Plant-based diets, rich in fiber, are linked to improved planetary and human health. A meta-analysis demonstrated a dose-response relationship between higher fiber intake and reduced risk of non-communicable diseases like obesity. Increasing global fiber intake is proposed to extend lifespan and reduce healthcare costs. Adopting plant-based diets could counter rising obesity and greenhouse gas emissions. However, the physiological and psychological factors influencing plant-based food intake remain understudied. Physiological mechanisms, involving gut-brain signaling and macronutrient content, likely modulate appetite control. Studies suggest that plant-based meals may induce greater satiety than meat-based meals, independent of weight status. Dietary fiber improves body weight and activates appetite-regulatory pathways. Short-chain fatty acids, from fiber fermentation, reduce psychosocial stress responses. However, the timeframe of these effects is unclear. Psychological mechanisms, such as self-control and cognitive strategies, also influence food choices. While pre-meal planning impacts choices, its role in plant-based diets is uninvestigated. Restrictive diets, like vegetarianism and veganism, have been linked to increased depressive symptoms, although the reasons remain unclear. Conversely, adequate nutrient intake, including high fiber, is associated with reduced depressive symptoms and anxiety. Reverse causation in food-mood relationships remains unresolved, and the impact of single meals with varying fiber content on mood is unclear. Interventions like labeling, portion size reduction, and price adjustments can influence food choices. Increasing vegetarian meal availability has increased plant-based meal sales. However, the perceived physiological and psychological effects of these choices remain largely unknown. This study aimed to determine the effects of a single plant-based or animal-based meal on satiety and mood in a large-scale, real-world setting.

Existing research suggests a correlation between plant-based diets and various health benefits, primarily due to their high fiber content. Meta-analyses have shown positive effects on weight management and the reduction of chronic diseases. However, studies exploring the acute effects of a single plant-based meal on satiety and mood are scarce. While some studies indicate increased satiety from plant-based meals compared to macronutrient-matched meat-based meals, the role of fiber and other nutrients remains unclear. The influence of psychological factors, such as pre-meal planning and individual dietary habits, also requires further investigation. Existing literature on the relationship between dietary restrictions and mental health presents mixed findings, necessitating further research into the potential short-term effects of single meals on mood.

This study employed a series of three preregistered smartphone-based studies conducted in German university cafeterias. **Study 1 (App-based study):** This large-scale study utilized the iMensa and Mensaplan apps, reaching a large, diverse sample of university students. Participants (n=16,135) rated their hunger and mood before and after consuming a meal of their choice using a five-point Likert emoji scale. Additional data collected included meal choice (plant-based or animal-based), meal planning (planned or spontaneous), gender, usual dietary habits, fluid intake, social interaction during the meal, smartphone use, and sleep quality. The meal categories were specified based on the available information provided by the cafeterias. Taste ratings were also collected. **Study 2 (Browser-based study with free choice):** A smaller, more deeply phenotyped sample of omnivorous participants (n=173) rated hunger, mood, and stress before and after a meal of their choice using a ten-point Likert scale. This study included detailed questionnaires assessing dietary habits, personality traits, eating behaviors, and general well-being. **Study 3 (Browser-based study with randomized choice):** A randomized controlled trial (n=71) with omnivorous participants assigned randomly to either an animal-based or plant-based meal. Participants rated hunger, mood, and stress before and after the meal, similar to Study 2. Post-meal, contentment with the meal assignment was also assessed. In all studies, time stamps (from app data or meal photos) were used to monitor compliance. Data Analysis: Linear mixed-effects models were used to analyze the data, accounting for individual and cafeteria variation. Chi-square tests were used for categorical data. Exploratory analyses examined the roles of various factors (macronutrients, taste, gender, social interaction, meal planning etc.) influencing satiety and mood.

Across all three studies (n_all = 16,379), meal intake resulted in decreased hunger and increased mood. However, the type of meal (plant-based vs. animal-based) did not significantly affect post-meal hunger. Contrary to the hypothesis, post-meal mood increases were slightly smaller after plant-based meals compared to animal-based meals. Pre-meal mood was slightly higher for those choosing plant-based meals. Protein content showed a marginal positive association with post-meal satiety, while gender and taste ratings strongly influenced both satiety and mood. Exploratory analyses revealed several key points: * Macronutrient composition differed between meal categories: plant-based meals had higher carbohydrates and sugar but lower fat and protein than animal-based meals. * Taste ratings significantly influenced both satiety and mood, with higher ratings leading to greater satiety and improved mood. Animal-based meals were rated as tastier overall. * Choosing a plant-based meal was more often associated with planned decision-making. * Gender and finishing the meal significantly influenced hunger and mood ratings. Females reported lower hunger than males, and diverse gender reported lower mood and higher hunger. * Social interaction didn't affect post-meal hunger or mood. * Subgroup analysis based on dietary habits showed that the effect of plant-based meals on mood and hunger differed across groups: omnivores and vegetarians showed decreased mood and increased hunger, while vegans showed the opposite. * In Study 3 (randomized meal allocation), contentment with the assigned meal influenced post-meal hunger and mood, indicating that forced choice significantly impacts the subjective experience. * A correlation was found between higher meal-induced satiety and greater mood improvement.

This study's results do not support the hypothesis that single plant-based meals significantly improve satiety and mood compared to animal-based meals. The observed small differences in mood might be attributed to societal expectations regarding taste and satisfaction associated with animal-based meals. The higher pre-meal mood in the plant-based group might reflect the long-term positive effects of plant-based diets. The strong influence of taste on satiety and mood highlights the importance of improving the palatability of plant-based options. Protein content appears to play a larger role in satiety than previously anticipated. The finding that plant-based meals were more often chosen based on planned decisions suggests a greater consideration of dietary goals in plant-based eaters. This underscores the influence of long-term dietary patterns and individual preferences on subjective experience. The impact of gender and finishing the meal indicates the complexity of eating behavior and the need for gender-sensitive approaches in promoting healthy dietary changes.

This large-scale study reveals that the type of meal (plant-based or animal-based) has a relatively minor impact on post-meal satiety and mood compared to factors like taste, protein content, and gender. Improving the taste and protein content of plant-based meals is crucial for increasing their acceptability and promoting sustainable dietary shifts. Future research should focus on longitudinal studies examining the long-term effects of different dietary patterns on satiety, mood, and overall well-being, investigating the role of internal beliefs and psychological factors influencing dietary choices.

The study's limitations include the reliance on self-reported subjective ratings, potential for bias in online surveys, limited nutritional data available for analysis (including the lack of fiber content data), and a predominantly young, student population. The sample size of the diverse gender group was small in comparison, which limited the generalizability of the findings to this group. The relatively short timeframe of the study may not fully capture the long-term effects of different dietary patterns. Data curation and exclusion of specific datapoints reduced sample sizes in certain analyses, potentially impacting statistical power.