Figure 3.

PET/MRI scanning of subjects in response to perception-based cooling or a thermal comfort state

PET/MRI scanning was part of the perception-based cooling or thermal comfort state protocols described in Figures 1 and 2, and data are shown from WS (n = 7) and C (n = 8). n represents the number of human individuals in each group and is consistent throughout this figure unless otherwise specifically stated.

(A) Representative images of a subject (winter swimmer) at a thermal comfort state (left) and during cooling (right).

(B) Glucose tracer (FDG) uptake in BAT measured as mean standardized uptake value (SUVmean) in the regions shown in (A).

(C) SUVmean uptake multiplied by the volume of BAT with SUV above or equal to 2.0 during a TC state or during cooling as a measure of BAT metabolic volume (BMV).

(D) Representative image showing glucose tracer uptake in the perirenal BAT (left), which was quantified for each subject and compared between groups and in response to a TC state and cooling (right).

(E) Resting energy expenditure (REE) in winter swimmers and controls in response to a thermal comfort state or cooling. The actual analysis was performed for 30 min, and the energy consumption during 24 h represents a calculated value if the energy consumption remains constant during this period.

(F) Cold-induced thermogenesis, calculated as the delta increase in energy expenditure induced by cooling.

(G) Lean body mass as measured by DEXA scan as an estimation of muscle mass.

(H) Electromyography (EMG) measurements recorded during 5 min at a thermal comfort state and during 5 min of cooling. Values are the area under the curve, including negative peaks, calculated per individual.

(I) A subset of subjects displayed glucose tracer uptake in intercostal and/or neck muscles following cooling (left). The signal in intercostal muscles was graded 1–4 depending on intensity, and 0.5 was added if the subject also had a signal in the neck. The scale bars thus represent estimated arbitrary values for cold-induced glucose uptake in the intercostal and neck muscles.

(J) Example of an MRI scan on a cooling day. Delineation of the region of interest (ROI) for calculation of the water percentage in areas with BAT (upper and middle panels), subcutaneous WAT (upper and lower panels), and skeletal muscle (middle panel).

(K) Water percentage in WAT, BAT, and skeletal muscle as calculated from MRI data.

(L) Water percentage in BAT in response to cooling.

(M–O) Gene expression analysis of LIPE, PLIN1, and ELOVL6 in WAT between groups and in response to cooling.

Differences between groups and treatments were assessed using mixed models and subsequent post-tests. Differences in AUC were assessed with an unpaired t test. When not otherwise specified, data are presented as mean ± SD. ∗p < 0.05, ∗∗p < 0.01, ∗∗∗p < 0.001, ∗∗∗∗p < 0.0001. See also Figure S2.