Positron emission tomography (PET)-computed tomography (CT) has identified metabolically active supraclavicular fat in adult humans based on uptake of labeled glucose and confirmed to be brown adipose tissue (BAT) histologically. However, PET-CT has estimated a prevalence of BAT as low as 5% in adult humans, casting doubt on its significance. The true prevalence of BAT is unknown because of the suboptimal sensitivity of standard PET-CT.

The objective of the study was to determine whether BAT is present in PET-negative supraclavicular fat.

This was a prospective cohort study.

The study was conducted at a tertiary referral hospital.

Seventeen patients who underwent preoperative PET-CT for staging of head and neck malignancy participated in the study.

The main outcome was signature BAT gene transcripts and protein in biopsies of supraclavicular fat with sc fat as negative control.

PET-CT was positive in three and negative in 14 patients. PET-positive fat harbored multilobulated lipid droplets and stained strongly for uncoupling protein 1 (UCP1). These features are absent in sc fat. By contrast, PET-negative fat contained a predominance of cells with unilobulated lipid droplets, with scattered cells containing multilobulated lipid droplets and variable UCP1 staining. Molecular analyses of fat biopsies showed lower but clear expression of UCP1, NDUFS3 (NADH dehydrogenase (ubiquinone) iron-sulfur protein 3), β₃-adrenoceptor, and PRDM16 (PR domain containing 16) transcripts.

BAT is present in supraclavicular fat, regardless of PET status. BAT is highly prevalent in adult humans, and its abundance determines PET status.