This study aims to characterize how individuals differ in their metabolic response to food intake, specifically focusing on early diet-induced thermogenesis (DIT). DIT refers to the increase in energy expenditure that occurs after eating and reflects the body's metabolic response to processing nutrients.

Healthy adult participants will complete a single laboratory visit under standardized conditions. Resting metabolic rate will first be measured, followed by consumption of a standardized liquid meal (550 kcal). Energy expenditure will then be continuously monitored for 3 hours using indirect calorimetry.

The primary objective is to quantify inter-individual variability in early postprandial thermogenesis. Rather than estimating total daily energy expenditure, this study focuses on the early metabolic response following meal ingestion. Findings will improve understanding of differences in metabolic efficiency between individuals and support future research in metabolism and personalized nutrition.

Diet-induced thermogenesis (DIT) represents the increase in energy expenditure above resting levels following food intake and reflects the energetic costs of digestion, absorption, transport, and metabolism of nutrients. Although DIT typically accounts for a relatively small proportion of total daily energy expenditure, it plays an important role in postprandial metabolism and may contribute to inter-individual differences in energy balance and body weight regulation.

Most prior research has focused on estimating total DIT over prolonged postprandial periods (e.g., 4-6 hours or longer). However, these approaches may obscure meaningful variability in the early phase of the thermogenic response, which is characterized by rapid and dynamic metabolic changes. The most pronounced increase in energy expenditure typically occurs within the first 1-3 hours following meal ingestion, driven by early physiological processes including gastrointestinal activity and neuroendocrine responses.

Emerging evidence suggests that individuals vary substantially in their metabolic response to a standardized meal, even under controlled conditions. However, this variability is often averaged out in group-level analyses or diluted by long measurement windows. The present study is specifically designed to characterize observed inter-individual variability in early DIT, rather than to estimate total thermogenesis or test intervention effects.

This study is a controlled, cross-sectional physiological study in healthy adults. Each participant will complete a single study visit under standardized laboratory conditions. After an initial resting period, resting metabolic rate will be measured using indirect calorimetry. Participants will then consume a standardized liquid meal providing 550 kcal, after which energy expenditure will be continuously monitored for 180 minutes.

Strict pre-test standardization procedures (including fasting, avoidance of exercise, caffeine, and alcohol) will be implemented to minimize non-biological variability. Body composition will also be assessed to support interpretation of metabolic responses.

The primary outcome is early DIT, quantified as the increase in energy expenditure above baseline over the postprandial period. Secondary analyses will describe the distribution, magnitude, and temporal characteristics of the thermogenic response, including peak response and time to peak.

By focusing on the early postprandial phase, this study aims to provide detailed characterization of variability in metabolic responses under tightly controlled conditions. These findings will contribute to a better understanding of human metabolic heterogeneity and may inform future research in metabolic physiology, obesity, and personalized nutrition.

Although a standardized liquid meal is administered as part of the protocol, the study is designed to characterize physiological variability in metabolic response rather than to evaluate the effect of an intervention.

Inclusion Criteria:

• Adults aged ≥18 years
• Stable body weight (±2 kg over the previous 3 months)
• Generally healthy individuals
• Ability to comply with study procedures, including fasting and remaining at rest - during measurements

Exclusion Criteria:

• Use of medications known to affect metabolism (e.g., GLP-1 receptor agonists, systemic corticosteroids, uncontrolled thyroid disease)
• Insulin-treated diabetes
• Pregnancy or lactation
• Acute illness at the time of testing
• Inability to comply with pre-test requirements (e.g., fasting, avoiding caffeine, alcohol, or physical activity)