Activation and quantification of human brown adipose tissue: Methodological considerations for between studies comparisons: Comment on: Hot heads & cool bodies: The conundrums of human BAT activity research

Infrared Thermography for Estimating Supraclavicular Skin Temperature and BAT Activity in Humans: A Systematic Review

OBJECTIVE

Brown adipose tissue (BAT) is a thermogenic tissue with potential as a therapeutic target in the treatment of obesity and related metabolic disorders. The most used technique for quantifying human BAT activity is the measurement of 18 F-fluorodeoxyglucose uptake via a positron emission tomography/computed tomography scan following exposure to cold. However, several studies have indicated the measurement of the supraclavicular skin temperature (SST) by infrared thermography (IRT) to be a less invasive alternative. This work reviews the state of the art of this latter method as a means of determining BAT activity in humans.

METHODS

The data sources for this review were PubMed, Web of Science, and EBSCOhost (SPORTdiscus), and eligible studies were those conducted in humans.

RESULTS

In most studies in which participants were first cooled, an increase in IRT-measured SST was noted. However, only 5 of 24 such studies also involved a nuclear technique that confirmed increased activity in BAT, and only 2 took into account the thickness of the fat layer when measuring SST by IRT.

CONCLUSIONS

More work is needed to understand the involvement of tissues other than BAT in determining IRT-measured SST; at present, IRT cannot determine whether any increase in SST is due to increased BAT activity.

Supraclavicular skin temperature measured by iButtons and 18F-fluorodeoxyglucose uptake by brown adipose tissue in adults

Currently, 18 [F]-Fluorodeoxyglucose (¹⁸F-FDG) in combination with a positron emission tomography/computed tomography (PET/CT) scan analysis is the most commonly used method to quantify human BAT volume and activity. However, this technique presents several drawbacks which negatively affect participant's health. The aim of the present work is to determine whether supraclavicular skin temperature can be used as an indirect marker of cold-induced BAT and skeletal muscle ¹⁸F-FDG uptake in adults, while taking into account body composition. We performed a personalized cooling protocol just before an ¹⁸F-FDG-PET/CT scan, and we measured supraclavicular skin temperature before (in warm conditions) and after the cooling protocol in 88 adults (n = 57 women, mean age: 21.9 ± 2.1 years old, body mass index: 24.5 ± 4.3 km/m²). We found that supraclavicular skin temperature at the warm and cold periods was weakly and positively associated with BAT activity (SUV_mean and SUV_peak: β = 3.000; R² = 0.072; P = 0.022 and β = 2.448; R² = 0.060; P = 0.021), but not with skeletal muscle ¹⁸F-FDG uptake, after controlling for body composition. We performed further analyses and the positive associations persisted only in the group of women. In conclusion, supraclavicular skin temperature in warm and cold conditions seems to be related with cold-induced ¹⁸F-FDG uptake by BAT only in women, although the low explained variance of these associations means that there are other factors involved in the supraclavicular skin temperature.

Association between brown adipose tissue and bone mineral density in humans

BACKGROUND

Brown adipose tissue (BAT) seems to play a role in bone morphogenesis. A negative association has been reported between BAT and bone mineral density (BMD) in women, but not in men. A panel of experts has recently published a set of recommendations for BAT assessment, and thus, to re-address previously reported associations is needed. This study aimed to investigate the association between cold-induced BAT ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG) uptake and BMD in young healthy adults.

METHODS

Ninety-eight healthy adults (68 women; 22 ± 2.2 years old; 24.3 ± 4.5 kg/m²) cold-induced BAT was assessed by means of an ¹⁸F-FDG positron emission tomography-computed tomography (PET-CT) scan preceded by a personalized cold stimulation. The cold exposure consisted in 2 h in a mild cold room at 19.5-20 °C wearing a water perfused cooling vest set 4 °C above the individual shivering threshold. Total body and lumbar spine BMD were assessed by a whole-body DXA scan.

RESULTS

We found no association between BMD and cold-induced BAT volume, mean, and maximal activity (all P > 0.1) in neither young and healthy men nor women. These results remained unchanged when adjusting by height, by body composition, and by objectively assessed physical activity. Sensitivity analyses using the criteria to quantify cold-induced BAT-related parameters applied in previous studies did not change the results.

CONCLUSIONS

In summary, our study shows that there is no association between cold-induced BAT and BMD in young healthy adults. Moreover, our data support the notion that previously shown associations between BAT and BMD in healthy non-calorically restricted individuals, could be driven by methodological issues related to BAT assessment and/or sample size limitations.

Brown Adipose Tissue and Skeletal Muscle 18F-FDG Activity After a Personalized Cold Exposure Is Not Associated With Cold-Induced Thermogenesis and Nutrient Oxidation Rates in Young Healthy Adults

