Journal September 27, 2026 5 min read

How is brown adipose tissue measured?

Brown adipose tissue (BAT) is measured in research settings by imaging, indirect calorimetry, and tissue biopsy, with each method capturing a different aspect of BAT amount or activity.

peptides-pro Editorial

Brown adipose tissue (BAT), also called brown fat, is a thermogenic adipose depot whose formal study in humans relies on a small set of measurement modalities: positron emission tomography combined with computed tomography (FDG-PET/CT), indirect calorimetry, and adipose tissue biopsy. BAT is not a compound with a sequence, formula or molecular weight; it is a tissue, and the identifiers that matter for measurement are the radiotracer used, the anatomical depot sampled, and the physiological stimulus applied. In human adults, functional BAT is typically identified by radiotracer uptake on PET/CT, most often after cold exposure, because cold is the major physiological activator of the tissue [4].

The work centres on the sympathetic nervous system and its beta-adrenergic receptors, particularly the beta3-adrenergic receptor, which is the receptor family most directly tied to BAT thermogenesis and to the pharmacological tools used to probe it [7].

Research Studies

FDG-PET/CT and cold exposure in human BAT measurement

The standard human measurement approach combines a radiolabelled glucose analogue with anatomical imaging. In a study of chronic beta3-adrenergic receptor agonist treatment, participants underwent scans in which a radioactive substance was injected into the arm, alongside exercise testing and blood, urine and heart tests, in adults aged 18–40 with a body mass index between 18 and 40 [7]. The same study design included an overnight baseline visit and repeat screening tests, indicating that BAT measurement is embedded in a broader metabolic phenotyping protocol rather than performed in isolation [7].

Indirect calorimetry and fat biopsy in hyperthyroid patients

An observational study of patients with thyrotoxicosis used FDG-PET/CT, indirect calorimetry and fat biopsies in the acute phase of the disease and again in the euthyroid phase after treatment [8]. The investigators hypothesised that high levels of circulating T3 might affect the amount and function of brown adipose tissue [8]. This design illustrates the two axes of BAT measurement: amount, captured by imaging and biopsy, and activity, captured by calorimetry.

NAD+ metabolism and human BAT function

A human study addresses whether the decline in brown fat functionality with age relates to declining availability of nicotinamide adenine dinucleotide (NAD+), a molecule central to several metabolic processes [6]. The project is designed to clarify whether NAD+-based molecules affect brown fat metabolism in humans [6]. The rationale given is that fully functional brown fat helps maintain a healthy weight and decreases the risk of metabolic diseases such as type II diabetes, and that brown fat functionality declines with age in human adults [6].

Capsinoid supplementation and BAT in obese adolescents

A clinical study examines capsinoid supplementation and brown adipose tissue in obese adolescents, motivated by the discovery of functional BAT in humans and by the observation that BAT oxidises fatty acids within its multiple lipid droplets to produce heat [4]. The tissue is described as playing an anti-obesogenic rather than a storage role, owing to its high metabolic and energetic activity [4]. Cold exposure remains the major physiological stimulus referenced in that work [4].

FSTL1 and white adipose tissue browning in male mice

Follistatin-like 1 (FSTL1) has been considered a vital adipokine, and prior evidence linked it to white adipose tissue (WAT) adipogenesis, brown adipose tissue involution and thermogenesis [1]. In male mice at ambient room temperature, FSTL1 overexpression induced WAT browning, enhanced energy metabolism and promoted glucose uptake, while overexpression specifically in WAT prevented metabolic dysfunction in mice with diet-induced obesity [1]. Mice with FSTL1 deficiency exhibited cold intolerance and were prone to obesity [1]. The report states that FSTL1 cannot directly induce the effect, indicating an indirect mechanism [1]. These findings are in mice.

Follistatin and adipose browning in mice

Follistatin (FST) binds to and neutralises members of the transforming growth factor-beta (TGF-β) superfamily, regulating skeletal muscle, adipose and bone homeostasis [2]. FST promotes adipose browning and enhances energy metabolism, leading to improved plasma lipid profiles and metabolic health in mice [2]. Transcriptomic and metabolomic analyses of the Hybrid Mouse Diversity Panel found that Fst expression was negatively correlated with aortic lesion area, and the anti-atherogenic potential of FST was investigated in that context [2]. These findings are in mice.

GDF-11 and cold-induced browning in mice and 3T3 cells

Growth differentiation factor 11 (GDF-11), a circulating member of the TGF-β superfamily, was investigated for its role in adipose tissue plasticity [3]. Mice were exposed to cold to induce browning, and GDF-11 expression and SMAD2/3 activation were assessed in brown adipose tissue, visceral and subcutaneous white adipose tissue, and skeletal muscle [3]. The study combined in vivo results with in vitro approaches using 3T3 cells [3]. The work is therefore animal and cell-culture based.

Erythropoietin and adipose metabolic regulation

Erythropoietin (EPO) is a key regulator of erythrocyte production, promoting erythroid progenitor cell survival, division and differentiation in the fetal liver and adult bone marrow, and mice lacking EPO or its receptor die in utero due to severe anaemia [5]. Beyond haematopoiesis, EPO influences non-haematopoietic tissues including glucose and fat metabolism in adipose tissue, skeletal muscle and the liver [5]. EPO enhances lipolysis while inhibiting lipogenic gene expression in white adipose tissue, brown adipose tissue and skeletal muscle [5]. EPO is used clinically to treat anaemia associated with chronic kidney disease and plays a role in maintaining metabolic homeostasis and regulating fat mass [5].

Comparison of human BAT measurement modalities

Modality	What it captures	Example context
FDG-PET/CT	Radiotracer uptake, interpreted as BAT amount and location	Thyrotoxicosis study, acute and euthyroid phases [8]; beta3-agonist study in adults aged 18–40 [7]
Indirect calorimetry	Energy expenditure, interpreted as BAT activity	Thyrotoxicosis study [8]
Fat biopsy	Tissue sampled directly for molecular analysis	Thyrotoxicosis study [8]
Cold exposure as stimulus	Physiological activation before measurement	Described as the major physiological activator [4]

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