This prospective observational study aims to characterize within-individual variability in resting metabolic rate (RMR) over a 12-week period using high-frequency repeated measurements under standardized conditions. Participants will undergo RMR assessments three times per week using whole-room indirect calorimetry.

The study will quantify the magnitude of physiological variability in RMR, distinguish true biological variation from measurement error, and estimate the minimal detectable change required to identify meaningful metabolic adaptations.

Resting metabolic rate (RMR) is a major component of total daily energy expenditure and is widely used in metabolic research. However, most studies rely on single or infrequent measurements, assuming RMR is stable within individuals.

Emerging evidence suggests that RMR exhibits measurable within-individual variability, but the temporal structure and magnitude of this variability remain poorly characterized.

This study will implement a high-frequency repeated-measures design, with RMR measured three times per week over 12 weeks using whole-room indirect calorimetry under standardized conditions. This approach will allow detailed characterization of short-term variability and provide estimates for within-subject variability, measurement error, and minimal detectable change.

The findings will inform interpretation of longitudinal metabolic studies and improve study design in future research.

Inclusion Criteria:

• Adults aged 18-65 years
• Clinically healthy individuals
• Stable body weight (±2 kg) over the past 3 months
• Ability and willingness to attend morning measurements three times per week for 12 weeks
• Ability to provide informed consent

Exclusion Criteria:

• Known uncontrolled endocrine or metabolic disease
• Use of medications known to significantly affect resting metabolic rate (e.g., systemic corticosteroids, unstable thyroid hormone therapy)
• Acute illness within 14 days prior to enrollment
• Pregnancy or lactation
• Any medical condition that, in the opinion of the investigators, may interfere with safe participation

Healthy adult men and women from the community, aged 18-65 years, who are clinically healthy and able to participate in repeated metabolic measurements under standardized laboratory conditions.