Effect of Caloric Restriction on Organ Size and Its Contribution to Metabolic Adaptation: An Ancillary Analysis of CALERIE 2

mechanisms vary between cold exposure and meal intake, yet direct comparisons using non-invasive imaging in humans remain limited.Methods: We assessed BAT activity in 40 healthy Caucasian volunteers (20 males, 20 females; age 24.5 AE 3.2 years; BMI 22.3 AE 2.3 kg/m 2 ) under cold and meal interventions.Cold exposure involved a pre-test to determine individual non-shivering temperature, followed by a 120-minute cooling suit intervention, while the meal intake was a high-fat meal.Infrared thermography (IRT) measured supraclavicular skin temperature changes (T), and multispectral optoacoustic tomography (MSOT) was used to determine tissue biomarkers, including total blood volume (TBV), oxygenated (HbO 2 ), and deoxygenated hemoglobin (Hb), and fat blood ratio (FBR).Also, energy expenditure (EE) and forehead temperature (T F ) were measured by indirect calorimetry (IC) and iButton sensors, respectively.All measurements were collected over 180 min.Results: Meal intake led to a T elevation within the first 30 min, whereas TBV exhibited a continuous rise, suggesting prolonged BAT activity.Cold exposure triggered a sustained T increase lasting 120 min, indicating persistent BAT activation, with TBV showing more nuanced changes, displaying a biphasic pattern peaking at 15 and 90 min.These TBV changes correlated positively with HbO 2 and Hb but inversely with FBR between peaks.TBV and T F were significantly correlated at 60 and 90 min during cold exposure, showing sustained core temperature maintenance, but no correlation was found postprandially.Conclusions: We provide a comprehensive comparison of MSOT and IRT for assessing BAT activity during cold exposure and meal intake.The differing activation patterns emphasize IRT's focus on surface heat dissipation, which could arise from cumulative thermal signals, while MSOT captures deeper tissue perfusion and metabolic activity.These findings highlight the importance of non-invasive imaging to understand BAT dynamics and metabolic mechanisms under cold and meal challenges, requiring further study.