The goal of this clinical trial is to learn how food processing level affects appetite and gastrointestinal transit time when meal palatability is changed. Participants will complete one screening visit and four test visits. During each test visit, participants will consume one of four meals that differ by food processing level and palatability: high-palatability ultra-processed food, low-palatability ultra-processed food, high-palatability non-ultra-processed food, or low-palatability non-ultra-processed food. The main questions this study aims to answer are whether food processing and palatability affect appetite sensations and gastrointestinal transit time. Participants will rate appetite sensations during each test visit and provide breath samples to measure gastrointestinal transit time.

This is a randomized crossover trial examining the effects of food processing level and palatability on appetite and gastrointestinal transit time. Participants will complete a screening visit followed by four test visits, with each test visit separated by at least one week. Each participant will consume all four test meals: high-palatability ultra-processed food, low-palatability ultra-processed food, high-palatability non-ultra-processed food, and low-palatability non-ultra-processed food. Palatability will be reduced by adding bitter melon powder to the low-palatability meals. During each test visit, participants will consume the assigned test meal containing lactulose as a marker of gastrointestinal transit time. Breath samples will be collected to assess orocecal transit time using hydrogen analysis. Participants will also rate meal palatability and appetite sensations, including hunger, fullness, desire to eat, amount they could eat, and thirst, using visual analog scales during the test visit.

Inclusion Criteria:

• Individuals aged 18 to 60 years.
• Any sex, gender, or orientation.
• Self-reported good health.
• No medication use.
• No allergies or food sensitivities to any of the foods used in the study.
• No anticipated change in physical activity during the study period.
• Willingness to complete all planned study activities, including the screening visit and four test visits.
• At screening, participants must rate the selected high-palatability foods between 60 and 90 on a 100-unit visual analog scale, where 100 = extremely palatable.
• At screening, participants must rate the selected low-palatability foods between 10 and 40 on a 100-unit visual analog scale.

Exclusion Criteria:

• Younger than 18 years or older than 60 years.
• Current medication use.
• Allergies or food sensitivities to any study foods.
• Not meeting the required palatability rating ranges during screening.
• Anticipated change in physical activity during the study period.
• Not self-reporting good health.
• Unwillingness or inability to complete the planned study activities.

Individual participant data will not be shared because the current IRB-approved protocol does not include a formal plan for external individual participant data sharing. Study data will be de-identified for analysis, and identifying information will be stored separately from study data. Code numbers linking participants to their data will be destroyed according to the approved protocol. No information by which participants can be identified will be released or published.