Cold induced thermogenesis (CIT) in humans results mainly from the combination of both brown adipose tissue (BAT) and skeletal muscle thermogenic activity. The relative contribution of both tissues to CIT and to cold induced nutrient oxidation rates (CI-NUTox) remains, however, to be elucidated. We investigated the association of BAT and skeletal muscle activity after a personalized cold exposure with CIT and CI-NUTox in 57 healthy adults (23.0 ± 2.4 years old; 25.1 ± 4.6 kg/m²; 35 women). BAT and skeletal muscle (paracervical, sternocleidomastoid, scalene, longus colli, trapezius, parathoracic, supraspinatus, subscapular, deltoid, pectoralis major, and triceps brachii) metabolic activity were assessed by means of a ¹⁸Fluorodeoxyglucose positron emission tomography-computed tomography scan preceded by a personalized cold exposure. The cold exposure consisted in remaining in a mild cold room for 2 h at 19.5–20°C wearing a water perfused cooling vest set at 3.8°C above the individual shivering threshold. On a separate day, we estimated CIT and CI-NUTox by indirect calorimetry under fasting conditions for 1 h of personalized cold exposure. There was no association of BAT volume or activity with CIT or CI-NUTox (all P > 0.2). Similarly, the skeletal muscle metabolic activity was not associated either with CIT or CI-NUTox (all P > 0.2). The results persisted after controlling for sex, the time of the day, and the date when CIT was assessed. Our results suggest that human BAT activity and skeletal muscle ¹⁸F-FDG activity are not associated to CIT in young healthy adults. Inherent limitations of the available radiotracers for BAT detection and muscle activity quantification may explain why we failed to detect a physiologically plausible association.

Role of Human Brown Fat in Obesity, Metabolism and Cardiovascular Disease: Strategies to Turn Up the Heat

Human brown adipose tissue (BAT) was re-discovered in 2009 by several independent groups, who showed that it is present and active in adults, as judged from the profound uptake of the glucose analogue radiotracer ¹⁸F-fluorodeoxyglucose in positron-emission tomography and computed tomography scan analysis after cold exposure. A potential clinical implication of activating BAT relates to its high metabolic activity and its potential role in stimulating energy expenditure (i.e. resting energy expenditure, meal-induced thermogenesis, and cold-induced thermogenesis), which makes it an attractive target to reduce adiposity. Moreover, due to its ability to oxidise glucose and lipids, BAT activation may also potentially exert beneficial metabolic and cardiovascular effects through reducing glucose and lipid levels, respectively. This review describes the potential role of human BAT in the prevention and treatment of obesity, metabolism, and cardiovascular disease focusing on its impact on energy expenditure and management of body fat accumulation as well as on glucose and lipid metabolism. This article also summarises the strategies that are currently being studied to activate human BAT.

The impact of using BARCIST 1.0 criteria on quantification of BAT volume and activity in three independent cohorts of adults

Human brown adipose tissue (BAT) is commonly assessed by cold-induced 18F-fluorodeoxyglucose (FDG) PET-CT using several quantification criteria. Uniform criteria for data analysis became available recently (BARCIST 1.0). We compared BAT volume and activity following BARCIST 1.0 criteria against the most commonly used criteria [Hounsfield Units (HU):-250, -50, standardized uptake value (SUV):2.0; HU: Not applied, SUV:2.0 and HU:-180, -10, SUV:1.5] in a prospective study using three independent cohorts of men including young lean adults, young overweight/obese adults and middle-aged overweight/obese adults. BAT volume was the most variable outcome between criteria. While BAT volume calculated using the HU: NA; SUV: 2.0 criteria was up to 207% higher than the BAT volume calculated based on BARCIST 1.0 criteria, it was up to 57% lower using the HU: -250, -50; SUV: 2.0 criteria compared to the BARCIST 1.0. Similarly, BAT activity (expressed as SUVmean) also differed between different thresholds mainly because SUVmean depends on BAT volume. SUVpeak was the most consistent BAT outcome across the four study criteria. Of note, we replicated these findings in three independent cohorts. In conclusion, BAT volume and activity as determined by 18F-FDG-PET/CT highly depend on the quantification criteria used. Future human BAT studies should conduct sensitivity analysis with different thresholds in order to understand whether results are driven by the selected HU and/or SUV thresholds. The design of the present study precludes providing any conclusive threshold, but before more definitive thresholds for HU and SUV are available, we support the use of BARCIST 1.0 criteria to facilitate interpretation of BAT characteristics between research groups.

A New Personalized Cooling Protocol to Activate Brown Adipose Tissue in Young Adults

Brown adipose tissue (BAT) activity is induced when humans are exposed to cold. Therefore, cold exposure prior to the ¹⁸F-FDG-PET/CT scan is used as a tool to quantify BAT. Several cooling protocols, including fixed and personalized ones are currently in use. The aim of the present study was to determine the effect of a new personalized cooling protocol where the shivering threshold was measured on a separate day, on BAT volume and activity in young adults. A total of 47 adults (n = 28 women) aged 22 ± 2 years participated in the study. We determined participants' shivering threshold (visually and self-reported) using a water perfused cooling vest in an air-conditioned cold room. 48–72 h later, participants wore the cooling vest set at ~4°C above the shivering threshold for 60 min prior to injection of ¹⁸F-FDG and ~5°C above the shivering threshold for ~60 min after injection, until PET/CT scan. We quantified BAT following BARCIST 1.0 recommendations. We identified 40 participants (85%, n = 25 women) as PET+ and 7 (n = 3 women) as PET–. The PET+ group presented significantly higher BAT volume and activity than PET– group (all P < 0.05). PET+ women had higher BAT mean activity than PET+ men (SUV_mean: 5.0 ± 1.6 vs. 3.6 ± 0.9 g/ml respectively, P = 0.003), and there were no significant sex differences in BAT volume (P = 0.161). A total of 9 out of 47 participants did not shiver during the shivering threshold test. Our findings are similar to previous cold-stimulated human BAT studies; therefore, we conclude that our personalized cooling protocol is able to activate BAT in young adults.