affected by food temperature. Cold and hot temperatures have been shown to slow gastric emptying by altering antropyloroduodenal motility and gastric electrical activity. However, the consequent effects on blood glucose, insulin, and gut hormones remain largely uninvestigated. Given the significant variations in food and drink temperatures across different cuisines, this study aimed to investigate the impact of hot and cold temperatures on glucose metabolism, specifically examining blood glucose and glucose-responsive hormones in both normal subjects and newly diagnosed, untreated T2DM patients. Continuous glucose monitoring (CGM) was employed to assess the overall glucose profiles throughout the day under different food temperature conditions.

Several studies have highlighted the impact of meal timing and frequency on diabetes prevalence and blood glucose control. Jakubowicz et al. demonstrated that fasting until noon impaired insulin response and increased postprandial hyperglycemia. Research also points to the influence of eating rate on energy intake and hunger. Previous studies have shown that cooling cooked rice increases resistant starch content and reduces glycemic response, and that hot potatoes induce a different glycemic and insulinemic response compared to cooled potatoes. Animal studies by Shinozaki et al. indicated a relationship between taste-induced reflexes and temperature of sweet tastes. Moreover, the impact of food temperature on gastric emptying is well-established. Sun et al. demonstrated that both hot and cold drinks empty more slowly than room-temperature drinks, with cold drinks taking longer to reach body temperature and showing greater differences in stomach contents compared to hot drinks. The interplay between gastric emptying and GLP-1 release is also documented, with faster gastric emptying potentially influencing GLP-1 secretion. The effect of temperature on perceived sweetness has also been studied, with cooling potentially reducing perceived sweetness intensity. The role of sweet taste receptors in GLP-1 and GIP secretion, while suggested, needs further investigation. Existing studies on GLP-1 levels in T2DM patients lack stringent controls for food temperature, prompting further research to understand the role of temperature in these variations.

This crossover, self-controlled study recruited 19 normal subjects and 22 newly diagnosed, untreated T2DM patients. Inclusion criteria included age (18-60), BMI (18.0-28.0 kg/m²), and absence of any hypoglycemic agent use, liver or kidney abnormalities, recent infections, or corticosteroid use. Patients with T2DM had HbA1c < 86 mmol/mol (10.0%). Participants underwent two OGTTs on consecutive days, one with a hot (50°C) and the other with a cold (8°C) glucose solution (75g glucose in 300ml water). Assignment to hot or cold OGTT on the first day was randomized. On the day following the hot OGTT, participants consumed hot food and water (>42°C), and vice versa for the day following cold OGTT. Food composition and caloric content were consistent between days. Blood samples were drawn at 0, 5, 10, 30, 60, and 120 min during each OGTT for measurements of glucose, insulin, GIP, GLP-1, and cortisol. CGM was conducted for 3 consecutive days, with 4 daily capillary blood glucose measurements for calibration. Statistical analyses included ANOVA for repeated measures for within-group comparisons and unpaired t-tests or Mann-Whitney U tests for between-group comparisons. A pre-study with six normal subjects determined the required sample size.

Compared to cold OGTT, hot OGTT resulted in significantly higher mean blood glucose levels in both normal subjects (P = 0.002) and T2DM patients (P = 0.028). Post-hoc analysis revealed higher blood glucose levels in the hot OGTT compared to the cold OGTT at multiple time points (5, 10, 30, and 120 min in normal subjects; 5 min in T2DM patients). Significantly higher insulin and GLP-1 levels after hot OGTT compared to cold OGTT were observed only in normal subjects (P < 0.05 for both), not in T2DM patients. No significant differences in GIP or cortisol were found between hot and cold OGTTs in either group. CGM data showed significantly higher 24-hour mean blood glucose (MBG) in both groups on the day of hot meal intake compared to the day of cold meal intake. The standard deviation of MBG (SDBG) was significantly higher on the day of hot meal intake in normal subjects only. No significant differences were observed in the coefficient of variation (CV), time in range (TIR), or 24-hour largest amplitude of glycemic excursion (LAGE) between hot and cold food intake days in either group. Regression analysis revealed no significant correlation between ΔIAUC glucose/GLP-1 and age, sex, BMI, HbA1c, or temperature difference between cold and hot OGTTs.

This study provides novel evidence that food temperature significantly affects post-OGTT blood glucose levels and GLP-1 response. Hot food resulted in higher blood glucose in both normal subjects and T2DM patients, although the differences were small but consistent across OGTTs and CGM data. These findings align with previous studies suggesting a link between gastric emptying rate and blood glucose response, where slower gastric emptying with cold drinks could lead to lower initial glucose levels. However, the significantly higher GLP-1 response to hot OGTT in normal subjects, absent in T2DM patients, requires further investigation. This discrepancy between groups might be related to differences in incretin sensitivity or other glucose-regulatory mechanisms. The potential impact of temperature on sweetness perception and its influence on GLP-1 and GIP release is also an area for future exploration. The mechanisms underlying the differential response to temperature could involve factors such as altered gastric emptying, changes in peripheral glucose uptake, effects on hormone degradation, or the influence of temperature on taste receptor activation.

This study demonstrates that food temperature is a significant factor influencing glucose absorption and GLP-1 response. Hot food led to higher blood glucose levels and a greater GLP-1 response in normal subjects, but this effect was diminished in T2DM patients. These findings highlight the need for further research to elucidate the underlying mechanisms and explore the potential implications for diabetes management. Future studies could investigate optimal food temperatures for glucose control and GLP-1 secretion, explore intragastric glucose administration to minimize orosensory effects, and examine the role of temperature in incretin secretion and sensitivity.

This study had some limitations. A third OGTT with a control temperature (room temperature or body temperature) was not included due to concerns about potential severe hyperglycemia in the T2DM group with multiple consecutive OGTTs without hypoglycemic treatment. The potential influence of temperature on the oral cavity, salivary glands, and orosensory effects was not controlled and could be further investigated. The relatively small sample size and specific study population might affect the generalizability of findings to other populations.