Ultra-processed Food Reducing Intervention and Continuous Glucose Monitoring

The goal of this trial is to investigate whether reduction in ultra-processed food intake through diet counseling and education can improve postprandial glucose levels and glycemic variability among Korean healthy adults aged 20-30 years. The main questions it aims to answer are: * Does the reduction in ultra-processed food intake lower postprandial glucose levels and glycemic variability in healthy adults without a history of diabetes? * Does the reduced intake in different ultra-processed food subgroups and items differentially influence postprandial glucose and glycemic variability? * Does other dietary and lifestyle factors (physical activity, sleep, smoking) alter the association between ultra-processed food intake reduction and glycemic variability? Participants will: * Undergo the 10-day pre-intervention monitoring period, during which each participant will wear a continuous glucose monitoring (CGM) device and concurrently report their daily dietary intakes (all food and beverage consumptions) and other lifestyle behaviors (sleep, smoking, physical activity) * After the 10-day pre-intervention monitoring period, participants will be randomized to either intervention or control group * Intervention group: Participants will visit the research site to receive dietitian-led nutrition education and personalized diet counseling targeting reduction of ultra-processed food intake, as well as improving diet quality. Personalized diet counseling will be provided by study dietitian based on participant's records of dietary intakes during the 10-day pre-intervention monitoring period. * Control group: Participants will receive dietitian-led nutrition education and personalized diet counseling targeting general improvement in nutrient intake (based on the national dietary guidelines). * After the intervention, participants will undergo the 10-day post-intervention monitoring period, during which participants will wear a new CGM device for an additional 10 days and continue daily reporting of dietary intakes (all food and beverage consumptions) and lifestyle behaviors (sleep, smoking, physical activity).

Diabetes is one of leading causes of mortality in Korean population. The increasing trend in diabetes prevalence is also observed among young Korean adults aged 19-29 years, imposing a substantial societal and economic burden. To mitigate the burden of diabetes, nutrient-based dietary strategies, such as increasing the intake of dietary fiber, while reducing total energy intake, and the consumption of refined carbohydrates, trans and saturated fats, and sodium, have been widely recommended to improve glycemic control and reduce glucose fluctuations. However, recent studies examining the association between ultra-processed food (UPF) intake and type 2 diabetes have suggested that the degree and methods of food processing may be independently associated with glycemic dysregulation, and this association is not fully considered in current nutrient-based dietary guidelines. According to the Nova classification, UPFs represents the most highly processed category of foods, including items such as sugar-sweetened beverages, packaged snacks, fast foods, and instant noodles. UPFs are typically characterized by poor nutritional quality (e.g., high in energy density, dietary fat, and sodium; low in dietary fiber and essential micronutrients) along with the use of ingredients not commonly used in household cooking, such as artificial flavorings, emulsifiers, and colorants. Excessive energy intake and nutrient imbalance resulting from UPF intake have been associated with an increased risk of obesity and chronic diseases, including diabetes. However, food additives commonly found in UPFs may also contribute to chronic inflammation, insulin resistance, and gut microbiome imbalances, all of which can promote glycemic dysregulation leading to the development of type 2 diabetes, independently of their nutrient quality. However, some UPFs with modified nutrient profiles, such as reduced saturated fat or added artificial sweeteners, are often considered as 'healthy' under the current nutrient-based dietary guidelines despite their high food additive contents. In Korea, from 1998 to 2018, the contribution of UPFs to total energy intake continuously increased, whereas that of minimally processed foods declined. Despite the increasing trend of UPF intake, evidence from randomized controlled trials (RCT) directly examining its association with glycemic outcomes including postprandial glucose level and glycemic variability remains limited. As dietary and lifestyle factors may act as potential confounders, RCTs are needed to establish a causal relationship. Additionally, a previous meta-analysis showed that the magnitude of the associations with type 2 diabetes varied across different UPF subgroups, with processed meats and artificially- and sugar-sweetened beverages showing the greatest magnitude of positive associations. The findings of the previous study suggest that not all UPFs are equally harmful and address the limitations of current Nova classification. While the Nova classification provides a useful framework for identifying UPFs based on the methods and degrees of processing, it does not fully capture the heterogeneity within the group, such as differences in nutrient profiles, amount and types of additives, and specific processing techniques. Continuous glucose monitoring (CGM) devices provide more accurate and real-time glycemic assessment. This study aims to investigate whether reduction in UPF intake can improve postprandial glucose levels (measured 2 hours after meal initiation) and glycemic variability (coefficient of variation \[CV\], standard deviation \[SD\], time in range 70-180 mg/dL \[TIR70-180\], time in range 90-126 mg/dL \[TIR90-126\], time above range \>126 mg/dL \[TAR126\], and time below range \<90 mg/dL \[TBR90\], in healthy adults aged 20-30 years. Glycemic responses under free-living conditions will be assessed using CGM, allowing for detailed observation of glucose fluctuations in response to daily dietary intake. In addition, this study aims to evaluate the potential role of lifestyle behaviors including sleep, smoking, and physical activity as effect modifiers in the association between reduction in UPF intake and glycemic outcomes. This study is a single-site, individually randomized trial conducted in Seoul, Korea, by the Health Promotion and Chronic Disease Epidemiology Lab at the College of Health Science, Korea University (KU). The study protocol was designed in accordance with the Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) guidelines. In summary, 40 adults aged 20-30 years, with no personal history of diabetes and whose energy intake from UPFs constitutes 25% or more, will be recruited from the community. Participants will be observed for 10 days to assess baseline (pre-intervention) glucose levels, dietary intake and lifestyle. To ensure rigorous glucose monitoring, all participants will be wearing a CGM device. This will be followed by a dietary education and counselling session. Participants assigned in the control group will receive conventional dietary education based on nutrient-based national dietary guidelines, whereas those in the intervention group will receive education aimed at reducing UPF intake. After the education and counselling sessions, all participants will be observed for an additional 10 days to collect CGM-derived glucose data, dietary intake, and lifestyle in order to evaluate changes compared to the baseline period.